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Chapter 7
Nutrition:
Energy Yielding
Nutrients
IN THIS CHAPTER
Carbohydrates
Fiber
Sugars
Glycemic Response
Fats
Trans Fatty Acids
Protein
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Introduction
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into carbohydrates, or proteins and carbohydrates into
fat by arranging the carbon chains and elements to
reflect the desired energy substrate. The rearrangement
of carbons by the body is an important function, as it
supports the ability of the body to reformulate nutrients
that are needed but may not be present in necessary
quantities. This process allows for energy needs to be
met during work and energy to be stored when not
currently needed by the body.
An optimal diet is one that supplies the necessary
nutrients in adequate amounts to maintain, repair, and
grow tissue without providing excess energy that the
body stores in the form of adipose tissue. Human
nutritional needs are relative to individual genetic
predisposition; affected by normal variations in nutrient
digestion, absorption, and assimilation; needed to meet
specific requirements for energy expenditure of physical
activity; and influenced by individual dietary practices
and preferences.
Carbohydrates
Although all nutrients are necessary for the body to
function properly, carbohydrates are the most important
nutrient related to physical activity. They represent the
primary fuel source for intense work and are necessary
for the formation of ATP used by the central nervous
system, making the nutrient an indispensable part of a
healthy diet. Carbohydrates fall into three general
categories: monosaccharides, disaccharides, and
polysaccharides. Monosaccharides are the most basic
form of carbohydrate. They represent a single sugar
moiety. Different monosaccharides vary from each
other based on their carbon-to-hydrogen-to-oxygen
linkage sequences.
These four factors indicate that there is no single,
universal diet for optimal nutrition (26; 36; 50; 51).
However, careful evaluation of food consumption,
coupled with food intake planning in accordance with
sound nutritional guidelines, will result in general and
specific improvements in overall health, fitness, and
performance.
Nutrients are divided into six classes and two
subcategories. Carbohydrates (CHO), proteins, and
lipids are individually classed and categorized as
energy-yielding nutrients. The non-energy-yielding
category includes the other three classes of nutrients:
vitamins, minerals, and water. Each plays a vital role in
the proper function of the human body in response to
varying physiological conditions. This chapter will
review the energy-yielding nutrients.
These differences in sequence determine a
carbohydrate’s biochemical characteristics.
For
example, glucose and galactose are monosaccharides
with the same number of carbon, hydrogen, and oxygen
atoms, but they are arranged differently, thus making
them behave and taste slightly different from each
other. Disaccharides, as their name implies, are formed
by the joining of two separate sugar molecules
(monsaccharides). Lactose is a disaccharide formed by
the combination of a glucose molecule with galactose.
Both mono- and disaccharides are referred to as simple
sugars because they require little manipulation by the
cell for use as energy
Finally, polysaccharides are
chains of monosaccharides linked together in sequences
from as few as three sugars, to as many as several
thousand. As the chains of monosaccharides sugars
increase in length and diversity, the complexity of the
nutrient increases. Polysaccharides are more commonly
referred to as complex carbohydrates and have chain
linkages of tens to thousands of monosaccharide
Energy-Yielding Nutrients
Energy-yielding nutrients are those nutrients that
provide the body tissues with usable energy to form
ATP. All of the energy-yielding nutrients contain the
chemical element carbon. The carbon atoms are bound
together into varying carbon chain skeletons. Carbon
chains become linked with atoms of other elements to
form lipids, carbohydrates, and proteins. When
consumed, the process of digestion breaks food down to
its most basic energy form. In the small intestine, the
nutrients are absorbed into the blood and transported to
tissues to perform their respective jobs throughout the
body.
Once the energy is in the blood stream,
chemoreceptors identify the nutrients and
determine a specific course of action for the
energy. The body has the ability to manipulate the
energy to meet any particular demand or to store
the energy for later use. The metabolic organs
determine the outcome of the energy consumed in
the diet via hormonal regulation. The liver is the
primary metabolic organ that can manipulate the
energy form based on the acute internal needs of
the body. If needed, the liver can convert proteins
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recommended 20-35 grams, the Western diet
predisposes consumers to an elevated disease risk (65).
~Key Terms~
Monosaccharides- The simplest form of
carbohydrates, comprised of one saccharide
molecule.
In less industrialized nations, the fiber content of the
average diet is between 40-100g per day (17). Not
surprisingly, the rate of related diseases is dramatically
lower in those countries than in America. Fiber
enhances gastrointestinal function and reduces irritation
to the intestine wall, mobilizes harmful chemicals and
compounds inhibiting their activity, shortens the time
for intestinal transport to excretion, decreases the length
of time carcinogenic materials stay in the intestines, and
slows down the absorption rate of carbohydrates, acting
positively on blood glucose dynamics. Additionally, the
foods associated with a high fiber diet often support
health by being lower in calories and more nutrient-rich
(6; 7). For this reason, fiber should be consumed
through food sources rather than as a supplement (66).
Sample foods high in fiber are whole grain products,
fruits, and leafy
green vegetables.
Disaccharides- A simple form of carbohydrate,
comprised of two monosaccharides.
Polysaccharides- A form of carbohydrate,
consisting of a number of monosaccharides.
Complex Carbohydrates- Sugar molecules that are
strung together in long, complex chains.
Glycogen- The main storage carbohydrate found
primarily in the liver and muscles.
residues. These complex chains are classified as either
plant or animal polysaccharides depending on their
dietary source.
Due to the fact that
carbohydrates serve
as the primary fuel
source for physical
activity, it makes
sense that they
represent the largest
portion of energy
from the diet. The
recommended
dietary intake for
carbohydrates
is
55%-60% of the
total diet, with the
majority
being
derived
from
polysaccharide
(complex) sources.
However,
the
average American
consumes
only
about 40-50% of their total diet from carbohydrate
sources with 50% of those calories being derived from
mono- and disaccharides (simple sugars). It has been
Glycogen and starch are two common forms of
polysaccharides in our diets. Glycogen is the storage
form of carbohydrates in animal tissue, while starch is
the term used for the storage form of carbohydrates in
plants. Starch is the larger component in seeds, corn,
potatoes, beans and the various grains that make up
common foods like spaghetti, bread, and cereals. Plant
starch remains the most important source of
carbohydrates for most Americans, accounting for more
than 50% of the total carbohydrates consumed. This
number has decreased by 30% since the turn of the
century, when starches comprised about 80% of
carbohydrate sources. This significant decline has been
met by an equally significant increase in simple sugar
consumption.
Fiber
Fiber is a non-starch polysaccharide, and in the form of
cellulose, is the most abundant organic molecule found
on earth. It is a tough material that resists enzymatic
breakdown, making it indigestible by humans, except
for a small portion that is fermented by intestinal
bacteria. Fiber is commonly categorized as either
soluble or insoluble. The bacterial breakdown of fiber
generally contributes less than 2 kcal per gram, making
it an excellent addition to calorie-controlled diets. Its
consumption is linked with lower occurrences of several
“Western” diseases including obesity, diabetes,
hypertension, intestinal disorders, some cancers, and
heart disease (1; 19; 55; 61). Due to the fact that most
Americans consume far less (12-15g) than the
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estimated that at least 25% of the American diet is
simple sugar, a trend that significantly contributes to the
increased risk for obesity, diabetes, and heart disease.
Most people consume more than 100 lbs. of sugar
annually, mainly in the form of sucrose and high
fructose corn syrup (23).
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conjunction with obesity is linked to the development of
Type II diabetes.
As mentioned earlier, a concern with high amounts of
insulin is that abnormally high levels facilitate the
conversion of sugar into triglycerides in the liver, which
are then stored as fat in adipose tissue. Diets high in
sugar are directly linked to an increased risk for obesity.
Contrary to this, foods high in dietary fiber and protein
slow the absorption rate of the sugars and minimize
surges of blood glucose. A reduction in blood sugar
causes a decrease in the concentration of insulin
secreted into the blood. Additionally, when fat is
combined with simple carbohydrates, the glycemic
effect is reduced, compared to the effects of the
independent sugar source (29; 59). Regular exercise
also exerts a notable positive influence on the body’s
sensitivity to insulin making regular physical activity a
key component in the prevention and management of
diabetes.
Sugars
Sugars fall into the categories of monosaccharides and
disaccharides. Glucose, fructose and galactose are all
monosaccharides. They are similar in chemical formula
but differ by specific carbon, hydrogen, and oxygen
linkage. Glucose is the main monosaccharide used by
the body. It can be split for ATP, stored for later use as
glycogen, or converted into lipid form following its
absorption into the cells. Fructose is nature’s fruit sugar.
It is the sweetest of the sugars per gram. When ingested,
it is converted into glucose by the liver. Galactose is an
animal monosaccharide found in dairy products.
The disaccharides include lactose, maltose, sucrose,
and trehalose.
Sucrose is the most commonly
consumed sugar form, representing up to 25% of some
American diets (24). It is commonly consumed in baked
products or in the form of table sugar. Lactose is formed
from glucose and galactose. It is often referred to as
dairy sugar. Many people around the world are lactose
intolerant, lacking sufficient enzymes to break the
disaccharide down in the body. When two glucose
molecules are combined, they form maltose, which is
present in cereals, germinating seeds, and beer.
The following glycemic index table identifies foods
according to their specific insulin responses.
The
glycemic value in the table is determined after ingesting
50 grams of a carbohydrate source and comparing the
resulting blood sugar levels over a two-hour period to a
standard for carbohydrates, usually white bread or
glucose, which has an assigned value of 100. The
corresponding value expresses the relationship or
percentage of total area under the curve set by the
reference standard. If white bread represents 100% of
the curve, then a food encompassing 45% of the total
area would be assigned a glycemic index value of 45.
International values for glycemic index do exist, but
slight discrepancies arise when comparing specific trials
due to variations in the foods used.
Glycemic Response
When sugars enter the blood, they raise the blood
glucose levels. The specific response is based on the
amount and type of sugar consumed. The elevation in
blood glucose concentrations is quantified using a
measurement system called glycemic indexing. The
glycemic index describes the effect carbohydrate
sources have on circulating blood glucose levels by rate
and concentration. The rate at which the food increases
blood glucose is called the glycemic response. The
amount of glucose in the blood is referred to as the
glycemic load.
~Quick Insight~
Monosaccharides can be broken down to form
derivatives which can be used by the body. Sugar
acids, sugar alcohols, and amino sugars are formed
when monosaccharides are metabolized. Amino
sugars and sugar acids are present in connective
tissue and assist in supporting the tissue’s metabolic
needs. Sugar alcohols have become a popular additive
to supplements and foods to provide flavor with a
decreased energy value. Glycerol and sorbitol, among
others, are used in energy bars and foods that attempt
to provide reduced sugar content and glycemic effect
in the diet (2). They are often not counted under the
total carbohydrates measured on the labels of food or
they are advertised on the label as “non-impact carbs”
for
carbohydrate-conscious
consumers.
They
generally provide 1-3 calories and have much less of
an effect on blood glucose dynamics (52).
In low fiber diets, manufacturer processed starches and
simple sugars are digested quickly and enter the blood
stream at a rapid rate. This causes the release and often
over-release
of
insulin,
which
heightens
hyperinsulinemia (a state of high insulin release by the
pancreas) (4; 9; 12; 18). Consistently eating high
glycemic foods may lead to reduced insulin sensitivity
(3; 6; 7; 9). This reduced insulin sensitivity causes the
pancreas to release higher amounts of insulin in
response to the blood sugar; this response in
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absorbed into the blood at a faster rate,
which can quickly elevate blood
glucose levels. This may be good for
post-exercise consumption, but for the
average, inactive person, this translates
to excessive circulating blood sugars,
leading to increased fat storage.
Ideally, the body attempts to keep blood
sugar concentrations at a constant level
throughout the day without dipping too
low, which can cause fatigue, or going
too high, such as with hyperglycemia.
However, most Americans have large
fluctuations in their blood glucose
levels within a 24-hour period. When
plotted on a line graph it looks
somewhat erratic, with many peaks and
valleys. After meals, especially those
high in simple sugars, a quick rise in
blood glucose concentration occurs,
which is often referred to as “spiking.” These large
fluctuations occur because most Americans consume
the majority of their calories in two to four sittings. The
body’s blood glucose response from three or four large
meals exceeds the stable regulatory functions because
the glycemic load is so high. As mentioned earlier, the
pancreas releases insulin during periods of high blood
sugar to increase glucose uptake by cells. Glucagon is
released during low levels of blood sugar, causing the
liver to release glucose into the blood stream so that it is
constantly available to cells that do not store glycogen,
Processed Carbohydrates
The manufacturing process manipulates the biological
integrity of some foods and consequently will often
affect the glycemic response. For instance, whole grains
are processed to create a more desirable consistency, as
seen with white flour. The reduction in biochemical
complexity increases the glycemic index of the food
from its original form. Essentially, the complex chains
are broken down to reflect a more simplified food
source. The end product from grain processing produces
a carbohydrate that is more rapidly digestible and
~Quick Insight~
Certain sugars, such as sucrose and high fructose corn syrup, can wreak havoc on blood glucose stability. These
saccharides are collectively identified as dietary troublemakers. Naturally occurring fructose is touted as dietetic, or a
more desirable sugar, because of its sweetness and reduced glycemic index (see below), particularly when consumed
from non-juiced, whole fruit sources. But unlike naturally occurring fructose, the commercially formulated highfructose corn syrup is quickly absorbed and causes a rapid rise in the body’s blood glucose concentration. Research
trials identified increased serum cholesterol and low-density lipoprotein (LDL) concentrations in the blood when 20%
of the dietary calories came from this monosaccharide.
It is estimated that the largest part of the American population routinely exceeds 20% of total dietary calories from
sucrose and high fructose corn syrup. The high consumption of sugars disrupts normal metabolic pathways due to the
response of insulin in attempts to prevent hyperglycemia. The hormones that control the level of circulating blood
sugar are regulated by beta- and alpha-cells in the pancreas. Elevated blood glucose causes the pancreas to release
insulin in proportion to the rate and quantity of sugar absorption. Insulin facilitates an uptake of the circulating blood
sugar into skeletal muscle and the liver, where it is stored as glycogen.
When high amounts of simple sugars are consumed on a regular basis, the body works hard to remove the glucose
from the blood. Once glycogen stores are filled, the body may metabolize the sugar as fuel or convert it in the liver
into triglycerides. Triglycerides are then stored as fat in adipose cells. Insulin is lipogenic, meaning it increases fat
storage. Consistently elevating blood glucose and the consequent release of insulin causes an increased susceptibility
to fat gain. This phenomenon is considered a major contributor to weight gain in America.
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relieved and blood glucose levels are
appropriately re-established. If hunger is
deferred, it soon turns into appetite.
Appetite is the psychological perception of
caloric need. The brain’s psychological
discernment of need exceeds the
physiological requirements for food and
predisposes an individual to over consume
calories. Anyone who has ordered and
consumed appetizers before the main course
at a restaurant probably realized they were
not as hungry as they originally thought
when the main dish arrived. Avoiding
appetite-related eating patterns is important
for preventing overeating habits (8).
When carbohydrate selections are made
intelligently, they can effectively aid in the
reduction of body weight and improve
health. Carbohydrates account for most of
the thermic effect of food, which can
represent up to 10% of daily caloric
expenditure. As previously stated, complex
carbohydrates are nutrient dense and
provide dietary fiber, which reduces the risk of
digestive disorders, and can be low in calories when
such as the brain. Eating patterns that cause the body to
work in constantly fluctuating blood glucose level
conditions cause higher concentration release of
pancreatic hormones. These conditions manifest
themselves in poor utilization of energy nutrients and
cause changes in the cellular use of glucose. For the
ideal blood glucose situation to occur, we need to take
in smaller quantities of food at roughly 90-120 minute
intervals, reflecting a grazing style of eating. This
suggests the average meal size should be lower in
calories to maintain optimal blood glucose levels (20;
62; 67). Grazing animals such as deer follow this eating
pattern and maintain low levels of body fat and do not
develop type II diabetes, in part because their foods are
high in fiber and low in calories, but it is also because
regular intake normalizes blood glucose (30). Although
7-9 meals may be unrealistic, many nutritionists
recommend eating small meals more frequently
throughout the day to mirror the grazing pattern of
eating to avoid large spikes in blood glucose levels.
~Key Terms~
Starch- A complex carbohydrate found in seeds, fruits,
and stems of plants and more notably, in corn, rice,
potatoes, and wheat.
Fiber- Indigestible plant matter, consisting primarily
of polysaccharides that when consumed increase water
absorption and intestinal peristalsis.
Soluble Fiber Sources- Oats/oat bran, dried beans,
nuts, barley, and vegetables such as carrots.
Insoluble Fiber Sources- Dark green/leafy vegetables,
fruit skins, corn bran, and seeds.
Glucose- A simple sugar (monosaccharide) used as the
primary fuel source by most cells in the body to
generate energy.
Fructose- A sweet sugar (monosaccharide) found
primarily in fruits.
A second benefit to increasing the consumption
frequency of carbohydrates and consuming calories in
smaller portions is to thwart hunger. When chemical
receptors in the tissues identify reduced levels of blood
glucose,
the
hypothalamus
stimulates
the
physiologically controlled hunger mechanism to
communicate the need for food. When this occurs, the
required calorie intake to meet the current need is often
low, equal to maybe a couple hundred calories. If food
is consumed at this time, the hunger mechanism is
Galactose- A simple sugar (monosaccharide) found in
dairy products.
Lactose-A disaccharide in dairy products that
hydrolyzes to yield glucose and galactose.
Maltose- A white sugar formed during the digestion of
starch.
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consumed from appropriate sources. Likewise,
phytochemicals and other compounds found in whole
grains can reduce the risk of certain diseases, such as
cancer and cardiovascular disease (31; 48). Compliance
with the suggested guidelines for carbohydrate
consumption seems to be one of the problems with the
American diet. Due to the fact that the majority of
calories in the average diet come from processed
carbohydrates and sugars, the added health benefits
from the foods are minimal, and the thermic effect of
most diets is low. Authors of diet books have
recognized this pattern and recommend reducing
carbohydrates to decrease the effects of sugar and
processed
carbohydrates
for
weight
control.
Carbohydrate selection can play a major role in weight
management and disease risk when intelligently
applied.
carbohydrates reduces the stored glycogen in the body
and is often not appropriately replaced. Due to the fact
that glucose bonds with water molecules when forming
glycogen in the cell, reducing glycogen stores within
the body also reduces the body’s water content. The
body will significantly diminish its carbohydrate stores
in about 96 hours under an extended fast, which can
account for significant metabolic water loss (21; 54).
This mechanism accounts for most of the initial weight
loss in low carbohydrate diets. Whereas calorie control
and exercise have been touted as the only way to
effectively lose weight and keep it off, most low
carbohydrate dieters gain the weight back soon after
finishing the diet. In fact, attempting to lose weight by
diet alone usually is only successful for approximately
2-5% of the population (13; 33). Most dieters gain the
weight back within 6-9 months. Additionally, low
carbohydrate diets are contrary to the metabolic
functions of the body for activity. When glycogen stores
are low, so is work capacity. In general, low
carbohydrate diets should not be recommended for
physically active people. It should be noted that
controlling sugar and processed carbohydrates is not the
same as a low carbohydrate diet.
The current trend toward cutting carbohydrates should
refer to reducing simple sugar and processed
carbohydrate consumption, not cutting complex
carbohydrates from the diet. The problem lies in the
dissemination of information to the general public. In
many cases, the popularized low-carbohydrate diets
cause weight loss for all the wrong reasons. Cutting
Carbohydrate Depletion
~Key Terms~
Sucrose- A disaccharide found in many plants and used as
a sweetener, which is more commonly known as table
sugar.
Trehalose- A sweet tasting disaccharide.
Glycemic Indexing- A rating system for evaluating how
different foods affect blood sugar levels.
Glycemic Response- A measure of the increase of blood
sugar after food consumption.
Glycemic Load- A ranking system for carbohydrate
content in food portions based on their glycemic index and
portion size.
Hunger Mechanism- A physiological response to hunger
controlled by chemical receptors being stimulated by the
hypothalamus in response to reducing levels of blood
glucose.
Appetite- An instinctive physical desire or caloric need.
Thermic Effect of Food- The increment in energy
expenditure above resting metabolic rate due to the cost of
processing food for storage and use.
Protein-Sparing Mechanism- The body’s preferential
utilization of fats and carbohydrates instead of protein for
energy.
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Carbohydrate availability affects more than energy
output alone; it also affects metabolic biochemistry.
When carbohydrate consumption is reduced by poor
eating habits, high protein diets, reduced energy
intake, starvation, or fasting, the body reacts to
compensate for the deficit. Reduced glycogen
reserves and low plasma glucose concentration trigger
glucose synthesis through a process called
gluconeogenesis in the liver and, to a lesser extent, in
the kidneys. Amino acids and triglyceride molecules
are rearranged to form sugar to fuel the body’s need.
The gluconeogenic conversion breaks down both lean
mass and fat mass to augment carbohydrate
availability. Normally the body has a natural proteinsparing mechanism, maintained by an adequate
consumption of carbohydrates and fat. Preferably, the
body does not want to use protein for fuel. Even if
adequate protein is consumed to meet functional
requirements, protein-sparing may not occur because
it is regulated by neural and hormonal assessment of
carbohydrate stability. Without protein-sparing, amino
acids are liberated from lean mass, primarily skeletal
muscle, and reformulated into carbohydrates in the
liver for energy. Prolonged effects of lean mass
catabolism can lower the body’s total metabolic rate.
This problem is exacerbated by the fact that the body
relies on carbohydrates as the primer for fat
metabolism. As mentioned earlier, with the byproducts of carbohydrate breakdown being reduced,
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amount of energy in higher-intensity aerobic training.
Energy derived from the breakdown of absorbed
glucose, and glycogen stored in the muscles and liver,
powers the protein contractile elements during muscle
contractions and additionally required biological work.
When exercise is intense or prolonged, glycogen stores
are rapidly depleted. For physically active people,
carbohydrate intake should mirror the training volume
on a daily or weekly basis. Low carbohydrate
consumption can lead to premature depletion of
glycogen, causing the storage volume to become too
low to support activity. The following graph illustrates
the varying fuel source contribution to glycogen storage
and its role in prolonging steady-state exercise.
there is an increased fat mobilization and decreased
capacity for fat oxidation. Under this state of
insufficient carbohydrate metabolism, more fat is
mobilized than can be oxidized. This produces an
incomplete breakdown of the fat and the build up of
acetone byproducts referred to earlier as ketone bodies.
For this reason, low carbohydrate diets are also referred
to as ketogenic diets. Only consumption of adequate
amounts of carbohydrates will prevent this phenomenon
from occurring.
After analyzing the relationship between energy
metabolism and available carbohydrates, it should be
obvious that the consumption of carbohydrates is
relative to specific need but has a definite lower limit.
The exact quantity consumed depends on the amount
and type of physical activity an individual engages in on
a regular basis. If a person is sedentary, carbohydrate
consumption may be as low as 45% of the total diet
because the level of activity they routinely engage in is
equally low. For individuals who exercise regularly,
carbohydrates should comprise roughly 55-60% of the
total diet (27; 38; 43). It is recommended that the
majority of carbohydrates come from complex sources
Carbohydrate consumption proves to be of further
significance due to the fact that the brain and central
nervous system require carbohydrates for proper
function. Under normal conditions, the brain uses blood
glucose almost exclusively for fuel. When the body is
deprived
of
carbohydrates,
gluconeogenesis is used to maintain
blood glucose concentrations for
nerve tissue metabolism and as a
fuel for red blood cells. In a low
carbohydrate environment, blood
glucose can drop to hypoglycemic
levels. In extreme situations, this
condition can lead to central fatigue
and may trigger unconsciousness,
and even irreversible brain damage.
Carbohydrate Need
Earlier
text
indicated
that
carbohydrates are stored in the liver
and muscle tissue in the form of
glycogen.
When consuming a
normal diet, the storage capacity
for
glycogen
is
between
approximately 300 to 400 grams or
1200-1600 kcal of energy, of which
about 80% is stored in the skeletal
muscles.
When
exercise
is
performed, carbohydrates serve to
fuel both high- and moderateintensity exercise anaerobically,
and contribute to a substantial
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Two ways exist to estimate the
grams
of
carbohydrates
necessary for daily intake
needs.
One method is to
calculate the value using a
predicted daily need and
subjective percentage rate. The
other method is to use
recommended
grams
per
kilogram of body weight,
based on estimations of
activity status. Based on the
prediction outcomes, the two
are similar in predictability
when calculated correctly.
This number will vary as
relative carbohydrate needs
change based on current
activity status. If training
volume and intensity are
increased or decreased, the
CHO intake should reflect the
shifts in training.
Fats
Fats, also known as dietary
lipids, are the other primary
source of energy the body uses
to fuel biological work. Lipid
molecules have the same
structural
elements
as
carbohydrate
molecules
(carbon,
oxygen
and
hydrogen), but differ in the
way the atoms are linked. This
marked difference determines
the way lipids are metabolized
and stored in the body. Like
carbohydrates, lipids differ in
chain length and molecular
complexity, which places them in three distinct
categories: simple lipids, compound lipids, and
derived lipids. The following table identifies the
specific characteristics related to each category.
with no more than 10% derived from simple sugar.
Total carbohydrate intake recommendation jumps to
70% for individuals who engage in regular intense
training (3; 56). The idea behind “carb-loading,” which
is the intake of high amounts of carbohydrates prior to a
race, is to optimize glycogen reserves, much like
topping off the fuel tank of a car before a trip (22; 34;
57). Frequently, however, athletes overeat the amount
of carbohydrates required, with the excess amount being
stored as fat (58). Additionally, diets that overemphasize protein often fail to meet the carbohydrate
needs of most physically active individuals.
Lipids are necessary for normal function of the body.
Most people view fat as a negative constituent of the
diet because of the perceived link to obesity. But this is
unwarranted, as fat actually serves many important
functions in the body.
The aforementioned roles that fat plays in the body
identify the importance of adequate consumption in the
diet. Problems with fat in the diet occur when more fat
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In the average American diet, lipids represent
approximately 35% of total caloric intake (64). About
34% of the dietary fats consumed come from nonanimal sources while the majority, 66%, comes from
animal sources. Dietary fat is classified as either
unsaturated or saturated fatty acids. Monounsaturated
and polyunsaturated fatty acids are commonly found
in plant sources of fat. Animal sources of fat often
contain higher amounts of saturated fats. It is
generally recommended that 30% of a individual’s diet
be derived from fat sources, with less than 7-10% from
saturated fat (40). On average, approximately 15% of
total calories are consumed as saturated fatty acids by
Americans everyday (39). This is very relevant, as a
relationship exists between fat intake, specifically
saturated fat, and coronary heart disease (16). Diets high
in saturated fats lead to a pronounced change in the
blood lipid profile, particularly in LDL cholesterol,
whereas a diet higher in monounsaturated fats has
been linked with lower coronary heart disease risk (37;
63).
is consumed than is required. Fat is high in calories and
therefore, accounts for a good part of the over
consumption predicament experienced by many
Americans. At 9 kcal per gram, the energy in fat is more
than twice that of carbohydrates and protein, a
characteristic that can contribute significantly to a
positive caloric balance. Additional problems occur
because fats are more easily stored as adipose tissue
throughout the body than their carbohydrate and protein
counterparts. Unlike carbohydrates, where stores are
limited and oxidation occurs rapidly, fat storage goes
relatively unregulated because the human body has an
almost unlimited ability to store lipids. Additionally,
biological mechanisms promote fat storage in situations
where high amounts of calories are consumed.
Unsaturated and saturated fats differ by the structural
bonds and respective carbon element make-up.
Saturated fatty acids do not have double bonds between
the carbons, which maximizes the hydrogen attachment
sites (making them saturated with hydrogen).
Monounsaturated fatty acids have a single double bond
within the carbon chain, and polyunsaturated fatty acids
~Key Terms~
have multiple double bonds, thereby
reducing the number of hydrogen
Simple Lipids- Oils or fats containing one or two different types of
attachments. Additionally, the location of
compounds.
the double bond plays a role in the
dynamics of the fatty acid. For instance,
Compound Lipids- Phospholipids and glycol-lipids, which frequently
when the first double bond is next to the
contain three or more chemical identities.
sixth carbon, the fatty acid is an omega-6,
Derived Lipids- Includes sterols and fatty acids.
and when the first double bond is next to
the third carbon it is referred to as an
Monounsaturated Fatty Acids- Any of a large group of monobasic
omega-3 fatty acid. The amount of
acids found in animal and vegetable fats and oils.
hydrogen and double bond number and site
all play a role in how the fatty acid reacts
Polyunsaturated Fatty Acids- An unsaturated fatty acid with a carbon
in the body. The following illustration
chain containing more than one double or triple valence bond per
demonstrates the chemical differences
molecule.
between saturated and unsaturated fats.
Notice the differences in the bonds and
Saturated Fats- A fat, most often of animal origin, that is solid at room
hydrogen concentration.
temperature, which contains chains of saturated fatty acids.
LDL Cholesterol- A complex of lipids and proteins that functions as a
transporter of cholesterol in blood, which, at high levels, is associated
with an increased risk of atherosclerosis and coronary heart disease
(CHD).
Monounsaturated Fats- Fatty acids with one double bonded carbon in
the molecule.
113
When fatty acids are consumed in the diet,
they enter in varying proportion based on
the type and quantity of foods consumed.
Fatty acids enter circulation through the
intestine and are picked up by lipoproteins
called chylomicrons. The lipids are
transported to the liver where they are
metabolized, attached to very low density
lipoproteins (VLDL), and delivered to fat
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cells to be stored or utilized as
fuel in the form of triglycerides.
The VLDL gives up the
transported lipids and is turned
into a low density lipoprotein
(LDL). The density of the
lipoprotein refers to the protein to
fat ratio. VLDLs are 95% fat.
LDL cholesterol circulates in the
blood en route back to the liver
where it can form bile. LDL is
considered to be hazardous
cholesterol because it is so small
it can collect on the lining of
damaged arteries. This affinity
for the cells of the arterial wall
increases risk of oxidation and
the proliferation of smooth muscle cells, thereby adding
to plaque in the artery, known as atherosclerosis (see
Chapter 12). High density lipoprotein (HDL) has a
protective role, inhibiting arterial plaque formation,
making HDL the more desirable cholesterol in
circulation.
compounds. The molecular manipulation, called
hydrogenation, occurs when one hydrogen atom of the
cis-monounsaturated fatty acid is moved from its
naturally occurring position to the opposite side of the
double bond that separates two carbon atoms. In doing
so, food manufacturers create more desirable texture
consistencies for foods like margarine, cookies, and ice
cream. Consequently, this change in the molecular
structure changes the way the lipids act in the body.
Trans fatty acids have a profound effect on serum
lipoproteins and can have possible adverse health
affects by increasing LDL cholesterol and triglycerides,
while at the same time reducing HDL cholesterol (4547). Trans fatty acids do not occur in nature, and their
intake may have other unknown detrimental effects. The
CDC estimates that trans fatty acids account for at least
30,000 deaths annually from heart disease. In 2006, the
American Heart Association recommended that 0% of
the diet come from trans fat sources (68).
Cholesterol is produced in the body, but it is also found
in food products. The body produces about 70% of its
approximately 1000 mg need per day. The remaining
300 mg comes from dietary sources (35; 71).
Cholesterol is needed to perform complex bodily
functions, including the formation of plasma
membranes and hormones. It also works in digestive
functions, forming bile and serves as a precursor for
vitamin D. Although an important lipid in the body, it
can also be problematic if consumed in abundance in
the diet. For this reason the American Heart Association
recommends consuming no more than 300 mg per day
(35). For individuals at higher risk for heart disease, that
recommendation drops to between 150 and 200 mg.
Dietary cholesterol is common in animal products that
contain higher amounts of fat. One egg yolk, for
instance, contains the full daily amount of cholesterol.
Likewise, red meat, shell fish, and dietary liver are also
high in cholesterol.
Trans Fatty Acids
Identifying which fats to consume has become
increasingly difficult due to the manipulation of fats in
processed food. Saturated fats are solid at room
temperature, whereas unsaturated fats are liquid.
Scientists working on ways to improve upon the
dynamics of fatty acids when manufacturing food
products created a specialized fat that takes advantage
of the desirable chemical components of both saturated
and unsaturated fats. Trans fatty acids have become an
ever-present component in many manufactured food
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Not all lipids have detrimental effects for
coronary artery disease. Studies performed on
Eskimos have shown a low incidence of
coronary heart disease associated with the
omega-3 fatty acids found in the high
quantities
of
fish
they
consume.
Polyunsaturated fatty acids may benefit the
blood lipid profile by reducing triglycerides
and reducing risk for heart disease (15; 25; 69;
70). Theoretically, the cold water fish oils
reduce the formation of clots on the arterial
wall in conjunction with decreasing plasma
triglycerides levels. Monounsaturated fats
have also been implicated as a healthier fat
choice. Mediterranean diets are often high in
monounsaturated fat, such as found in olive
oil, representing sometimes more than 50% of the
dietary fat consumed. Even though the average person
in Greece consumes at least 40% of their diet from fat,
they experience a much lower incidence of heart disease
(14; 42). It is important to note that the Mediterranean
diet is low in animal products and high in vegetables,
fruits, olives, and legumes. Scientists are not clear
whether the Mediterranean diet itself is the cause or if
the elevated amounts of monounsaturated fat is an
independent variable. Most conclude it is the totality of
the dietary consumption which is important.
Even with the beneficial effects some fats have on
reducing coronary risk factors, these fats still need to be
consumed in moderation. Dietary fats can be converted
to adipose tissue very efficiently when the diet is too
rich in lipid consumption (5; 49). Fat intakes that
exceed 30% of the total diet increase the risk for
developing many diseases, even if the diet is within
recommended caloric intake for the individual. To the
contrary, cutting fats out of the diet is not a prudent
decision either. Diets that drop dietary fat below 20%
of the recommended intake can cause insufficient
consumption of the essential fatty acids, linolenic, and
linoleic acids (28; 41). This is not uncommon in
dieting, as most people prefer to reduce fat in the diet
over practicing a balanced calorie restriction. For the
majority of the population, following the dietary
guidelines will help reduce the risk of heart disease
associated with the Western diet.
Dietary Fat and Disease
Cardiovascular disease is the number one killer in the
United States. Although other factors contribute to
heart disease, such as smoking and physical inactivity,
fat tends to claim a large segment of the responsibility.
Directly, dietary fat has a strong effect on blood lipid
profiles. Additionally, dietary fat contributes to
increases in fat mass and risk for obesity, which is
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linked to both heart and metabolic diseases. A high
amount of saturated fat in the diet increases blood
cholesterol dramatically, which can lead to coronary
artery disease. This is further compounded by an
excessive consumption of dietary fat, trans fatty acids,
and cholesterol. When high amounts of fat are
consumed on a regular basis, body fat is often
increased, and the risk for disease is further elevated.
For this reason, the recommendation is to consume less
than 30% of total calories from fat. Categorizing the fat
intake is a useful step that can further serve dietary
improvement.
The American Heart Association suggests a ratio of
fatty acids of 10:10:10. A healthier split of 30% dietary
fat intake may be 15% monounsaturated, 10%
polyunsaturated, and 5% or less from saturated sources
(11; 53). This recommendation alone would drastically
reduce the risk of coronary artery disease in many
people.
~Quick Insight~
When excess fat accumulates in the body, the risk for
cardiovascular disease dramatically increases. For this
reason, manufacturers produce fat-replacers, which serve as
tasty substitutes for the higher calorie lipids. These replacers
have actually caused more problems in the American diet
than they have solved. People seem to confuse fat-free
foods with calorie-free foods and consume larger volumes
of the food product than they would if real fat was an
ingredient. This leads to excessive caloric intake and a
positive caloric balance. Although the percentage of fat in
the diet has declined from just over 40% in the early
eighties to approximately 36% today, the total amount of fat
has increased because many diets are higher in calories.
Additionally, higher sugar intake is common because sugar
is used to improve the taste lost from the removal of the fat,
further contributing to the problem. In most cases,
moderation of fat and non-fat products will help reduce the
chances of overeating calories.
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particular characteristics of the molecule and the role
they will serve in the body. The body can combine
elements to create amino acids and combine the amino
acids to form an almost infinite number of proteins. As
is clear from the following picture, amino acids are
carbon chains with different linkage options, which
ultimately determine the type of amino acid and their
specific characteristics.
~Key Terms~
Chylomicrons- One of the microscopic particles
of fat occurring in chyle and in the blood,
especially after a meal high in fat.
Very Low Density Lipoproteins (VLDL)- A
lipoprotein containing a very large portion of
lipids which carry most of the cholesterol from the
liver to the tissues.
The body can synthesize almost all of the 20 different
amino acids from other amino acids. However, there
are eight amino acids that the body cannot synthesize.
These are labeled essential amino acids and must be
consumed through the diet. They are isoleucine, leucine,
lysine,
methionine,
phenylalanine,
threonine,
tryptophan, and valine. Infants and children have
additional essential amino acid needs because infants
cannot synthesize histidine, and children have been
identified to have a reduced capacity to form arginine,
lysine, histidine and cysteine (32). The source of the
amino acid does not matter, so the proteins can be
consumed as animal or plant foods. A protein termed
complete is considered to be higher quality because it
contains all eight essential amino acids in appropriate
quantity and correct ratio to maintain nitrogen balance,
whereas an incomplete protein lacks one or more of
the essential amino acids.
Trans Fatty Acids- An unsaturated fatty acid
produced by the partial hydrogenation of
vegetables oils.
Hydrogenation- The act of combining with
hydrogen
Essential Fatty Acids- Any of the
polyunsaturated fatty acids which are required in
the diet of mammals.
Linolenic Acid- An Omega-3 unsaturated fatty
acid considered essential to the human diet.
Linoleic Acid- An Omega-6 unsaturated fatty
acid, considered essential to the human diet.
Animal sources generally receive the highest rating
among dietary proteins. For the average American,
animal proteins account for up to 70% of the total
dietary protein consumed (60). This is an increase of
20% from the early 1900’s. One problem associated
with the increased intake of animal proteins is that they
are often accompanied by higher amounts of saturated
fats and cholesterol. Plant proteins, although often
incomplete, can supply all the essential proteins when
Protein
Proteins serve thousands of functions within the human
body, representing the primary structural components of
non-bone tissues. Proteins are made up of building
blocks called amino acids. In the same way many
monosaccharides are chained together to form a
complex carbohydrate, multiple amino acids are also
linked together in a chain to form a protein. Amino
acids are substances composed of carbon, hydrogen,
and oxygen, much like the other two energy nutrients,
but amino acids also have nitrogen added to form an
amine group (NH2) and side chains which give each
amino acid a unique identity. Proteins are comprised of
different amino acid sequences, making up
approximately 50,000 different protein-containing
compounds in the body. The order and number of the
amino acids in a protein give the protein its unique
properties. There are not many physiological reactions
that occur within the body that do not require the
presence of a protein at some point during the process.
As mentioned above, biochemical function and the
properties of each protein differ by how the amino acids
are structurally arranged. Twenty (20) different amino
acids are currently recognized, each differing from the
side chains that attach to the amine group and
associated carbon skeleton. The side chains dictate the
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consumed through a variety of sources and
often contain fiber and other beneficial
nutrients. This suggests that vegetarians can
get adequate protein from the foods they
consume, as long as their diets maintain a
variety of plant food sources that together
provide the eight essential amino acids.
Dietary recommendations established by the
Food and Nutrition Board of the National
Research Council and National Academy of
Science reflect the nutritional needs of the
total population, not an individual’s specific
requirements. The recommendation of 0.8-0.9
grams per kilogram of bodyweight meets the
protein requirements for most individuals.
Those who are physically active may require increased
intake. For most active people, the increased need for
protein is already met by the higher caloric intakes
associated with extra calories consumed to meet the
heightened energy needs. Individuals who engage in
high volume resistance programs or participate in
endurance training programs often need more protein to
meet the tissue re-synthesis requirements associated
with recurring microtrauma, or in the case of the
endurance athlete, protein lost to energy metabolism.
The chart above identifies recommended intake options
for different activities and physiological demands.
It should be noted that protein consumption beyond 1.6
grams per kilogram of body weight has been associated
with increased risk of elevated saturated fat in the diet
due to higher amounts of animal foods consumed and
renal and liver distress due to excessive nitrogen in
certain individuals.
~Quick Insight~
Most people consume two times the protein they need for
the day. If Americans complied with the recommended
10-15% of total calories coming from protein they would
likely find their nutritional needs would be met. The
American diet relies heavily on protein as a staple to most
meals, making the diet rich in nitrogen. For most
exercisers, additional protein is not warranted based on
their current consumption level and relative intensity of
the activities they regularly complete. Many new
exercisers want to buy protein supplements to encourage
muscle gain. Increasing protein alone will add no new
benefits and is likely to contribute to weight gain from a
positive caloric balance. Likewise, increasing protein
intake requires an equal increase in water consumption to
manage excess nitrogen. If protein supplements are
deemed necessary to meet training demands, many
experts recommend whey protein as a first choice.
A 175 lb. male with 15% body fat, training four days a
week and using moderate resistance training would have a
recommended protein intake of approximately 110 grams
and a total calorie requirement of 2912 kcal per day. His
protein needs can easily be met by consuming 15% of his
calories from protein.
3000 calorie diet x 10-15% = 300-450 kcal or 75-113
grams
118
Individual requirements are based on the frequency,
duration, and intensity of exercise. High intensity
endurance and resistance training activities increase
protein requirements significantly, compared to
sedentary and low level physical activity participation.
To calculate the estimated protein requirements of an
individual use the formula on the next page and the
previous protein intake chart.
Protein intakes above 2.0 g/kg of bodyweight often
cause increased renal stress due to the high quantities of
nitrogen (10; 44). Protein consumption that exceeds
requirements for anabolic processes can force the body
into additional work to deal with the overflow. Protein
is degradated into amino acids during digestion.
Deamination occurs in the liver where the amino acid
loses its nitrogen group to form urea. When protein is
needed in the body, the deaminated amino acids
formulate into new amino acids. Excessive protein
intakes can cause the deaminated amino acid to
formulate carbohydrates or fat, depending on the
physiological environment. When protein is overconsumed, the kidneys and liver must process the
excess, and therefore, become stressed from additional
biochemical reactions. Furthermore, the kidneys require
more water to create urine when the solute
concentration of urea is high. This can lead to increased
fluid loss and dehydration. Excessive protein consumed
over an extended period of time can be detrimental to
the body.
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When manipulating protein intake for weight gain or
weight loss some key recommendations should be
followed. As described earlier, a high protein diet does
not increase rate of weight loss, compared to balanced
diets of similar calorie content. Most early weight loss
is due to dehydration from the loss of water. Protein
diets do offer some fat loss, but again, the weight
reduction is due to calorie control, not the specific
energy involved. To ensure adequate protein is
consumed during dietary restriction, protein should be
consumed at approximately 15% of the total calories.
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~Key Terms~
Essential Amino Acids- Eight of the 20 amino
acids that the body cannot synthesize that must
be consumed in a diet.
If weight gain is intended, the additional calories can
come from protein sources. In general, adding 30-50
grams (120-200 kcal) of protein is recommended as
long as the total amount reflects a value less than 2.0
grams per kilogram of body weight. Additional calories
beyond a positive caloric balance of 200 kcal per day
may lead to fat gain in many individuals by the
conversion of excess proteins into carbohydrates and
fat. If protein supplements are consumed, the
appropriate time is post-exercise. Cellular use of protein
is heightened as is cellular uptake of carbohydrates in
the presence of insulin.
Complete Protein- A food source that contains
adequate amounts of the essential amino acids.
Incomplete Protein- A food source that does
not contain adequate amounts of the essential
amino acids.
~Quick Insight~
Nitrogen balance occurs when the amount of nitrogen in the protein consumed equals the nitrogen excreted by the
body. When nitrogen balance is positive, the amount of nitrogen intake exceeds the amount of nitrogen excreted.
Positive nitrogen balance occurs when the tissues utilize the largest amount of protein for anabolic processes, such as
growth, maintenance and repair. This is commonly seen during the stages of developmental growth, pregnancy,
illness and with resistance-trained individuals through protein synthesis. When the nitrogen balance becomes
negative, the output of nitrogen exceeds the intake, and protein is utilized as an energy source. Inadequate
consumption of carbohydrates and fats can lead to negative nitrogen balance, even in the presence of adequate protein
intake. Low carbohydrate diets, inadequate nutrition, or excessive exercise have all been implicated in negative
nitrogen balance and consequential loss of lean mass proteins.
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