Download Though Val Thinks His Meth

Human Nutrition
Agriculturally-produced foods can meet human nutritional
requirements. What are those requirements?
1. Caloric need: ~2,000 – 2,600 Kcal/day depending on sex,
size, and activity level
2. Essential amino acids: isoleucine, leucine, lysine, threonine,
valine, tryptophan, histidine, methionine, phenylalanine. All
these must be obtained from the diet. A helpful mnemonic
(thanks to Dr. Crosby): I Love Lysine, Though Val Thinks
His Meth’s Preferable.
3. Essential Fatty Acids: linoleic acid, linolenic acid,
arachadonic acid. These, too, must be consumed in the diet.
4. Vitamins: 2 groups – fat soluble and water soluble
Fat soluble: A (retinol), D (calciferol), E (tocopherol) and K
(phylloquinone)
Water soluble: C (ascorbic acid), folic acid, B vitamins – B1
(thiamine), B2 (riboflavin), B3 (niacin), B5
(pantothenic acid), B6 (pyroxidine), B12
(cobalamin)
5. Minerals: Major – Ca, P, K, S, Na, Cl, Mg
Minor – Fe, I, Zn, Se, Mn, Cr, Cu, Co, Mb, F
Think about how the typical North American diet compares to
that of the !Kung San bushmen and to the requirements
indicated in this list. You may want to check out Chapter 10
and the USDA Food Pyramid in fig.10.5.
To understand the importance of a proper diet, let’s examine
the ‘diseases’ associated with various dietary lacks and
excesses:
The first problem is what happens when the diet provides
insufficient calories (and probably also a similar insufficiency
in protein intake and other nutrients). The ‘disease’ is called
marasmus.
This image is of a child
during WWII.
When the diet includes sufficient caloric intake, but
insufficient protein, the ‘disease’ is called kwashiorkor. This
is common in underdeveloped countries where the diet is
almost all starch after weaning.
Symptoms include edema (caused by hypoalbuminemia)
evident in puffy skin and swollen belly, frequently dermatitis,
behavioural listlessness, liver diease (fatty liver), loss of
muscle mass, and changes in pigmentation
Both marasmus and kwashiorkor, if not reversed in growing
infants, will result in mental retardation and stunted growth.
Once these conditions occur, they are permanent. Earlier
symptoms are readily reversible with improvement in the
diet.
What about the opposite general condition – overnutrition?
That’s what’s common in North America. About 40% of us
are now either overweight or obese. That is a serious health
problem. Obesity greatly increases the risk of many serious
problems, including: hypertension, cardiovascular disease,
non-insulin dependent (type II) diabetes mellitus, asthma,
glaucoma, and a number of cancers.
Type II diabetes used to be a disease of middle age and
beyond. Now it is appearing in many young adults (age ~20)
and even in adolescents. Lifetime cost of medication is high.
Your textbook presents the latest measure to indicate
overweight or obese conditions: body mass index or BMI.
BMI = body weight in kg/(height in m)2
Optimum is a BMI of 20-25
Overweight is a BMI of 25-30
Obese is a BMI of >30
Severe and dangerous obesity is a BMI >40
Underweight is a BMI <20
With no intention of embarrassment, let’s check the BMIs of a
few members of the class…
Fiber in the diet: There are two basic types of fiber – soluble
and insoluble.
Soluble fiber includes materials like:
• pectins (used to thicken jams),
• gums (Look at the ingredients on your next package of ice
cream. You are likely to find guar gum being used as a
thickener and ‘smoothing’ agent. It’s also what makes
oatmeal porridge thick)
• hemicelluloses (alginates, made from hemicelluloses in cell
walls of algae, are also frequently found as thickeners in
yogurt and ice cream)
Some soluble fibers are thought to lower cholesterol levels,
but may have their main effect as bulk, moving food through
the gut faster and lowering time for cholesterol uptake.
Insoluble fiber is not soluble in water. As such, most is
indigestible and adds no nutrient to the diet, but does make
for ‘regularity’ and may (??) reduce the occurrence of polyps
in the large intestine. Those polyps are considered percursors
of colon cancer.
Now on to proteins. Some amino acids are essential. Those,
listed earlier, cannot be synthesized by metabolic pathways
we (and most other animals) possess.
Proteins from animal sources are almost all complete,
meaning they include those essential amino acids (think
logically: if animals need them in the diet to grow and
prosper, when we harvest them, they must have obtained
those essential amino acids). Plant proteins are mostly
incomplete, lacking one or more of the essential amino acids.
How much protein do we need?
Much less than most of us eat! About 10% of caloric intake
should come from protein.
For a 2000 kcal diet, about 200 kcal should come from
protein. At 4 kcal/g, that means about 50g of protein per day.
Assume your average “1/4 pounder” is just less than 20%
protein. ¼ pound is ~115 g. 20% of that is ~24 g, or about ½
your needed daily intake of protein. If you have it as a
cheeseburger, with a 50 g slice of cheese, you’ve added about
another 15 g protein.
You only need another 10 – 15 g of protein over the rest of
your day to meet daily requirements.
Fats and Cholesterol: it is suggested that 30% of caloric intake
should come from fats and oils.
Most fats and oils in foods are in the form of triglycerides – a
glycerin to which 3 fatty acids are bound. Here’s the basic
structure:
This is a saturated fat. There are no double bonds in the fatty
acid chains
Saturated fats (and fatty acids) can pack tightly together, and
thus are generally ‘solid’. Double bonds along the carbon
chains (one = monounsaturated, >1 = polyunsaturated) cause
bends in the chains. They cannot pack tightly together, and are
therefore typically liquid (oils).
Note that both of these fatty acids are in cis configuration.
Lately there has been a lot of press about trans fats. The trans
configuration means that where there is a double bond, the
hydrogens attached to the carbons are ‘pointed’ in opposite
directions:
H
…-C=C- …
H
Trans fatty acids mostly act like saturated fats, and are
formed when lipids are ‘hydrogenated’ partially or fully to
manufacture products like margarine. This kind of trans fat is
now being eliminated (by KFC, MacDonald’s, and others).
Some trans fats in animal food products (red meats, milk,
cream, and butter) cannot be eliminated. Conjugated linoleic
acid is a major component, and may be an anticarcinogen.
We need fats in our diet.
They are raw materials for the biosynthesis of cell membrane
components, a number of hormones (various sex hormones are
all cholesterol derivatives), and cholesterol itself. Some
cholesterol is synthesized in our livers from saturated fatty
acids; some is absorbed from animal products in our diets:
eggs, butter, cheeses and meats.
Fats, however, are insoluble in water. They must be bound to
water-soluble protein to move in the circulation. So bound
they are called lipoproteins. They fall into two classes: low
density lipoproteins (LDL) typically called “bad cholesterol”
and high density lipoproteins (HDL) called “good
cholesterol”.
Reason for those common names: LDL is deposited on
arteries, slowly clogging them. HDL tends to remove LDL
from arterial walls and prevent accumulation.
When LDL deposits develop in an artery, they partially block
it.WThey also collect other junk, and a plaque develops.
h
Platelets
clump, trying to repair the arterial wall. The whole
e hardens. Slowly, blood flow beyond the blockage is
mess
n
reduced.
It may finally become a complete blockage. If it
L
occurs
in an artery supplying the
D a ‘heart attack’ (myocardial
heart,
L
infarction)
occurs, and downstream
d of the heart muscle die. If it
parts
e
occurs
in the brain, a stroke occurs,
p downstream brain cells die.
and
o
The
s U.S. FDA has set a standard
that
i total cholesterol:HDL as a
ratio
t should be less than 5:1.
s
Vitamins and Minerals: These are collectively called
micronutrients, since we only need a little of each one in our
daily diet.
We’ll consider vitamins first. Remember, there are two types:
water soluble and fat soluble vitamins.
Too little of any vitamin causes some form of deficiency
disease, but too much can also cause problems.
Water soluble vitamins: the vitamin B complex are mostly
coenzymes, necessary to the proper function of various
enzymes in the body. Since they are water soluble, they can
be lost in food preparation that uses excessive amounts of
water, then discards the water. Some can also be destroyed by
high temperature cooking.
Vitamin B1 is otherwise called thiamine. It is a critical
coenzyme in basic metabolism of carbohydrates. The disease
of thiamine deficiency is beriberi. The symptoms are: muscle
weakness, fatigue, confusion, cramping, numbness in the
legs.
This woman is so weakened by
beriberi that she cannot stand.
Major sources of thiamine: cereal bran, whole grains, seeds,
nuts, legumes, and, to a lesser extent, eggs, milk and meat.
Vitamin B2 or riboflavin is another important coenzyme. You
may recognize the electron transport component FAD – the F
is for flavin. Deficiency symptoms include a number of
ocular problems (eye strain and fatigue), sensitivity to light,
and lesions of the mouth and lips.
Riboflavin deficiency – angular
stomatitis of the mouth and lips,
evident as irritation and fissuring
in the corners of the mouth
Rich food sources include liver, meat, eggs, green leafy
vegetables, enriched and whole grain breads and cereals.
Vitamin B3 or niacin (or nicotinamide) forms the coenzyme
NAD, phosphorylated NADP, and reduced as NADPH. This
coenzyme is critical in both photosynthesis and metabolism.
Niacin deficiency results in pellagra. The sequence of effects
are called the 4Ds: dermatitis, diarrhea, depression (or
dementia) and death.
Rich sources include meat, peanuts and legumes, whole grain
and enriched breads and cereals. Part of the niacin
requirement is met by synthesis of niacin from
tryptophan.
This ‘necklace’ is typical in the early
stages of pellagra dermatitis. Facial
lesions are also evident. His tongue would
be ‘strawberry’ in colour.
Corn is poor in niacin and tryptophan. When corn was
brought back from the New World, nearly epidemic pellagra
resulted. In the New World, corn was prepared with lime,
which releases niacin from the grain and prevented pellagra.
Vitamin B12 or cobalamin does not occur in plant-derived
foods. It must come from meat, dairy products and eggs in
the diet. Vegans must take dietary supplements.
The most common symptoms of deficiency are weakness,
fatigue, palpitations, dizziness, and pallor. These are all
related to the disease which can result: pernicious anemia. In
pernicious anemia, red blood cells are malformed, poor
oxygen transportation results, and there can be eventual
peripheral nerve damage. Cobalamin is also involved in DNA
synthesis, most evident in red blood cell formation.
pernicious
anemia
Note that very few of these red blood cells look like the
healthy ‘donuts’ we’d expect.
The last of the water soluble vitamins is folic acid. Basically, it
functions with B12, and has the same deficiency effects (except
peripheral neuropathy).
The fat soluble vitamins:
Vitamin A – there are plant and animal sources. From plants
the source is β-carotene (e.g. from carrots). From animals the
source is retinol. It is an important anti-oxidant.
Vitamin A deficiency leads to ‘night blindness’ due to
insufficient synthesis of opsin proteins (rhodopsin – black
and white vision and iodopsins for colour vision). Long term
and severe deficiency leads to corneal degeneration and
epithelial keratinization
Vitamin D is critical in the regulation of calcium and
phosphorus: 1) it is important in absorption of these minerals
from the gastrointestinal tract (from food), 2) it acts as a
cofactor with parathyroid hormone (see below) to release
calcium from bone when the blood level drops, and 3) it acts
on the kidney to cause calcium retention.
Vitamin D is the only vitamin we can make. It is made from a
cholesterol derivative in the skin under exposure to UV light
(the product is a ‘preliminary’ form – provitamin D). The
provitamin is converted to the active form in the liver.
Vitamin D deficiency leads to rickets (pictures with calcium
regulation). Vitamin D excess can cause calcium deposition
in the circulation and kidneys, even leading to death.
Vitamin C or ascorbic acid comes (obviously) from citrus
fruits, but also from organ meats. It is involved in the
synthesis of collagen, a protein of the connective tissue.
Deficiency causes scurvy, which was rampant in sailors and
explorers before the vitamin and its sources were identified.
Scurvy caused bleeding from the gums and loss of teeth. It
also causes blood vessel rupture in the skin, so that small
wounds do not heal and bruises formed and remained. It can
be fatal.
Explorers who depended on pemmican and other dried meat
suffered from scurvy until they learned that fresh meat and
blubber acted as antiscorbutics. In the exploration of the
Antarctic, Scott’s polar party suffered from scurvy, but
Amundsen, who started with extra dogs, and when they
couldn’t pull, they were slaughtered and eaten (by both
humans and remaining dogs) did not.
How much do we need? 10 mg/day prevents scurvy; 60
mg/day is the RDI. However, Linus Pauling claimed vitamin
C could prevent the common cold, and suggested taking
2,000 mg/day. Others think saturation occurs with 100
mg/day, and anything more is excreted in urine. Expensive
waste. The evidence that excess vitamin C prevents colds is
weak.
Lastly, the minerals.
Calcium – bones and teeth are made up of calcium phosphate,
but calcium is also critical in muscle contraction, blood
clotting, and nerve impulse transmission. Calcium in the body
fluid is kept within a narrow range under the control of two
hormones: calcitonin and parathyroid hormone.
Calcitonin, made in the thyroid, reduces circulating Ca.
Parathormone, made by the parathyroid glands, increases
circulating Ca. It causes cells in bone (osteoclasts) to release
Ca from bone mineral into circulation. Calcitonin has the
reverse effect.
Calcium deficiency at advanced age leads to osteoporosis, or
degeneration of bone due to loss of calcium. In osteoporosis,
bones become fragile and easily broken.
When calcium deficiency occurs during bone growth, the
disease rickets occurs; bone weakness leads to deformation,
which commonly is seen as ‘bending’ of the long bones of the
legs.
Milk, milk products, and dark green leafy vegetables are the
best sources of calcium. The RDI is 1,200 – 1,500 mg/day.
Excess calcium (generally more than 12,000 mg/day) can
also lead to problems. The most painful one is kidney stones.
Iron – Fe is vital to the formation and function of red blood
cells. We can get iron from plants (green leafy vegetables,
fruits, legumes, and whole grains and the products made from
them. We also get iron from animal tissues, both in the form
of heme-iron (hemoglobin, myoglobin) and non-heme-iron.
Vitamin C influences absorption of non-heme iron.
Deficiency leads to iron deficiency anemia. Fatigue, paleness,
and weakness are its symptoms (also low hematocrit).
deficient
normal
Finally, iodine – iodine is critical to the formation of thyroid
hormone (thyroxine) and its precursors (tri- and
tetraiodothyronine). Today, in areas that were once centers of
thyroid insufficiency (e.g. the Great Lakes area), goiters are
rare. Our table salt is iodized, and that is the source of enough
iodine to prevent goiter. Many of us use sea salt, which is rich
in iodine, and/or eat seafood, that is similarly rich in it. In
other areas of the world, goiter persists as a problem.
Here, there are a number of reasons, including autoimmune
diseases like Hashimoto’s syndrome (thyroiditis), for
thyroxine insufficiency. Treatment uses adjusted doses of Lthyroxine to treat the diseases.
Hashimoto’s thyoiditis is an example of a disease that is being
explored using molecular tools. It seems that there are
particular genes in the MHC (major histocompatibility
complex) that are much more commonly found in individuals
who have the thyroiditis. There are also associated problems:
rheumatoid arthritis (another autoimmune condition),
problems in the kidneys, and other problems.
There was a time, in the Renaissance, when a moderate
swelling due to goiter was considered a mark of beauty.
Now that you know what we need to have in our diets, what
do we eat?
Too many calories. We need ~2000 kcal per day, but consider
a typical McDonalds lunch:
A Big Mac - 585 calories
Large fries - 554 calories
Large vanilla shake - 1108 calories.
Total: 2,247 calories.
That's enough for the whole day and it's only lunch.
Obesity is an increasing problem. Data from your text
indicates an increase in obesity from 14.5% of the U.S.
population in 1971 to 31% in 2000, and now (from more
recent news reports) ~40%.
Repeating (and expanding) the risks of obesity, you may be at
risk for a wide range of serious diseases and conditions
including:
• hypertension or high blood pressure;
• coronary heart disease;
• Type 2 diabetes;
• stroke;
• gallbladder disease;
• osteoarthritis;
• sleep apnea and other breathing problems;
• some cancers such as breast, colon and endometrial cancer;
and
• mental health problems, such as low self-esteem and
depression.