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Carbohydrates
Carbohydrates
• One of the three macronutrients
• A primary energy source, especially
for nerve cells & RBC’s
• CH2O
• Grains, fruits and vegetables
Where do Carbs originate?
• Glucose
– The most abundant
carbohydrate
– Produced by plants
through
photosynthesis
• Simple or complex
Simple & Complex
• Simple contain one or two molecules
– Monosaccharides contain one molecule
– Disaccharides contain two molecules
• Complex contain hundreds to thousands
of molecules
– Starches, glycogen, fibers
Monosaccharides
• Glucose: usually occurs with other sugars
• Fructose: fruit and vegetable sugar
• Galactose: Always occurs with glucose in
foods
Disaccharides
• Lactose: Milk sugar (glucose + galactose)
• Maltose: Malt sugar (glucose + glucose); by-product of
fermentation in our bodies; substrate for
fermentation in beer and liquor)
• Sucrose: Another naturally occurring fruit sugar
Complex
• Starches: Storage form of glucose in plants
• Glycogen: Storage form of glucose in animals
– Muscles and liver
• Fiber: Form structural support elements of plants
Complex Carbohydrates
• Fiber
– Dietary fiber: the non-digestible part of
plants
• Grains, rice, seeds, legumes, fruits
– Functional fiber: carbohydrate extracted
from plants and added to food
• Cellulose, guar gum, pectin, psyllium
– Total fiber = dietary + functional fiber
Digestion of Carbs
• Begins with salivary amylase in the mouth
– Breaks carbohydrates down to maltose
– What kind of macromolecule is amylase?
• No digestion of carbohydrates in the
stomach
– HCl denatures amylase
Digestion of Carbohydrates
Digestion of Carbohydrates
• Most chemical digestion occurs in small
intestine
• Pancreatic amylase
– Produced in the pancreas; secreted into
small intestine
– Digests carbohydrates to maltose
Digestion of Carbohydrates
• Additional enzymes in small intestine digest
disaccharides to monosaccharides
– Enzymes include maltase, sucrase, and lactase
• Monosaccharides are absorbed into cells
lining the small intestine, then enter the
bloodstream
Digestion of Carbohydrates
• All monosaccharides are converted to
glucose by the liver
– Which ones are those?
• Glucose circulating in the blood is our
primary energy source
• Excess glucose is converted to glycogen by
the liver
– Anabolic (building) process
Blood transports
nutrients
• Nutrients are
absorbed in digestive
tract
• Carried to the liver
for processing
• Then returned to the
body via the heart
Storage of Glycogen
Digestion of Fiber
• Don’t have enzymes to digest fiber!
• Bacteria in large intestine can
metabolize some fiber
– Produce fatty acids and gas
• Most fiber remains undigested and is
eliminated with feces
Regulation of Blood Glucose
• Level in the blood is closely regulated
• Pancreatic cells produce, store and
secrete two hormones, insulin and
glucagon, which control level of glucose in
the blood
Insulin
• After eating, pancreas releases Insulin
– Produced by beta cells of the pancreas
– Stimulates body cells to absorb glucose
from the blood
– Stimulates liver cells to absorb glucose and
convert it to glycogen
Blood Glucose & Insulin
Blood Glucose & Glucagon
• Glucagon
– Produced by alpha cells of the pancreas
– Stimulates breakdown of glycogen to glucose
to make glucose available to cells of the body
– Stimulates gluconeogenesis – the production
of glucose from amino acids (AA)
• Where do these AA come from?
• Body proteins!!
Regulation of Blood Glucose:
Glucagon
Regulation of Blood Glucose
• Glycemic index: Measure of a food’s
ability to raise blood glucose levels
– Foods with a low glycemic index:
• Are better for people with diabetes
– Don’t produce dramatic fluctuations in blood glucose
– May increase HDL/LDL ratio
• Are generally higher in fiber
• May reduce the risk of heart disease and colon
cancer
Glycemic indices
The Role of Carbohydrates
• Energy
– 4 kcal/gram
– Red blood cells rely only on glucose
– Especially important for energy during
exercise
The Role of Carbohydrates
The Role of Carbohydrates
• Energy
– Sufficient energy from carbohydrates
prevents production of ketones as an
alternate energy source
– Excessive ketones produce high blood
acidity (ketoacidosis) usually in diabetics
– High blood acidity damages body tissues
The Role of Carbohydrates
• Fiber
– May reduce the risk of colon cancer
– May reduce the risk of heart disease
• May block or delay absorption of dietary
cholesterol
– May enhance weight loss
• Occupies space so you feel full; also absorbs
water and expands
– Help prevents hemorrhoids, constipation,
and diverticulosis
How Much Carbohydrate?
• Recommended Dietary Allowance (RDA) is
130 grams/day just to supply the brain
with glucose
• 45-65% of daily calorie intake should be in
the form of carbohydrates
• Focus on foods high in fiber and low in
added sugars
Too much sugar
• Most American eat too many simple
sugars
– Most of these come from added sugars
– Added sugars = sugars and syrups added to
food during processing
• Common source is soda pop (~ 10 tsp. sugar per 12
oz. can)
• Average American consumes 40 gallons each year!
Diets high in simple sugars:
• Contribute to dental problems such as
cavities and gum disease
– Cavity causing bacteria thrive on simple
sugars
• Are associated with increased levels of
“bad cholesterol” and decreased levels of
“good cholesterol”
– Cause heart disease
Complex Carbohydrates:
• Come associated with fiber
– Adequate Intake (AI) = 14 grams for every
1,000 kcal in the diet
– Most Americans eat only 1/2 the
recommended amount of fiber
• Whole grain foods are more healthful
than foods with added sugar
– Come stocked with lots of fiber & necessary
vitamins and minerals
Alternative Sweeteners
• Nutritive sweeteners
– Contain 4 kcal energy per gram
– Sucrose, fructose, honey, brown sugar,
sugar alcohols
• Non-nutritive (alternative) sweeteners
– Provide little or no energy
Alternative Sweeteners
•Acceptable Daily Intakes (ADI) have been
established for:
•
aspartame
•
sucralose
•
acesulfame-K
•No ADI has been set for saccharin but it
has been removed from the list of cancercausing agents.
Alternative Sweeteners
Health Disorders
• Three health disorders related to
carbohydrate metabolism
– Diabetes
– Hypoglycemia
– Lactose intolerance
Diabetes
Inability to regulate blood glucose levels
– Three types:
• Type 1 diabetes
• Type 2 diabetes
• Gestational diabetes
– Untreated diabetes can cause nerve
damage, kidney damage, blindness, and
death
Symptoms of Diabetes
Type 1
• Accounts for 10% of all cases
• Patients do not produce enough insulin
• Causes hyperglycemia – high blood
sugar (glucose)
• Requires insulin injections
• May be an autoimmune disease
Type 2
• Most diabetics have Type 2
• Body cells are insensitive or
unresponsive to insulin (deafened)
• Excess insulin is often produced
• Causes hyperglycemia because cells do
not hear the signal to absorb glucose
Type 2
• Cause is sometimes unclear but genetics,
obesity, and physical inactivity play a
role
• Treated with diet, exercise, and possibly
oral medications
• Healthy lifestyle choices delay onset or
may prevent onset of type 2 diabetes
Hypoglycemia
• Low blood sugar (glucose)
– Reactive hypoglycemia results when too much
insulin is produced after a meal
• Causes shakiness, sweating, anxiety
– Fasting hypoglycemia results when too much
insulin is produced even when the patient has
not eaten
Lactose Intolerance
• Insufficient lactase production
• causes an inability to digest lactose
found in dairy products
• We all make this enzymes as infants, but
lose it as we age…why?