Digestion
• Purpose is to mechanically (with
teeth and muscular contractions)
and chemically (with enzymes)
break down food to allow nutrients
to be absorbed into the
bloodstream
• Food is broken down into macroand micronutrients
• Calories (a measure of energy) are
obtained that are needed to power
all cellular processes
Macronutrients
Classification:
• Carbohydrates (sugars,
starches, fiber)
• Fats (saturated and
unsaturated (mono- and poly-))
• Proteins
*See handout for more specific
information
Micronutrients
• Vitamins - compounds required for life in
small amounts and must be obtained via
diet (they are involved in many
biochemical reactions)
• Minerals - elements that cannot be made
by our bodies and are required in many
biochemical reactions (they exist as ions
in the body: Na+, K+, Ca2+, Mg2+, etc.)
• Phytochemicals - wide variety of plant
compounds found in fruits and vegetables
that have many beneficial effects on
health but are not required nutrients for
survival
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Carbohydrates
• Function: Used as energy source (glucose) in
cellular respiration and used as a stored energy
source (glycogen in liver and skeletal muscle)
• Stimulate secretion
of insulin by beta-cells of the
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• Once liver and muscle glycogen is sufficient, excess
carbohydrates are converted to fat and stored in
adipose tissues
Hormonal Blood Sugar
Regulation
• Insulin and glucagon
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•
•
(both released from
pancreas) regulate blood
sugar
Insulin lowers blood
sugar if it is too high
Glucagon raises blood
sugar if it is too low
Carbohydrates stimulate
insulin release (fats and
proteins have little effect
on insulin)
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Effect of Carbohydrates on Blood Sugar
(Glycemic Index and Glycemic Load)
• All carbohydrates digest
•
into sugars to be released
into the blood (except for
fiber)
Glycemic index (GI)
measures how fast a
carbohydrate is digested
and raises blood sugar
and insulin levels
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Glycemic Index (continued)
• Foods with high GI have very little fiber,
protein, and fat
• Examples: Starchy carbohydrates (white
bread, white rice, pasta, crackers,
potatoes) and sugary drinks (regular
sodas, sports drinks, juices)
• Avoid foods where the main ingredients
are white flour, high-fructose corn syrup,
sugar
Glycemic Load
• Measures the amount of carbohydrate per
serving and the impact it has on blood sugar
• Total carbohydrate - fiber = net carbs
GL = Net carbohydrates/serving X GI/100
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Glycemic Load (continued)
• Some foods with high GI have low GL and
therefore have little effect on blood sugar
when eaten in moderation
• Ex. Carrots and watermelon (both have a
very high GI, but a very low GL)
Benefits of low GI and GL diet
• Lower insulin levels
• Stable blood sugar levels
• Improved blood lipid profile (lower LDL, higher HDL,
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•
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•
lower triglycerides)
Less accumulation of body fat and weight gain
Improved satiety (fullness)
Improved gastrointestinal health (due to higher fiber
content)
Reduced risk of cardiovascular disease, obesity and type
II diabetes due to the above factors
Effects of chronically elevated
blood sugar levels
• Leads to insulin resistance (cells won’t respond
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•
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to insulin since pancreas is constantly producing
it to deal with the excess sugar)
Low HDL and high triglycerides (fats) - both
contribute to atherosclerosis
Elevated blood pressure
Higher cancer risk
Higher risk of blood clots
Higher risk of kidney stones
Fats
“The idea that all fat is bad for you, with the
exclusive focus on adverse effects of fat
may have contributed to the obesity
epidemic… The emphasis on total fat
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health in general.”
- Walter Willett
Chair of Department of Nutrition
Harvard School of Public Health
Fats (Triglycerides)
• Are composed of 3
fatty acids bonded to
glycerol
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Different Types of Fatty Acids
Saturated fatty acid (no double bonds between carbons)
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Monounsaturated fatty acid (one double bond)
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Polyunsaturated fatty acid (more than one double bond)
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Trans-fats are polyunsatured fats that have been “hydrogenated”
(behave more like saturated fats except far worse)
How does the type of fat affect
blood cholesterol levels?
Type of fat
HDL levels
LDL levels
saturated
monounsaturated
polyunsaturated
trans
When substituted for refined carbohydrates and excess saturated fat
Cholesterol
• Used to synthesize
hormones, vitamin D, and
is a component of cell
membranes
• Most (75-80%) is
produced by the liver. The
remainder comes from diet
• Dietary cholesterol is less
of a factor in determining
blood cholesterol ratios
than dietary fat intake
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Lipoproteins and Cholesterol
Transport (a review)
• Lipoproteins are molecules that
carry cholesterol to and from
tissues (are produced in the liver)
• LDL (low density - BAD) particles
carry cholesterol to the tissues
(excess cholesterol ends up in
arterial walls forming plaque)
• HDL (high density - GOOD)
particles take cholesterol from
tissues and blood vessels and
bring it to the liver for disposal
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Lipid transport in the body:
Carry dietary fats from
small intestine
to tissues
Produced in liver
VLDL
Carry triglycerides
to tissues and become
LDL
produces
Liver
produces
HDL
Picks up
cholesterol from
tissues and blood
Brings cholesterol to cells
(when cells have enough
cholesterol, LDL
deposits cholesterol in arterial walls (especially if LDL particle is small))
Can lead to
Plaque formation and beginning stages of
atherosclerosis
Soluble fiber, blood sugar and
cholesterol
• Soluble fiber prevents bile acids from being reabsorbed
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•
into the body (liver must take cholesterol from the blood
to make more)
Soluble fiber also slows digestion and slows the
absorption of sugar into the bloodstream, lessening the
need for large amounts of insulin at once
Good sources - oat fiber (oatmeal, oatbran), apples,
peas, legumes (beans), citrus fruits, fiber supplements,
barley
Digestion
• Mechanical digestion - teeth,
churning of stomach, segmentation
of small intestine
• Chemical digestion - enzymes
break bonds of macronutrient
molecules
• Chemical digestion of
carbohydrates begins in the mouth,
protein digestion begins in the
stomach, and fat digestion begins
in the small intestine
• All digestion is completed in the
small intestine
3 phases of Gastric (Stomach)
Secretion
• Cephalic - smell, thought, sight, taste,
or texture of food stimulates gastric
secretions (HCl (acid) and pepsin
(protein digesting enzyme) in the
stomach)
• Gastric - Stretching of stomach lining
or presence of proteins or caffeine in
the stomach increases gastric
secretions
• Intestinal - Presence of chyme in the
duodenum inhibits gastric secretion
(especially if chyme is fatty or acidic
(ph below 2))
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animation
Digestive hormones
• Cholestokynin (CCK) - stimulates
bile release from gallbladder and
lipase release from pancreas
(released by cells in the
duodenum)
• Secretin - stimulates pancreas to
release bicarbonate to neutralize
acid in the duodenum (released by
cells in the duodenum)
• Gastrin - released by the stomach
to increase gastric secretion
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Hormonal Appetite Regulation
• Leptin - released by adipose
tissue and signals
hypothalamus that you are full
• Ghrelin - released by stomach
and stimulates appetite by
acting on the hypothalamus
• Orexins - released by
hypothalamus and stimulate
appetite (ghrelin increases
orexin levels while leptin
decreases them)
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Hypothalamic regulation of appetite
by ghrelin and leptin