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Digestion Powered By Docstoc
• Purpose is to mechanically (with
  teeth and muscular contractions)
  and chemically (with enzymes)
  break down food to allow nutrients
  to be absorbed into the
• Food is broken down into macro-
  and micronutrients
• Calories (a measure of energy) are
  obtained that are needed to power
  all cellular processes
• Carbohydrates (sugars,
   starches, fiber)
• Fats (saturated and
   unsaturated (mono- and poly-))
• Proteins
*See handout for more specific
• 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.)               QuickTime™ and a
• Phytochemicals - wide variety of plant      TIFF (Uncompressed) decompressor
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  compounds found in fruits and vegetables
  that have many beneficial effects on
  health but are not required nutrients for
• 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 and a
                                 by beta-cells of the
  pancreas. (Uncompressed)this picture.
          are needed to see

• Once liver and muscle glycogen is sufficient, excess
  carbohydrates are converted to fat and stored in
  adipose tissues
           Hormonal Blood Sugar
• Insulin and glucagon
    (both released from
    pancreas) regulate blood
•   Insulin lowers blood
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    sugar if it is too high       TIFF (Uncompressed) decompressor
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•   Glucagon raises blood
    sugar if it is too low
•   Carbohydrates stimulate
    insulin release (fats and
    proteins have little effect
    on insulin)
     Effect of Carbohydrates on Blood Sugar
          (Glycemic Index and Glycemic Load)
• All carbohydrates digest
  into sugars to be released
  into the blood (except for             QuickTime™ and a
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• 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,
                     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,
    lower triglycerides)
•   Less accumulation of body fat and weight gain
•   Improved satiety (fullness)
•   Improved gastrointestinal health (due to higher fiber
•   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
    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
   “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
             QuickTime™ and a
   reduction has been a serious distraction in
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   efforts to control see this picture.
   are needed to obesity and improve
   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
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            Qu ickTime™ a nd a
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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





When substituted for refined carbohydrates and excess saturated fat
• Used to synthesize
  hormones, vitamin D, and
  is a component of cell
• Most (75-80%) is
  produced by the liver. The              QuickTime™ and a
  remainder comes from diet     TIFF (Uncompressed) decompressor
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• Dietary cholesterol is less
  of a factor in determining
  blood cholesterol ratios
  than dietary fat intake
              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)                      QuickTime™ and a
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• HDL (high density - GOOD)                 TIFF (Uncompressed) decompressor
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                                        TIFF (Uncompressed) this picture.
  particles take cholesterol from          are needed to see this picture.
  tissues and blood vessels and
  bring it to the liver for disposal
Lipid transport in the body:   Produced in liver

 Carry dietary fats from
     small intestine
       to tissues
                Carry triglycerides
        VLDL                            LDL
               to tissues and become

Liver                            Brings cholesterol to cells
                                  (when cells have enough
produces                              cholesterol, LDL
                            deposits cholesterol in arterial walls -
                            (especially if LDL particle is small))
                                                Can lead to
     Picks up
 cholesterol from
                        Plaque formation and beginning stages of
tissues and blood
    Soluble fiber, blood sugar and
• Soluble fiber prevents bile acids from being reabsorbed
  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,
• Mechanical digestion - teeth,
  churning of stomach, segmentation
  of small intestine
• Chemical digestion - enzymes
  break bonds of macronutrient
• 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)
• Cephalic - smell, thought, sight, taste,
  or texture of food stimulates gastric
  secretions (HCl (acid) and pepsin
  (protein digesting enzyme) in the
• Gastric - Stretching of stomach lining
  or presence of proteins or caffeine in
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• Intestinal - Presence of chyme in the
  duodenum inhibits gastric secretion
  (especially if chyme is fatty or acidic
  (ph below 2))

                Digestive hormones
• Cholestokynin (CCK) - stimulates
  bile release from gallbladder and
  lipase release from pancreas
  (released by cells in the
• Secretin - stimulates pancreas to
  release bicarbonate to neutralize             QuickTime™ and a

  acid in the duodenum (released by   TIFF (Uncompressed) decompressor
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  cells in the duodenum)
• Gastrin - released by the stomach
  to increase gastric secretion
  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                            QuickTime™ and a
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  appetite (ghrelin increases
  orexin levels while leptin
  decreases them)

                                   Hypothalamic regulation of appetite
                                   by ghrelin and leptin

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