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Ch. 26: Nutrition and Metabolism
Nutrition: the study of interactions between organisms and food.
I. Nutrition
(A) Body Weight and Energy Balance – Body weight tends to stabilize
over time, the Set Point Theory. Overriding influence is energy (Calorie)
balance. 30-50% of BW variation genetic, the remainder environmental.
(B) Appetite – determined by hypothalamus. Influenced by hormones
(leptin, ghrelin), drugs (THC), emotions.
(C) Obesity – more than 20% over ideal body weight, approximately 1/3 of
American adults. Often determined by measuring BMI: Body Mass Index,
calculated from height and weight.
Adipocyte hyperplasia (increasing cell number) during childhood enhances
tendency. Adipocyte hypertrophy (enlargement) throughout life.
(D) Calories = kilocalories, each with enough energy to heat 1 kg water
one degree C. Equivalent to body fuel.
(E) Nutrients used for growth, repair, maintenance. Macronutrients
needed in large quantities: carbohydrates, proteins, lipids, water. Micronutrients
= vitamins and minerals. RDAs (recommended daily allowances) have been
long determined. Recent recommendations refine diet to prevent chronic
disease. Essential nutrients must be in diet, cannot be created in body.
(F) Carbohydrates should comprise ~ 50% of Calories. Most becomes
glucose. For energy (ATPs), few other structural functions. Better sources are
complex (starch) and associated with other nutrients (whole grain). From plants,
may include undigested fiber- for digestive health and possibly cholesterol
control.
(G) Lipids – stored energy, protection, insulation, some hormones,
membranes. Should comprise no more than 30% of consumed Calories. Two
essential fatty acids- deficiency possible, more likely in children. Saturated fats
from animals, unsaturated from plants- based on fatty acid structure.
Cholesterol and serum lipoproteins clinically monitered. LDLs with ‘bad
cholesterol’; HDLs with ‘good’. Major problem is high energy content, > 2x the
calories/ gram as carbohydrates and proteins.
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(H) Proteins should comprise 10-25% of consumed Calories. 8 essential
amino acids, difficult to obtain with vegetarian diets. Numerous and diverse
functions, the last resort for energy provision. Metabolic use results in ketone
production and excessive urea.
(I) Micronutrients not for energy, no Calories. Unique functions.
Deficiencies and excesses can be life threatening.
-1- minerals: inorganic. Most abundant is calcium. Includes electrolytes, iron,
iodine.
-2- vitamins: organic.
a. water soluble = Bs and C. Many are in coenzymes.
b. fat soluble = A, D, E, K
II. Carbohydrate Metabolism
(A) Glucose Catabolism
Begins with glycolysis – 12 steps divides glucose into two pyruvates.
Generates 2 ATPS. Usually followed by:
-1- cellular respiration – in mitochondria. Produces an additional ~34 ATPs/
glucose. Requires oxygen = aerobic.
a. Krebs Cycle: generates reduced coenzymes, which are used to extract
energy and create ATPs during
b. oxidative phosphorylation
-2- anaerobic fermentation, possible in few cell types, e.g. skeletal muscle.
Glycolysis is maintained for a short while without O2. Generates lactic acid,
which is later converted to pyruvate in liver.
Surplus glucose is stored as glycogen- limited due to bulk.
Glycogenesis vs. glycogenolysis.
Gluconeogenesis from fats and amino acids. Liver involved in regulation.
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III. Lipid and Protein Metabolism
(A) Lipids most are triglycerides. Lipogenesis vs. lipolysis. Fatty acids
created from acetyl CoAs, which can be derived from glucose or amino acids.
(B) Proteins provide the body’s amino acid pool. Protein synthesis is
genetically programmed. Chemical modifications include deamination,
transamination and amination. Deaminated amino acids become keto acids,
then pyruvate or Acetyl CoA.
IV. Metabolic States and Rate
(A) Absorptive State for four hours after a meal. Blood glucose available.
Insulin influential. Nutrient storage predominates.
(B) Postabsorptive State – stored nutrients provide energy. More complex
regulation. Glucagon and sympathetic Nervous system influential.
(C) Metabolic Rate - based on BMR (basal metabolic rate) and activity.
Average adult BMR = 2000 Calories/ day (men). BMR decreases with age, leads
to creeping obesity.
V. Thermoregulation
Normal body temperature is variable. Diurnal cycle with lowest in the early AM
(basal body temperature). Core vs. shell temperature (~ 37C).
Heat loss via radiation, conduction, evaporation. Thermostat within
hypothalamus. Regulation with dermal blood flow. Shivering vs. nonshivering
thermogenesis.
Hyperthermia: heat cramps (due to electrolyte losses), heat exhaustion, heat
stroke (potentially fatal).
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