Metabolism

1. Metabolism • The sum of all chemical reactions in the body • How the body uses the nutrition it receives • Includes breakdown and synthesis of biochemical compounds • Carried out by enzymatic steps called metabolic pathways

2. Carbohydrate Metabolism 1 Glycolysis Aerobic respiration Anerobic fermentation

3. Carbohydrate Metabolism 2 • Glycogenesis – formation of glycogen • Carbohydrate storage in liver & muscle • Stimulated by insulin if excess blood glucose • Glucose is linked together in branched chains by glycogen synthase

4. Carbohydrate Metabolism 3 • Glycogenolysis – hydrolysis of glycogen • stimulated by glucagon and epinephrine • Glycogen phosphorylase removes one glucose residue at a time, which enters glycolysis • Gluconeogenesis – synthesis of glucose from non-carbohydrates (fats/proteins)

5. Carbohydrate Metabolism 4

6. Protein Metabolism 1 • Free amino acid pool • From dietary protein, dead epithelial cells, broken-down enzymes, & tissue turnover • Deamination – removal of –NH2 group • Creates a keto acid that can be converted to pyruvic acid, acetyl Co-A or another Citric Acid Cycle component for fuel • Can be reversed for protein synthesis (amination) • Can be converted into glucose in gluconeogenesis

7. Protein Metabolism 2 • Transamination • When deamination occurs, the extra –NH2 group is transferred to α-ketoglutaric acid, creating glutamic acid • In the liver, the –NH2 group is removed and converted to ammonia, then urea • Ammonia → Urea via Orthinine Cycle

8. Protein Metabolism 3 • 9-10 essential amino acids (humans) • Must be obtained through diet • Remaining (nonessential) amino acids can be synthesized by the liver • Protein synthesis involves DNA, mRNA tRNA, ribosomes, etc. in Translation • Proteins are necessary for somatic tissues, enzymes, hormones, immunoglobulins & cellular structures

9. Lipid Metabolism 1 • Lipolysis – breaking down fat for fuel • Trigylcerides hydrolyzed into glycerol & fatty acids • Glycerol converted to PGAL → Citric Acid Cycle • Fatty acids broken down 2 carbons at a time (acetyl groups) via beta-oxidation • Converted to acetyl Co-A → Citric Acid Cycle • A 16-carbon fatty acid yields 129 ATP molecules • Excess acetyl groups converted into ketone bodies • Ketogenesis • Some cells can convert ketone bodies back into acetyl Co-A

10. Lipid Metabolism 2 • Lipogenesis – synthesis of new triglycerides • PGAL can be converted to glycerol • Acetyl Co-A can be converted to fatty acids • These are condensed to form a triglyceride • Lipogenesis occurs when excess glucose and amino acids are present, or when dietary fat intake is too low • Lipids are necessary for cell membranes, hormones, myelination & CNS function

11. Lipid Metabolism 3

12. Appetite Regulation • Short term – hunger/satiety • Ghrelin – secreted when stomach is empty • Stimulates hypothalamic secretion of growth-hormone-releasing hormone • Primes the body to receive nutrients • Peptide YY (PYY) – signals satiety • Secreted by ileum & colon, but before chyme arrives • Helps prevent stomach from emptying too quickly • Cholecystokinin (CCK) – signals satiety • Secreted by duodenum & jejunum • Stimulates brain & vagal nerves

13. Appetite Regulation • Long term – adiposity signals • Leptin – secreted by adipocytes • deficiency or resistance contributes to chronic overeating • Insulin – secreted by pancreas • Also signals brain of body fat status • Other appetite regulators • Gastric peristalsis – hunger contractions • Certain neurotransmitters cause specific cravings

14. Insulin and Glucagon • Glucagon mobilizes sugars, fats & proteins for the body’s use during fasting • Also stimulated by ingesting protein and fat • Insulin stores nutrients in cells • Protects the brain from high blood sugar • Signals the liver to absorb sugar, converting it to fat • Insulin-to-Glucagon Ratio • High = fat storage • Low = lean body mass destroyed • Balanced = lean body mass built up; appropriate body composition attained

15. Metabolic Postabsorptive State • Maintains blood glucose level between meals • Carbohydrates • Glycogenolysis - enough glycogen for 4 hours • After that, gluconeogenesis occurs • Fats • Glycerol from hydrolyzed fats converted to glucose • Free fatty acids converted to ketone bodies in liver • Proteins • Proteins (esp. from muscle) can be broken down for fuel when glycogen & fat reserves are depleted

16. Metabolic Postabsorptive State • As blood glucose levels drop, glucagon secretion from pancreas begins • Promotes glycogenolysis, gluconeogenesis, & lipolysis • Sympathoadrenal system secretes epinephrine in response to injury, fear, anger, or other stress • Promotes glycogenolysis & lipolysis • Cortisol is also secreted, promoting fat & protein break-down & gluconeogenesis

17. Metabolic Absorptive State • Carbohydrates • Ingested carbohydrates do not distend stomach and pass quickly into duodenum • Insulin is secreted directly into the portal vein • Absorbed sugars go directly to the liver • Excess sugar is converted into glycogen and lipids • Lipids produced in the liver go into circulation • Tightly regulated amounts of sugar go into circulation • Blood sugar is absorbed by the body’s cells • Sugar reaching the brain finally signals satiety

18. Metabolic Absorptive State • Lipids • Ingestion of fats releases CCK, signaling satiety • Excess CCK causes nausea • Ingestion of fats does not cause insulin secretion • Chylomicrons circulate, initially bypassing liver • Lipoprotein lipase hydrolyzes triglycerides in capillaries • Fats can be used for tissue regeneration or stored • Dietary cholesterol inhibits HMG Co-A Reductase • This enzyme converts carbohydrates to cholesterol in the liver - it cannot be inhibited by self-produced cholesterol

19. Metabolic Absorptive State • Proteins • Ingestion of proteins also releases CCK • Amino acids stimulate glucagon secretion • Some amino acids also stimulate insulin secretion • Amino acids also circulate to the liver first • Can be used for protein synthesis • Deaminated and used for fuel or fatty acid synthesis • Most amino acids leave the liver and are used for protein synthesis

20. Glycemic Index • How fast insulin rises in response to sugar • Simple sugars are absorbed most rapidly • High levels of insulin quickly enter portal vein • Complex carbohydrates are digested and absorbed at a somewhat slower rate • Sugars enter the portal vein more steadily • Proteins & fats slow down carbohydrate absorption even further • A balanced level of insulin remains stable over a longer period of time

21. Calories • 1 calorie = amount of heat that raises 1g of water 1°C • Calorie = kilocalorie (1000 calories) • A Calorie is not a Calorie • Calories are measured by burning food and measuring heat production • The body does not completely oxidize all ingested foods

22. Metabolic Rate • Amount of energy liberated per unit time • kcal/hr or kcal/day • Basal Metabolic Rate (BMR) • Awake, relaxed, room temperature, 12-14 hrs fasting • Total Metabolic Rate (TMR) • BMR + energy used for voluntary activity • MR affected by anxiety/stress/depression, fever, eating/starvation, hormones • Usually declines with age

23. Estimates of Body Fat % • Dual-energy X-ray absorptiometry (DXA) • Air-displacement plethysmography • Hydrodensitometry • Anthropometry • Body Mass Index • BMI = weight (kg) / height2 (m) • Bioelectrical impedence