Proteins • Sources of proteins

– Proteins are assembled from a pool of 20 different amino acids– The body synthesizes amino acids from other compounds in the

body– Only about half of the necessary types of amino acids can be

produced by the body– Essential amino acids must be supplied through diet

• Meats and vegetables

• Mixture of different vegetables

– Cereal grains

– Nuts

– Legumes

Proteins• Protein metabolism

– Anabolism is primary• Synthesis of proteins by ribosomes of all cells

• Different cells produce different proteins, determined by cell’s genes

– Catabolism is secondary• Deamination takes place in liver cells and forms an ammonia and keto acid molecule

(alpha-keto gultarate)– Ammonia is converted to urea and excreted

– Keto acid is oxidized or converted to glucose or fat (Figure 27-22)

– Protein balance• Rate of protein anabolism balances rate of protein catabolism

– Nitrogen balance• Amount of nitrogen taken in equals nitrogen in protein catabolic waste

– Two kinds of protein or nitrogen imbalance• Negative nitrogen balance—protein catabolism exceeds protein anabolism; more tissue

proteins are catabolized than are replaced by protein synthesis

• Positive nitrogen balance—protein anabolism exceeds protein catabolism

– Anabolic and catabolic hormone control protein metabolism

Vitamins and Minerals• Vitamins (Table 27-3)

– Organic molecules necessary for normal metabolism– Many attach to enzymes and help them work (Figure 27-3)– Variety of other roles

• Vitamin A – detecting light in sensory cells of the retina• Vitamin D – converted to the hormone calcitrol• Vitamin E – antioxidant (Figure 27-4)• Vitamin C – antioxidant

– Most or the necessary vitamins must be obtained through diet• Vitamin D can be made by the body• Bacteria make vitamin K and biotin• The body stores fat-soluble vitamins (A, D, E, and K) in the liver• The body does not store water-soluble vitamins (B, C)

Vitamins and Minerals• Minerals (Table 27-4)

– Inorganic elements or salts found in the earth– Attach to enzymes and help them work and function

in chemical reactions– Essential to the fluid/ion balance of internal fluid environment– Are involved in many processes in the body such as nerve

conduction, muscle contraction, nerve function, hardening of bone, respiration, etc.

– Too large or too small an amount of some minerals may be harmful

– Recommended mineral intakes may vary over the lifespan (Figures 27-26 and 27-27)

Metabolic Rates• Amount of energy released by catabolism• Expressed in two ways

– Number of kilocalories of heat energy expended per hour or per day

– As normal or as a percentage above or below normal

• Basal metabolic rate– Rate of energy expended under basal conditions– Factors (Figures 27-28 through 27-31)

• Size• Body composition• Sex• Age• Thyroid hormone• Body temperature• Drugs

Metabolic Rates• Total metabolic rate

– The amount of energy used in a given time

– Determined by multiple factors

• Basal metabolic rate

• Energy used to do skeletal muscle work

• Thermic effect of foods

• Energy balance and weight

– The body maintains a state of energy balance • Body maintains weight when the total calories in the food ingested

equals the total metabolic rate

• Body weight increases when energy input exceeds energy output

• Body weight decreases when energy output exceeds energy input

• In starvation, carbohydrates are used up first, then fats, then proteins

Mechanisms for Regulating Food Intake

• Feeding centers in hypothalamus exert primary control

over appetite– Appetite center

• Cluster of neurons in lateral hypothalamus that, if stimulated, brings

about increased appetite

• Orexigenic effects—factors that trigger appetite

– Satiety center

• Group of neurons in ventral medial nucleus of hypothalamus that, if

stimulated, brings about decreased appetite

• Anorexigenic effects—factors that suppress appetite (anorexia is loss

of appetite)

The Big Picture: Nutrition, Metabolism, and the Whole Body

• Every cell in the body needs the maintenance of the metabolic pathways to stay alive

• Anabolic pathways build the various structural and functional components of the cells

• Catabolic pathways convert energy to a usable form and degrade large molecules into subunits used in anabolic pathways

• Cells require appropriate amounts of vitamins and minerals to produce structural and functional components necessary for cellular metabolism

• Other body mechanisms operate to ensure that nutrients reach the cells