Nutrition, Metabolism, and Body Temperature Regulation
Nutrition is one of the most overlooked areas in clinical med-
icine. Yet, what we eat and drink inﬂuences nearly every phase
of metabolism and plays a major role in our overall health. Now
that we have examined the fates of nutrients in body cells, we are
ready to study the urinary system, the organ system that works
tirelessly to rid the body of nitrogen wastes resulting from me-
tabolism and to maintain the purity of our internal ﬂuids.
Check Your Understanding
List at least two reasons that metabolic rate declines
in old age.
List two types of drugs or over-the-counter products that can
interfere with the nutrition of elderly people.
For answers, see Appendix H.
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Diet and Nutrition
Nutrients include water, carbohydrates, lipids, proteins, vitamins,
and minerals. Te bulk of the organic nutrients is used as fuel
to produce cellular energy (A±P). Te energy value of foods is
measured in kilocalories (kcal).
Essential nutrients are those that are inadequately synthesized by
body cells and must be ingested in the diet.
Carbohydrates are obtained primarily from plant products.
Absorbed monosaccharides other than glucose are converted to
glucose by the liver.
Monosaccharides are used primarily for cellular fuel. Small
amounts are used for nucleic acid synthesis and to add sugar
residues to plasma membranes.
Recommended carbohydrate intake for adults is 45–65% of daily
Most dietary lipids are triglycerides. Te primary sources of
saturated fats are animal products, tropical oils, and hydrogenated
oils; unsaturated fats are present in plant products, nuts, and cold-
water ﬁsh. Te major sources of cholesterol are egg yolk, meats,
and milk products.
Linoleic and linolenic acids are essential fatty acids.
±riglycerides provide reserve energy, cushion body organs, and
insulate the body. Phospholipids are used to synthesize plasma mem-
branes and myelin. Cholesterol is used in plasma membranes and is
the structural basis of vitamin D, steroid hormones, and bile salts.
Fat intake should represent 30% or less of caloric intake, and
saturated and trans fats should be replaced by mono- and
polyunsaturated fats if possible.
Animal products provide high-quality complete protein
containing all 10 essential amino acids. Most plant products lack
one or more of the essential amino acids.
Amino acids are the structural building blocks of the body and of
important regulatory molecules.
Protein synthesis can and will occur if all essential amino acids
are present and suﬃcient carbohydrate (or fat) calories are
available to produce A±P. Otherwise, amino acids will be burned
Nitrogen balance occurs when protein synthesis equals protein
A dietary intake of 0.8 g of protein per kg of body weight is
recommended for most healthy adults.
Vitamins are organic compounds needed in minute amounts.
Most act as coenzymes. Te richest sources are whole grains,
vegetables, legumes, and fruit.
Except for vitamin D and the K and B vitamins made by enteric
bacteria, vitamins are not made in the body.
Water-soluble vitamins (B and C) are not stored to excess in the
body. Fat-soluble vitamins include vitamins A, D, E, and K; all
but vitamin K are stored and can accumulate to toxic amounts.
Besides calcium, phosphorus, potassium, sulfur, sodium,
chloride, and magnesium, the body requires trace amounts of at
least a dozen other minerals.
Minerals are not used for energy. Some are used to mineralize
bone; others are bound to organic compounds or exist as ions in
body ﬂuids, where they play various roles in cell processes and
Mineral uptake and excretion are carefully regulated to prevent
mineral toxicity. Te richest sources of minerals are some meats,
vegetables, nuts, and legumes.
Overview of Metabolic Reactions
Metabolism encompasses all chemical reactions necessary to
maintain life. Metabolic processes are either anabolic or catabolic.
Cellular respiration refers to catabolic processes during which
energy is released and some is captured in A±P bonds.
Energy is released when organic compounds are oxidized.
Cellular oxidation is accomplished primarily by removing
hydrogen (electrons). When molecules are oxidized, others
are simultaneously reduced by accepting hydrogen (or
Most enzymes catalyzing oxidation-reduction reactions require
coenzymes as hydrogen acceptors. ±wo important coenzymes in
these reactions are NAD
In animal cells, the two mechanisms of A±P synthesis are
substrate-level phosphorylation and oxidative phosphorylation.
Muscular System; Topic: Muscle Metabolism, pp. 3–8.