14.4: Summary

Muscles make up about 45% of the normal body weight in men and about 35% in women. Muscle is about 70% water and 20% protein; the rest is fat, minerals, and carbohydrate. Muscle cells are also called muscle fibers because they are long and cylindrical; there are two main kinds: white fibers (best at anaerobic metabolism) and red fibers (best at aerobic metabolism).

The kind and number of cells in a particular muscle seem to be genetically determined. Exercise increases the size of muscle fibers rather than their number, and the muscle becomes thicker and stronger. A muscle’s strength is proportional to its size. The size gained through exercise falls rapidly with disuse. After age 30, we gradually lose muscle protein, and the amount of fat in muscle goes up. This leads to the weaker muscle strength associated with aging.

Anabolic steroids are synthetic substances that are chemically similar to the male sex hormone testosterone. They are used illegally by some athletes to increase muscle size and strength. They are also used by men who aren’t athletes, but use the steroids to enhance body-building. Anabolic steroids promote increased muscle mass, but also have mild to severe side effects.

Muscles are made to contract by nerves (an exception is heart muscle, which is stimulated by local pacemaker cells). Energy for muscle action comes almost entirely from the ATP generated by the metabolism of glucose and fatty acids.

Bone provides our bodies with physical support, blood cell production, and a mineral reserve. Bone is about 30% protein and about 70% minerals (mainly calcium and phosphorus). Protein in bone gives it flexibility, whereas the minerals give it compressional strength.

Bones grow fast through adolescence, reaching peak density by about age 30; however, most of the density is acquired by the age of puberty. Bone density stays about the same for about the next 20 years, and then starts to fall beginning at about age 50.

Vitamin D is needed to absorb calcium from the intestine and to properly mineralize bone. With inadequate mineralization during childhood, bones are weak and easily bent.

Vitamin D is made in the skin by the action of ultraviolet light on vitamin-­D precursors found in the skin. Many people make enough vitamin D through normal exposure to sunlight. But certain situations (e.g., being housebound) can lessen sun exposure. Darker skin also hampers vitamin D production in skin. The efficiency of vitamin D production in the skin falls with age. In these situations, vitamin D is needed in the diet. Sources include eggs, fish, milk and milk products, and fortified cereals.

Much of the mineral content of bone is calcium. Calcium is needed for many functions, including blood clotting, muscle contraction, and proper nerve function. Through hormone action, calcium is constantly added to or removed from bone to keep a constant level in the blood. Most of the calcium in the U.S. diet comes from milk and milk products.

Vitamin C is needed to make the collagen portion of bone (it modifies an amino acid in collagen). Our food includes many sources of vitamin C, either as a natural component or as an additive, and the RDA is generous.

Osteoporosis is a bone disease associated with aging. Bone density and flexibility lessen as minerals and protein are lost. Bones become more porous and brittle and are easily fractured. Fractures of the spinal vertebrae cause the familiar stoop of old age.

Osteoporosis is seen on X-rays only after a lot of mineral loss. Commonly, it’s diagnosed only after a fracture. The aim of prevention is to achieve a high peak bone density and to slow the bone loss that occurs with aging. Peak density is reached in early adulthood; high peak density delays the onset of osteoporosis.

Women have a higher risk of osteoporosis than men, mostly due to smaller bones, lower peak bone density, and hormonal changes accompanying menopause. Women can lower their risk by regular exercise, a diet adequate in calcium, not smoking, and hormone-replacement therapy starting at menopause (which should be weighed against possible side-effects).

Teeth are structurally similar to bone, and their formation is influenced by many of the same factors that affect bone formation. The hard outer surface is the enamel, made almost entirely of mineral. Under the enamel is dentin, which has a mineral and collagen make-up like bone. The center of the tooth (pulp) has nerves and blood vessels.

Teeth are surrounded by the gums (gingiva), and the roots are attached to the surrounding bone by collagen fibers in the periodontal ligament.

Gingivitis is an inflammation of the gums that begins with dental plaque that extends into the crevice between tooth and gums. Gingivitis can progress to where the crevice widens to create a pocket, which in turn becomes the site of a more significant infection.

The spread of infection to the periodontal ligament and surrounding bone is called periodontitis. Periodontal disease is the major cause of tooth loss in adults. Good dental hygiene and regular professional care helps prevent gingivitis, periodontitis, and loss of teeth.