Passage III

Many individuals past the age of 45 develop osteoporosis, which makes the bones less dense and is characterized by a net loss of calcium in the bones. Although osteoporosis occurs in men, it is more common in women. Several hypotheses have been proposed to explain the onset of osteoporosis.

Dietary Hypothesis

Calcium from food is absorbed into the bloodstream by the small intestine. Vitamin D is necessary for this process. Most Americans ingest too little calcium and vitamin D in their diet. In a study of individuals 18-25 years old, it was shown that the majority had significantly low levels of calcium in their blood. When these individuals received daily supplements of 1,500 mg of calcium and 400 units of vitamin D, their blood calcium levels increased to normal levels. If insufficient levels of calcium and vitamin D are supplied by the diet, dietary supplements should be taken to avoid osteoporosis.

Estrogen Hypothesis

Estrogens, hormones produced primarily in the ovaries in women and, to a much lesser degree, in the adrenal glands in both sexes, and androgens, produced in the testes in men, are required for the deposition of calcium into bone. Androgen levels in men remain relatively constant throughout life; estrogen levels in women slowly decline after the onset of menopause (the time when a woman's ability to reproduce ends), which usually occurs between the ages of 45 and 55. As a result, the bones of postmenopausal women lose calcium.

Scientists compared the bone density of 4 groups of postmenopausal women. Each group took a dietary supplement. The results are shown in the table.

Exercise Hypothesis

Lack of exercise results in calcium loss from bones, whereas regular weight-bearing exercise can increase bone density. One study showed that 8 weeks of weight training added calcium and hardened bones in both postmenopausal women and men over the age of 45. Since body weight is supported by water, 8 weeks of swimming had no effect on bone density. Both groups followed the same high-calcium diet. Vitamin D intake was not measured.