A slender, uneven primer on the ""brittle bone"" disease, with plenty of emphasis on prevention and treatment--but surpassed overall by Patricia Hausman's The Calcium Bible (above). Reporting on the findings of a recent National Institutes of Health meeting, Smith defines osteoporosis as ""a disease that accelerates the natural loss of bone mass accompanying aging, until the skeleton is so porous and fragile"" that bones will break with little or no trauma. (It is the cause of so-called ""dowager's hump"" in older women, and, more seriously, of most fractured hips in the aging.) Discussing causes, Smith gets completely bogged down in technical terminology (""the other effect of lower blood calcium levels is that the increased secretion of PTH facilitates the production of 1, 25 (OH) D. . .""); on more practical matters, she is much better. Since prevention is the key, risk factors are covered at length (age, sex, family history, size, weight, habitat, milk allergy, lack of exercise); then, steps readers can take to minimize those risks are clearly outlined. Most important is ""Building Peak Bone Mass Before Menopause"" by a diet rich in calcium (menus and recipes provided) and a program of weight-bearing exercise. Preventing bone loss after menopause, and lessening its severity, require similar measures, with the addition of nutritional supplements and medication. Absolutely up-to-date, then, on a problem gaining increased public attention; Hausman has the edge, however, in consistency and general distinction.