2. During pregnancy, the prospective mother's diet must provide sufficient calcium for not only herself, but her developing baby. Likewise, lactation or milk production requires the intake of extra calcium. Milk production alone will use 160-300 mg/day of calcium, which must be replenished through the mother's diet. At present, there is a large group of pregnant, American women, mostly poor, who are not receiving enough dietary calcium daily.
3. In primitive times, virtually no population of humans had access to cow or goat milk, only human milk. No human child nursed more than a few years at most, being weaned from milk at an early age. Consequently, there is no biological reason why humans in general should retain the ability to digest milk throughout life, and, in fact, most don't. For the majority of the world's human population the best way to obtain sufficient dietary calcium is not through ingesting dairy products, but by eating green vegetables, calcium-set tofu, some legumes, fish, seeds, and nuts. Bread and cereals, not high in calcium, nevertheless can contribute significant calcium to the diet because of the frequency and amount in which they are eaten.
4. So many American men have osteoporosis because so many men have very poor diets, and get inadequate exercise, if any. This answer also has to do with the next question and answer.
5. It is estimated that the number of American men with osteoporosis will double before the year 2050 because the population of American men over 65 will double during that time.
6. Strength training. Fairly heavy duty exercise, such as two 45 minute weight training session a week can have the extraordinary effect of making you physiologically younger, whatever your age. However, assuming you have reasonable health, older people will show a more dramatic physiological reduction in age. After only one year of such exercise, you are likely to become 15 to 20 years physiologically younger. Walking is often recommended as a preventive to developing osteoporosis. Walking is certainly better than nothing, but the upper body needs to be exercised too, which weight training will do.