Osteoporosis is a disorder characterised by decreased bone mass, owing to loss of bone mineral, and increased susceptibility to fractures. This disorder primarily affects middle-aged and elderly people, especially women. The female hormone oestrogen protects against bone mineral loss during young adulthood. When oestrogen levels decline after menopause (or because of a disruption in the menstrual cycle) bone mineral content declines as well.

Women of northern European descent are especially at risk for the development of osteoporosis, while African-Americans have a much lower risk. Small, thin females have higher rates of osteoporosis than large women. Small women have smaller bones to start with, and thin women have less adipose tissue, which is a great source of estrone, an oestrogen that slows bone loss.

Though the loss of bone mineral may begin as early as the fourth decade of life, osteoporosis is often not diagnosed until so much bone mass has been lost that a person develops a fracture, which may have been caused by something no more traumatic than a sneeze. One of the most noticeable signs of osteoporosis is the rounded back posture known as "dowager's hump," so often depicted in the advertisements for calcium supplements. When the vertebrae lose bone mass, the spinal column loses strength, and individual vertebrae collapse because of what are known as crush fractures.

When these fractures accumulate in several vertebrae, the spine develops a hunchback that is often accompanied by back pain and loss of height.

While several factors affecting bone mineral density are outside your control, you can still make a significant impact on your lifelong bone density by cultivating a "high-density" lifestyle. When it comes to maintaining healthy bones, an ounce of prevention may be worth a pound of cure.

Calcium intake has been the most publicised factor for promoting healthy bones, and the one with which Americans are probably most familiar. Several scientific and government groups have recommended increasing calcium intake to prevent osteoporosis, and the calcium supplement industry has not been slow to jump on the bandwagon.

A great deal of controversy surrounds the issue of calcium supplementation. Studies of whether calcium supplements help prevent or treat osteoporosis have yielded mixed results. Some studies have shown an increase in bone mineral density when calcium intake was increased. Other studies have failed to show a clear association between either childhood or adulthood calcium intake and the development of osteoporosis.

Nevertheless, nutritionists still agree that an adequate intake of dietary calcium is important for good health, which includes the growth and maintenance of bone tissue. Good sources of calcium include milk and milk products, dark green vegetables such as broccoli, sardines, and canned salmon (eat the bones too). Tofu and soy milk are often fortified with calcium for people who avoid dairy products.

Consumption of excess protein may contribute to loss of bone mineral. Doubling protein intake increases urinary calcium excretion 50%. This appears to be due to decreased recovery of calcium by the kidneys. Epidemiological studies have found lower rates of osteoporosis in British vegetarians and female Seventh-Day Adventists in the U.S. than in meat eating individuals. These differences, however, could also be due to other factors, such as activity or heredity. But since most Americans consume more protein than they need, decreasing intake of concentrated protein sources such as meat and eggs (which are also high in cholesterol and fat) could be beneficial.

Some researchers have expressed concern that a very high-fiber diet may decrease calcium absorption from the intestine, since dietary fiber may bind with the calcium. Studies suggesting this concern have used fiber amounts much higher than those consumed by most people, so health professionals continue to recommend a moderate increase in fiber intake since such a diet is associated with many other health benefits. In countries where high-fiber diets are consumed, calcium balance and osteoporosis do not appear to be problems.

Vitamin D improves the efficiency of intestinal absorption of calcium. Enough vitamin D may not be synthesised, and it is difficult to obtain from natural dietary sources, so it is added to milk. Vitamin C, abundant in many fruits and vegetables, is essential for collagen production, and vitamin A, found in yellow and orange fruits and vegetables, also aids in bone development.

Several minerals in addition to calcium are important for bone development and maintenance. These include magnesium, found in seeds, grains, and green vegetables; manganese, found in blueberries, greens, and legumes; and boron, found in pears, apples, grapes, leafy vegetables, and legumes.

Frequent dieting can decrease bone density, as the body draws calcium from bone to make up for calcium missing from the diet. Therefore it is likely that frequent periods of low caloric intake contribute to the development of osteoporosis.

The association between a high level of alcohol consumption and osteoporosis may be a consequence of poor nutrition, lower body weight, liver disease, or other illness, or it may be a result of a toxic effect of alcohol. A moderate intake of alcohol does not appear to cause bone loss.

People who smoke cigarettes have a higher risk of osteoporosis than nonsmokers. This risk may be explained by the fact that smokers tend to be thinner, which is a risk factor. Female smokers also have lower oestrogen levels and undergo menopause at an earlier age than non-smokers, so they experience less of estrogen's protective effect.

Bone accommodates to the stresses imposed on it, so active people tend to have denser bones. Weight-bearing activity is strongly recommended to help increase bone mass during adolescence and young adulthood and to delay its loss thereafter. Weight bearing refers to activities in which the skeletal system must support body weight, for example, walking, hiking, and cross-country skiing as opposed to swimming. Progressive resistance training, the most common form of which is weight lifting, also appears to apply the mechanical stress required to increase bone mineral density.

Menstrual irregularity in premenopausal women is often caused by eating disorders and/or excessive exercise, and signals low levels of oestrogen. Several studies have documented osteoporosis in young women athletes who had stopped menstruating. Bone-density measurements have revealed that many of these young athletes had the bone density of women in their fifties in some cases, the bone damage sustained was considered irreversible.