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Nutrition and the Skeleton
The Role of Calcium and Other Nutrients

Nutrition is one of many factors that influence bone mass. Dietary intake can also impact the tendency to fall and plays a significant role in maintaining a soft tissue cushion to protect the skeleton from the impact of a fall.

Calcium and Calcium Balance
Bone serves as the reservoir for 99% of the body's total calcium. Calcium is an essential nutrient for bone health. It is also needed for the heart, muscles and nerves to function properly and for blood to clot normally. The body loses calcium every day through urine, feces, sweat and shed skin, hair and nails. The lost calcium is normally replaced by calcium in the diet. When the diet does not contain enough calcium to offset such losses, the body breaks down bone to release calcium needed to accommodate these physiologic demands.

Many other nutrients affect bone health, too, because they impact the absorption or excretion of calcium. Calcium balance-not just intake-is necessary for healthy bones. This balance is dependent on the absorption rate of calcium consumed as well as the rate of (mostly urinary) calcium excretion. For example, in a study of 560 healthy women, one researcher demonstrated that only about 10 percent of the variance in calcium balance among these women was explained by differences in their calcium intakes and absorption only explained another 15 percent. Urinary losses explained slightly more then 50 percent.

Nutrients Affecting Calcium Balance

Several nutrients significantly influence calcium balance:

* Vitamin D has a positive impact on calcium balance, as it increases calcium absorption in the gastrointestinal tract. The most readily available source of vitamin D is exposure to direct sunlight. Vitamin D is also found in fatty fish, eggs, liver, butter, fortified foods such as milk and multivitamins. Vitamin D deficiency may be a problem among some elderly, those in institutional settings, and some people with chronic neurological or gastrointestinal diseases.

* Protein is essential in our diets to build tissue during growth and to repair and replace tissue throughout the life cycle. It is also needed for fracture healing and proper function of the immune system. Protein deficiency is an important factor contributing to death, institutionalization, and loss of independence among the elderly following hip fracture.

However, protein also increases the body's need for calcium by increasing calcium excretion. Excess protein in the diet is used by the body for energy, just like fats and carbohydrates. However, as protein is burned for energy, it produces a chemical called sulfate, which the body excretes through the kidneys. Sulfate increases the excretion of calcium.

It is generally believed that most Americans exceed the recommended intake for protein--44 grams for women and 56 grams for men.

* Sodium (and chloride), the components of table salt, increase the calcium requirement by increasing urinary calcium excretion. Individuals with low salt intakes may be able to maintain calcium balance at low calcium intakes, while those with more typical U.S. salt intakes will have higher calcium requirements.

* Oxalate is a nutrient that increases the calcium requirement by interfering with calcium absorption in the same food (not in others). Spinach, for example, is an extremely nutritious food, but its calcium is not absorbed because it is chemically bound to the oxalates that are present. However, eating spinach with cheese would not affect the absorption of calcium from the cheese. Foods high in oxalates include spinach, rhubarb and sweet potatoes.

* Phosphorous. Some concern has been expressed that there may be too much phosphorous in the North American diet, particularly since phosphorous (in the form of phosphoric acid) is a constituent of cola beverages and because phosphate is added to many processed foods. Phosphorous may increase the body's need for calcium by interfering with calcium absorption. There is no scientific consensus that current levels of phosphorous intake are, by themselves, harmful. There is, however, general agreement that today's higher phosphorous intakes would be entirely safe in individuals with normal kidney function if calcium intakes were at recommended levels.

* Caffeine. The amount of caffeine in a cup of coffee can reduce calcium absorption by a few milligrams, but that loss can be easily offset by adding a tablespoon or two of milk. Much of the apparently harmful effect of caffeine appears to be due not to the caffeine itself, but to the fact that caffeine-containing beverages are often substituted for milk in the diet.

* Fiber. Adequate fiberintake is necessary to aid digestion and prevent several chronic diseases such as colon cancer and heart disease. Fiber has a minimal effect on the absorption of calcium. The fiber in wheat bran is most likely to interfere, but unless the intake level is extreme, this is not a significant problem.


The Effects of Nutrition on Bone Density and Fracture Risk

Calcium and Vitamin D
Many published studies have demonstrated that low calcium intake is associated with low bone mass and increased fracture risk. It has been suggested that calcium deficiencies in youth can account for a 5-10% difference in peak bone mass and can significantly increase the risk for hip fracture in later life.

Among postmenopausal women, studies have shown that supplemental calcium can decrease the rate of bone loss from the femoral neck, the spine and the total body. Women who have been postmenopausal for six or more years tend to reap a greater benefit from calcium supplementation than those who are within five years of menopause. Since calcium is a nutrient, not a drug, the positive effects of supplemental calcium are most pronounced among women with low to moderate calcium intakes. Recent clinical trials have suggested that supplementation with calcium or calcium plus vitamin D, can reduce fracture incidence by about 30-50% in subjects with low calcium intakes.

Vitamin D deficiency is also a concern in bone health. The nutrient is essential for calcium absorption and normal bone mineralization. Studies have shown that low levels of vitamin D can contribute to low bone density. Recent evidence suggests that deficiencies in this nutrient may additionally contribute to hip fractures in postmenopausal women.

Protein and Bone Health
It is known that high protein intake increases the calcium requirement. However, the association between protein and osteoporotic bone fractures in individuals has not been fully explored.

Reduced protein intake has been linked to low femoral neck bone density in elderly hospitalized patients. In these individuals, clinical outcomes following hip fracture were significantly enhanced when protein intake was normalized through nutritional supplementation.

Soy and Isoflavones
The soybean plant, a legume, contains specific phytochemicals known as isoflavones. Phytochemicals are non-nutritive substances, in that they contain no vitamins or minerals. Isoflavones are also phytoestrogens. Phytoestrogens are compounds that have mild estrogenic effects.

Chick peas and legumes are good sources of isoflavones. The legume, soy, has the greatest concentration of these chemicals.

Isoflavones have received a good deal of attention for their possible cancer and heart disease-preventive traits. Because of their estrogen-like properties, many believe that isoflavones and other phytoestrogens may one day play a role in postmenopausal health.

One isoflavone derivative, ipriflavone, is currently used outside of the United States as an osteoporosis therapy. Ipriflavone has had a bone-protective effect in several studies. Further research is needed to understand the systemic effects of this compound before it can be considered a therapeutic approach for osteoporosis prevention and treatment.

Nutrition and Weight
Nutritional status and body weight are additional considerations in skeletal health. Nutritional status can influence one's tendency to fall and is a factor in the maintenance of adequate soft tissue mass to protect the bones from a fall- related fracture. Of particular concern are thin, undernourished, elderly individuals who may lack sufficient muscle and fat mass in the hip region.

Body weight, is an important determinant of bone density. The skeleton of heavy individuals tends to benefit from it's increased load-carrying role. Studies have demonstrated that body weight is positively correlated with bone mineral density, and that weight loss is associated with bone loss. (Increasing calcium intake appears to reduce the bone loss that accompanies weight loss.)

Weight loss in older individuals has been linked to an increase in fracture risk. Researchers found that "extreme" weight loss (10% or more) beginning at age fifty, increased the risk of hip fracture in older women and men. Conversely, a weight gain of 10% or more decreased hip fracture risk. Such studies suggest that maintaining weight in later life may have a protective effect on bone.