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size. It is thus independent of growth. The DXA method is also limited by excessive soft tissue as present in massively obese individuals. Dual-energy computed tomography measurements, which are much more expensive and require larger X-ray doses can provide density as well as volumetric determinants and are useful for estimating the entire mineral component.

Direct estimation of calcium balance in older adults by BMD is highly dependent on other factors besides calcium intake, such as serum levels of estrogen and PTH, intake of other nutrients (e.g., phosphorus and sodium), as well as adequate intestinal absorption and normal kidney function. Indeed, bone remodeling is not directly regulated by calcium, although it can suppress PTH-induced increases in bone resorption under certain conditions. Circumstances that enhance bone resorption, such as estrogen deficiency, or glucocorticoid use, alter the organic matrix and reduce the thickness and density of trabeculae, independent of calcium intake. In short, density measurements do not directly reflect calcium stores.

OTHER FACTORS RELATED TO CALCIUM NUTRITURE

As described above, not all calcium consumed is absorbed once it enters the gut. In general, the efficiency of calcium absorption is in reverse proportion to the amount of calcium consumed at any one time. Other factors also affect the amount of calcium available to the body.

Bioavailability of Calcium

Humans absorb about 30 percent of the calcium present in foods, but this varies with the type of food consumed. Bioavailability is generally increased when calcium is well solubilized and inhibited in the presence of agents that bind calcium or form insoluble calcium salts. The absorption of calcium is about 30 percent from dairy and fortified foods (e.g., orange juice, tofu, soy milk) and nearly twice as high from certain green vegetables (bok choy, broccoli, and kale). If a food contains compounds that bind calcium or otherwise interfere with calcium absorption, such as oxalic acid and phytic acid, then the food source is considered to be a poor source of calcium. Foods with high levels of oxalic acid include spinach, collard greens, sweet potatoes, rhubarb, and beans. Among the foods high in phytic acid are fiber-containing whole-grain products and wheat bran, beans, seeds, nuts, and soy isolates. The extent to which these compounds affect calcium absorption varies, and food combinations affect overall absorption efficiency. Eating spinach with milk at the same time reduces the absorption of the calcium in the milk (Weaver and Heaney, 1991); in contrast, wheat products (with the exception of wheat bran) do not appear



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