Soy and Osteoporosis

Osteoporosis, the thinning and weakening of bone tissue resulting in brittle and fragile bones prone to facture.

Osteoporosis is a serious problem worldwide. However, rates vary in different parts of the word. For example, the United States has one of the highest rates; between 15 and 20 million Americans have osteoporosis.

While this condition afflicts both sexes, women are at greater risk than men. In fact, women suffered two-thirds of all the osteoporosis-related hip fractures in 1990. Women are particularly susceptible to osteoporosis because they have a smaller peak bone mass and experience dramatic increases in bone loss following menopause. Scientists estimate that women may lose as much as 15 to 50 percent of their bone ass in the first 10 years following menopause.

What Can Be Done?

Calcium Intake

Of the many factors that contribute to susceptibility to osteoporosis, including sodium and alcohol use, exercise, genetics, and smoking, calcium deficiency is one of the most crucial, Calcium deficiency is particularly high in the United States. Many Americans are calcium-deficient, especially girls and women, who only consume about 66 percent of the RDA for this mineral.

Dietary differences account for much of this discrepancy , especially those involving calcium intake and protein sources. While Americans depend primarily on dairy products, legumes, nuts, and seeds for much of their calcium, people in countries with lower incidence of osteoporosis subsist primarily on soy, a rich source of calcium. Soy's calcium value is well documented. For example, one cup of cooked soybeans contains 175 mg of calcium. Thus, we can increase the calcium in our diet by eating more soy.

Calcium Absorption

While calcium may enter the body in abundance, it is not always readily absorbed. Research confirms that the calcium in soy is absorbed better than that of many other plant foods. For example, soy contains more calcium than spinach or beet greens, and its calcium is more easily absorbed. While the calcium in these vegetables is poorly assimilated, the body absorbs an amazing 30 to 40 percent of the calcium in soy. These statistics confirm the fact that soy is linked to high calcium absorption.

Calcium Retention

Despite the importance of calcium intake and absorption in determining bone health, the amount of calcium retained by the body is even more important. Researchers have found that replacing the consumption of animal proteins with those from soy helps the body retain calcium by minimizing urinary calcium losses. For example, a 1988 University of Texas Health Sciences Center study showed that volunteers excreted 50 percent less calcium in their urine when they replaced the animal products in their diet with soy foods. In another study, when human subjects were fed diets that differed only in the type of protein consumed, subjects that consumed exclusively soy protein excreted 33 percent less calcium than subjects who are only animal protein. We can see from these studies that a diet rich in soy foods and help us retain the calcium in our bodies.

Calcium Resorption

While calcium intake and retention are vital to lifelong bone health, another factor in bone remodeling---bone resorption---can contribute to osteoporosis. During resorption, bones break down into tiny collagen fibers made of simple proteins that are released into the urine. When bone resorption exceeds bone formation, osteoporosis result. Daidzein and genistein, isoflavones in soy foods, have been found to successfully infibit the disintegration process. In one study using the isoflavone genistein, results showed a reduction in bone resorption. An additional observation supports the potential benefits of isoflavones in bone health: their similarity in chemical structure to ipriflavone. This synthetic isoflavone has been shown to inhibit bone break down and has been used successfully in both Asia and Europe to treat osteoporosis. Since both daidzein and genistein are present in foods, we can feel confident knowing that a diet rich in soy can help us avoid bone resorption.

Estrogen and Bone Loss

Estrogen deficiency is another factor affecting osteoporosis risk in postmenopausal women. For years these women have depended on estrogen replacement for bone loss protection. This is because estrogen combines with special estrogen receptors in the body. These complexes then affect certain genes, causing them to proliferate. A new estrogen receptor, ER beta, has been discovered that resides in bone tissue. This receptor binds with estrogen, inducing bone tissue health and proliferation. The soy isoflavone genistein acts like estrogen in the body by combining with ER beta almost as well as estrogen does, increasing bone density. In a six-month study performed in Illinois, three dozen menopausal women consumed 40 grams of enhanced soy protein every day. At the end of that time, the subjects exhibited an increased spinal bone density of 2.2 percent. Both mineral content and bone mineral density increased significantly. In another study, done in Australia, 52 postmenopausal women consumed 45 grams of soy grits daily for 12 weeks. They also experienced a significant increase in bone mineral content in the lumbar spine. This research supports the theory that soy protein isolates prevent bone loss induced by ovarian hormone deficiency. Therefore, a diet rich in soy foods can help us reduce bone loss. Clearly, soy foods address the crucial issues of osteoporosis. Rich in calcium, they encourage calcium retention and absorption. Their isoflavones minimized resorption and act estrogenic, reducing bone loss and even increasing bone density in some cases.