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What are isoflavones? Are they effective for treating osteoporosis?

Isoflavones[1] are phytoestrogens[2], weak estrogenic substances found in plants. Isoflavones are most notably found in soy and red clover. The isoflavones that are found in soy are different from the isoflavones found in red clover. Sometimes manufacturers mix the different isoflavones into one product. For example, Rimostil® is a blend of isoflavones derived from red clover, while Estroven contains 55 mg of isoflavones derived from soy and another plant extract. Most isoflavone products are available over the counter and are not held to the same Food and Drug Administration (FDA) regulations as prescription medications. One isoflavone product, Fosteum®, meets the FDA GRAS standard, meaning "generally regarded as safe," and is available by prescription as a medical food. Fosteum contains a combination of genistein (isoflavone purified from soy), zinc, and vitamin D. It has been shown to improve BMD, but fracture data are not available (see Table 7).

Manufacturers extract isoflavones from soy protein and red clover, but isoflavones are also found in foods such as soy products and other legumes such as chickpeas.

When you eat foods containing isoflavones, your intestines convert the plant chemicals into estrogen-like substances that are absorbed into the circulation and can then bind with estrogen receptors, making them act like estrogen in some ways. And we know that estrogen is a major component in building and maintaining strong bones, particularly in women, which is why there is such interest in the potential role of isoflavones and phytoestrogens in the prevention and treatment of bone loss.

Because isoflavones have been found to act like estrogen in the body, isoflavones are being studied not only for their effects on the hot flashes associated with menopause, but also for their effects on bone health. Several small studies have shown some promise in reducing bone loss and increasing bone mineral density without some of the side effects of estrogen observed in other scientific studies. For example, isoflavones don't seem to increase breast density, increase endometrial thickness, or exert the same negative effects on your heart health. Further study is needed to confirm the bone findings reported when isoflavone supplements are taken. Isoflavones are considered safe when taken with other medications, such as the prescription medications described in Questions 55 to 67.

Some people choose to eat soy instead of taking soy isoflavone supplements. The FDA has approved product labeling that says, "Diets low in saturated fat and cholesterol that include 25 grams of soy protein a day may reduce the risk of heart disease." One serving of soy protein burger (for example) provides 9 grams of soy protein. In order to use this claim on labeling, a soy product must be low in fat, cholesterol, and sodium and contain at least 6.25 grams of soy protein. But it's not clear how many milligrams of isoflavones are in each gram of soy protein. The degree to which you absorb isoflavones from soy will depend on the food product and the bacteria in your intestines. An 8-ounce glass of soy milk, when converted in your intestines, can provide 20 to 45 mg of isoflavones. There's no firm evidence at this time that eating soy or taking soy supplements will improve your bone density; however, eating soy is still a good strategy as part of a healthy diet, and many soy foods, like tofu, are a good source of calcium. Question 71 discusses other complementary and alternative therapies for osteoporosis, and Question 99 discusses future developments, including a synthetic isoflavone that is being researched for its effects on bone.

  • [1] A type of phytoestrogen found most notably in soy and red clover.
  • [2] Weak, estrogen-like substances that are in plants. Can be eaten in whole foods, such as soy, or extracted from red clover in the form of isoflavones and made into supplements.
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