Supplements containing soy isoflavones may do little to preserve women's bone mass after menopause.

In a study of more than 200 women ages 46 to 65, researchers found that the soy supplement did not appear to ward off bone-density loss over 3 years. In general, women on the supplement showed the same degree of bone loss as those given a placebo -- though there was some evidence that a higher dose helped protect bone density in the hip.

The findings, reported in the American Journal of Clinical Nutrition, add to a conflicting body of research on soy and postmenopausal bone health.

Some studies have suggested that soy-based foods, isolated soy protein or isoflavone-containing supplements may be beneficial, while others have found no advantage.

Isoflavones are natural chemicals found in soybeans and certain other plant foods that are structurally similar to estrogen, and may have certain estrogen-like effects in the human body. Since declining estrogen levels after menopause spur bone-density loss, isoflavone supplements could theoretically protect women's bone mass.

The current findings, however, do not support that theory.

"I would not be able to recommend that women should take soy isoflavone tablets -- extracted from soy protein -- since these have not been demonstrated convincingly to be effective for prevention of bone loss," lead researcher Dr. D. Lee Alekel, a professor of nutrition at Iowa State University in Ames, told Reuters Health in an email.

Still, she said, women should still try to eat a variety of soy-based foods -- such as tofu, tempeh and soy-derived versions of cheese and yogurt -- since these are "nutritionally sound" choices.

For their study, Alekel and her colleagues randomly assigned 255 postmenopausal women to one of three groups: one that took 80 milligrams （mg） of a soy isoflavone supplement each day; one that took a 120-mg dose; and one given inactive placebo pills.

All of the women also took calcium and vitamin D supplements.

Over three years, women in the isoflavone and placebo groups showed similar average declines in whole-body bone mass, as well as bone density in the spine and hip area.

Women in the higher-dose isoflavone group did, however, show less bone loss in the femoral neck -- an area at the top of the thigh bone, where it meets the pelvis. But, the researchers write, because the effect was "very modest," and limited to the femoral neck, "we cannot conclude that soy isoflavones hold potential promise in the prevention of postmenopausal osteoporosis."

The idea that soy or soy isoflavones may protect bone mass comes, in part, from studies showing lower rates of hip fracture among women in Southeast Asia, where the traditional diet is rich in soy.

Those types of studies do not prove cause-and-effect, however. Because the current study looked only at soy isoflavone supplements, Alekel said it cannot speak to the potential effects of soy foods on women's bone health.

含有大豆黄酮异构衍生物的补充品对防止女性绝经后的骨质流失几乎没有作用。

在一项对200多名年龄在46到65岁的女性调查研究中，研究者发现，在这3年当中大豆补充品并没有防止骨密度的减少。虽然有些证据表明大剂量的补充品有助于保持髋部的骨密度，但是从总体上看，给予这种补充物的女性与给予安慰剂的女性其骨质流失程度是一致的。

发表在美国临床营养杂志的这项发现，是对大豆与绝经后骨健康研究的一个反例补充。

一些研究提示，大豆食品、分离出的大豆蛋白或是黄酮异构体衍生物的补充品可能对人体有益，然而另一些研究却没有发现对人体有益。

黄酮异构体衍生物是存在于大豆和其他某种植物食物中的天然化学成分。其结构上与雌激素相似，并且对人体可能有一定的雌激素样作用。由于绝经后雌激素水平下降导致了骨密度的降低，理论上，含有黄酮异构体衍生物的补充品就可能保持女性的骨质重量。

然而目前的发现还不支持上述理论。

"我不能给出建议说，女性应该服用含有从大豆蛋白中提取的黄酮异构衍生物的片剂，因为还没有明确这些衍生物对预防骨质流失是有效的，"重要研究成员D.Lee Alekel博士（爱荷华州立大学营养学教授）在给路透社健康专栏的一封e-mail里这样写道。

但是她说，女性朋友们应该依然试着食用各种大豆制品的食物，例如豆腐、印尼豆豉以及各种来源于大豆的奶酪和酸奶。这些都是"天然的极佳"选择。

为了研究，Alekel和她的同事随机分配255名绝经后妇女到三个实验组中。一组每天服用80毫克的含有大豆黄酮异构体衍生物的补充品，一组每天服用120毫克，剩下一组给予没有作用效果的安慰剂。

这些妇女同时也全部服用钙剂和维生素D.

三年后，黄酮异构体衍生物实验组和安慰剂组的女性，其全身骨质重量平均下降水平相似，同样，脊椎和髋部的骨密度平均下降水平也相似。

不过，服用高剂量黄酮异构体衍生物组的女性，其股骨颈（大腿骨顶端位置，连接骨盆）骨质流失量要比其他组少。研究人员报告说，由于效果"很温和",且只对股骨颈有效，"我们不能作出大豆黄酮异构体衍生物对预防绝经后的骨质疏松症有潜在前景的结论。"

在某种程度上，大豆或大豆黄酮异构体衍生物有可能维护骨质增加的研究观点显示，东南亚的女性髋部骨折发生率较低。这些地方的传统饮食富含大豆。

然而那些各种类型的研究没能验证因果关系。因为目前的研究只关注于大豆黄酮异构体衍生物补充品，Alekel说这不能证明大豆食品对女性骨质健康有潜在效用。