The soybean has been prized for centuries in Asia as a nutritious, high-protein food with a myriad of uses, and today it's popular in the United States not only in Asian food, but also as a cholesterol-free meat and dairy substitute in traditional American foods. Soy burgers, soy yogurt, tofu hot dogs, and tofu cheese can be found in a growing number of grocery stores alongside the traditional white blocks of tofu, and soy is increasingly used as a protein filler in many prepared foods, including fast-food “hamburger.”
Soy appears to reduce blood cholesterol levels, and the US Food and Drug Administration has authorized allowing foods containing soy to carry a "heart-healthy" label.
Soybeans contain isoflavones, chemicals that are similar to estrogen. These are widely thought to be the active ingredients in soy, although, as discussed below, there is substantial evidence that other constituents may be equally or more important. Much of the information in this article overlaps with that in the Isoflavone article.
If you like Japanese, Chinese, Thai, or Vietnamese food, it's easy to get a healthy dose of soy. Tofu is one of the world's most versatile foods. It can be stir-fried, steamed, or added to soup. You can also mash a cake of tofu and use it in place of ricotta cheese in your lasagna. If you don't like tofu, there are many other soy products to try: plain soybeans, soy cheese, soy burgers, soy milk, or tempeh. Or, you can use a soy supplement instead.
The FDA allows soy foods containing 6-½ grams of soy to carry a heart-healthy label. Evidence suggests that a daily intake of 25 g of soy protein is adequate to noticeably reduce cholesterol. This amount is typically found in about 2-½ cups of soy milk or ½ pound of tofu.
Note: Soy is increasingly added to foods in the US as a protein filler, and there are concerns that some people here may be greatly exceeding the amount of soy eaten anywhere else in the world. Even the 25 g amount recommended for reducing cholesterol levels is relatively high. For comparison, in Asia, the average intake of soy is only about 10 g daily. 71
According to the combined evidence of numerous controlled studies, soy can reduce blood cholesterol levels and improve the ratio of LDL ("bad") versus HDL ("good") cholesterol. 1,122,136 At an average dosage of 47 g daily, total cholesterol falls by about 9%, LDL cholesterol by 13%, and triglycerides by 10%. Soy's effects on HDL cholesterol itself are less impressive. There is inconsistent evidence regarding whether soy might help reduce high blood pressure . 117, 124 Indeed, a comprehensive and careful review of studies investigating the influence of phytoestrogens (including soy meals) on blood pressure found no meaningful effect. 140 However, another review found that soy protein (as opposed to other soy products) could significantly reduce blood pressure. 143
Soy may reduce the common menopausal symptom known as "hot flashes," but study results conflict. 71,125 Soy has not been found helpful for improving the hot flashes that often occur in breast cancer survivors. 69,84
Unlike estrogen, soy appears to reduce the risk of uterine cancer. 4 Its effect on breast cancer is not as well established, but there are reasons to believe that soy can help reduce breast cancer risk as well, 5-7 possibly by reducing estrogen levels and lengthening the menstrual cycle. 70 (For more information, see the Isoflavone article.) Soy has shown inconsistent promise for helping to prevent prostate and colon cancers. 10-12,118
Soy may be beneficial in diabetes . A soy extract, pinitol, may improve blood sugar control in diabetics. 115 And, in addition to its favorable effects on cholesterol, the long-term consumption of a diet rich in soy may reduce blood sugar and improve kidney function in diabetic patients with renal dysfunction. 137
Yet another soy extract, pinitol, may improve blood sugar control in people with diabetes. 115
A very small study found hints that soy isoflavones might help reduce buildup of abdominal fat. 126
Numerous controlled studies indicate that soy can reduce LDL (“bad”) cholesterol by about 10%, and perhaps slightly raise HDL (“good”) cholesterol as well. 23,71,87,119,122,136,137,143,144
It has often been assumed that isoflavones are the active ingredients in soy responsible for improving cholesterol profile. However, studies that used purified isoflavones (as opposed to whole soy, or soy protein) have generally failed to find benefit. 96,145 It is possible that non-isoflavone constituents of soy, such as proteins, fiber, and phospholipids, may be equally or perhaps even more important than the isoflavones in soy. 71,97 In a careful review of 30 studies involving almost 3,000 subjects, researchers determined that isolated soy protein (in the range of 15-40 g per day) modestly reduced LDL cholesterol levels by an average of 6%. 142 In another study involving two soy milk products, one made from whole soy beans and the other from isolated soy proteins, researchers found that both were more effective than cow’s milk at reducing LDL cholesterol levels. 136 In addition, the substance pinitol appears to have cholesterol-lowering properties. 115
However, there are other possibilities as well. One study suggests that the isoflavone daidzein may be only effective for reducing cholesterol when it is converted by intestinal bacterial into a substance called equol. 98 It appears that only about one-third of people have the right intestinal bacteria to make equol.
Another study found that soy products may at times have an unusual isoflavone profile—containing high levels of the isoflavone glycitein rather than the more usual genistein and daidzein. 67 Glycitein could be inactive regarding cholesterol reduction; in other words, variations in the proportions of specific isoflavone constituents might have made some studied soy isoflavone products inactive.
It has also been suggested that soy protein must be kept in its original state to be effective. Ordinary soy protein extracts are somewhat damaged (“denatured”). In a double-blind study of 120 people, a special soy protein extract—in which the proteins were protected from damage—proved more effective for improving cholesterol profile than did standard denatured soy protein extracts. 135
There are other theories, as well. For more information, see the Isoflavone article.
Although study results are not entirely consistent, soy may be helpful for symptoms of menopause , especially "hot flashes." For example, a double-blind, placebo-controlled study involving 104 women found that isoflavone-rich soy protein provided significant relief of hot flashes compared to placebo (milk protein). Improvements in hot flashes, as well as other symptoms such as vaginal dryness, were seen in several other studies of soy or soy isoflavones as well. 3,31,32,83,88-90,125,99-101,116 A review of 13 randomized trials with 1,996 women showed a reduced frequency of hot flashes with soy isoflavones compared to placebo, with more than 12 weeks of treatment showing the greatest effect. Hot flash severity improved in 9 randomized trials involving 988 women with soy isoflavones compared to placebo. 146 However, about as many studies have failed to find benefit with soy or concentrated isoflavones. 34-36,68,71,92,102,120 Furthermore, in three double-blind, placebo-controlled trials, isoflavone-rich soy failed to reduce hot flashes among survivors of breast cancer. 69,84,103
To make matters even more complicated, a double-blind study of 241 women experiencing hot flashes found equivalent benefits whether isoflavone-free or isoflavone-rich soy products were used. 93
The high rate of the placebo effect seen in many studies of menopausal symptoms may account for these discrepancies. In addition, it is possible that certain formulations of soy contain as yet unidentified ingredients beyond isoflavones that play an important role.
At least two studies found that people who are equol producers (see previous section) may experience greater benefits. 138,139
In one study that evaluated the benefits of soy in osteoporosis, a total of 66 postmenopausal women took either placebo (soy protein with isoflavones removed) or soy protein with 56 mg or 90 mg of isoflavones daily for 6 months. 37 The group that took the higher dosage of isoflavones showed significant gains in spinal bone density. There was little change in the placebo or low-dose isoflavone groups. This study suggests that the soy isoflavones in soy protein may be effective for osteoporosis.
Very nearly the same results were also seen in a similar study. This 24-week, double-blind study of 69 postmenopausal women found that soy can significantly reduce bone loss from the spine. 38
Similar benefits with soy or soy isoflavones have been seen in other human and animal trials; however, other studies have failed to find benefit. 39-48,71,85,104-109,123 On balance it is probably fair to say that isoflavones (either as soy, purified isoflavones, or tofu extract) are likely to have a modestly beneficial effect on bone density at most.
Interestingly, one small but long-term study suggests that progesterone cream (another treatment proposed for use in preventing or treating osteoporosis) may decrease the bone-sparing effect of soy isoflavones. 110
Estrogen and most other medications for osteoporosis work by fighting bone breakdown. It has been hypothesized that soy may also work in other ways, by helping to increase new bone formation. 49,50
Studies in animals have found soy essentially nontoxic. 51 And it is reassuring to note that researchers found no evidence of ill effects when they gave healthy postmenopausal women 900 mg of soy isoflavones a day for 84 consecutive days. 141 However, soy or its isoflavones could conceivably have some potentially harmful effects in certain specific situations.
Soy appears to have numerous potential effects involving the thyroid gland. When given to individuals with impaired thyroid function , soy products have been observed to reduce absorption of thyroid medication. 58-72 In addition, some evidence hints that soy isoflavones may directly inhibit the function of the thyroid gland, although this inhibition may only be significant in individuals who are deficient in iodine. 73,74,111 However, to make matters even more confusing, studies of healthy humans and animals given soy isoflavones or other soy products have generally found that soy either had no effect on thyroid hormone levels or actually increased levels. 65,74-78 The bottom line: In view of soy’s complex effects regarding the thyroid, individuals with impaired thyroid function should not take large amounts of soy products except under the supervision of a physician.
Soy may reduce the absorption of the nutrients zinc, iron, and calcium. 60-64 To avoid absorption problems, you should probably take these minerals at least 2 hours apart from eating soy.
Other concerns relate to the estrogenic properties of soy isoflavones. For example, while soy is thought to reduce the risk of developing breast cancer, it is possible that soy might not be safe for women who have already had breast cancer. In addition, there are concerns that intensive use of soy products by pregnant women could exert a hormonal effect that impacts unborn fetuses. Finally, fears have been expressed by some experts that soy might interfere with the action of oral contraceptives . However, one study of 36 women found reassuring results. 55 For more information on these and other safety issues regarding the isoflavones in soy, see the full Isoflavones article.
One observational study raised concerns that soy might impair mental function. 79 However, observational studies are highly unreliable by nature, and experts do not consider this a serious issue. 71 Additionally, a number of studies looking at cognitive improvement have found that soy or soy isoflavones either have no effect on mental function, or perhaps minimally improve it. 105, 127-129
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