In October, 1999, the Food and Drug Administration (FDA) approved labeling claims for dietary soy protein stating that it may reduce the risk of heart disease.
This is the 11th health claim allowed by the FDA. The health claim that can be used on labels of products containing soy protein states: "Diets low in saturated fat and cholesterol that include 25 grams of soy protein a day may reduce the risk of heart disease."
This latest FDA health claim was based on animal studies, epidemiological studies and human studies demonstrating that diets high in soy protein and low in animal protein lead to decreased levels of total cholesterol, low-density lipoprotein (LDL) cholesterol and triglycerides.
Diets rich in soy protein have been found to reduce serum levels of total cholesterol, LDL-cholesterol, triglycerides and apolipoprotein B (apo B). The mechanism of the lipid-lowering activity of soy protein is unclear. There are a few possible explanations. Soy protein is much richer in L-arginine than is animal protein, which is richer in L-lysine.
Some animal studies indicate that dietary increases in L-arginine are accompanied by decreases in cholesterol levels. Further, some studies have demonstrated that, under certain conditions, e.g., hypercholesterolemia, high intakes of L-arginine could enhance endothelial-dependent vasodilation and nitric oxide or NO production (see L-arginine). This could contribute to the possible antiatherogenic activity of soy protein.
The soy isoflavones may also contribute to the lipid-lowering activity of soy protein as well as its antiatherogenic activity. Most soy protein products contain the isoflavones genistin, daidzin and glycitin, which have weak estrogenic effects and also may have antiestrogenic activity (see Soy Isoflavones). Oral estrogens have been shown to decrease total cholesterol and LDL-cholesterol. The soy isoflavones may have similar actions.
Interestingly, a few studies have shown that when the isoflavones are removed from the soy protein, the protein itself has little hypocholesterolemic activity. Soy isoflavones themselves do not have the same hypocholesterolemic activity as the combination of soy protein and soy isoflavones. There are probably synergistic effects of these substances that are not understood at this time.
There are also other substances associated with soy protein, including saponins, trypsin inhibitor and bioactive peptides, which may also contribute to the lipid-lowering activity of soy protein. The soy isoflavones are antioxidants, and their antioxidant activity may contribute to the possible anti-atherogenic effect of soy protein.
The antioxidant, anticarcinogenic and antiosteoporotic activities of soy protein are probably due, in large part, to the soy isoflavones (see Soy Isoflavones). Soy protein has been found to reduce intestinal mucosa polyamine levels in rats, which may be another anticarcinogenic mechanism. Also, a bioactive peptide has recently been isolated from soybeans and has been found to have potent antimitotic activity.
The FDA-approved health claim that soy protein, in adequate amounts, may help protect against heart disease is based upon numerous in vitro, animal, epidemiological and human studies. Evidence has accumulated over many decades showing that soy protein, but not animal protein, has significant cholesterol-lowering properties in animal studies.
In a meta-analysis of clinical studies, most of them well-controlled, investigators concluded that soy protein significantly lowered serum concentrations of total cholesterol, LDL-cholesterol and triglycerides without significantly altering HDL-cholesterol concentrations.
Since the meta-analysis cited above was conducted, other clinical research has continued to confirm the lipid-lowering ability of soy protein. Recently it was demonstrated that administration for six weeks of as little as 20 grams of soy protein per day, in place of animal protein, achieved significant reductions of non-HDL-cholesterol and apolipoprotein (apo) B in moderately hypercholesterolemic men.
Soy protein supplements are contraindicated in those who are hypersensitive to any component of a soy protein-containing product.
Women with estrogen receptor-positive tumors should exercise caution in the use of soy protein supplements and should only use them if they are recommended and monitored by their physicians.
Unquestionably, soy protein foods help squelch bad cholesterol, says James Anderson of the University of Kentucky, who reviewed 38 human studies in the New England Journal of Medicine. He found eating an average 47 grams (1.6 ounces) daily in place of animal protein depressed "bad" LDL cholesterol 13 percent and triglycerides 10 percent, and raised "good" HDL 2.5 percent. That's a lot of soy (more than a quart of soy milk), but eating less can lower cholesterol to a lesser degree.
The more soy you eat and the higher your cholesterol, the greater the benefit. Anderson concluded that eating soy protein regularly could cut heart disease risk 25-30 percent. It's not yet known how soy cuts cholesterol.
Soy may neutralize the ability of LDL cholesterol to infiltrate artery walls and trigger plaque buildup. In experimental animals, soy's genistein blocks this process.
Vitamin E, shown to ward off heart attacks and strokes, works identically. Confirming soy's powers: Monkeys fed soy protein have less arterial plaque and better arterial dilation, also crucial in avoiding heart attacks.
Why do Americans have several times the rate of breast and prostate cancer of the Japanese? Why do these cancers grow more slowly in the Japanese?
Researchers suspect one reason is that the Japanese eat 15-25 times more soy isoflavones, including genistein, than do Americans.
Genistein suppresses the growth of cancer cells, says Stephen Barnes of the University of Alabama. Also, soy's estrogenic activity may thwart hormone-dependent cancers, such as breast and prostate cancer.
Indeed, soy's genistein is similar to the breast cancer drug tamoxifen, Barnes says. In men, soy estrogens may block testosterone, slowing prostate tumor growth.
There is new evidence soy foods can cut the risk of osteoporosis and fractures in later years. John Erdman Jr. at the University of Illinois found that postmenopausal women who ate 40 grams daily of isolated soy protein (with 90 milligrams isoflavones) increased lumbar-spine bone density by 2.2 percent in six months. In other research, bone density increased in women who ate 11/2 ounce soy flour daily.
Eases hot flashes
Because soy has estrogen-like activity, and only one-third as many Asian as American women report menopausal symptoms, such as hot flashes, the question is: Can eating soy act as "estrogen replacement therapy"?
A first-of-its-kind study at the Bowman Gray School of Medicine in North Carolina suggests soy may help. Forty-three women suffering hot flashes or night sweats added 20 grams (3/4 ounce) powdered soy protein to their morning juice or cereal. In six weeks, the severity, but not the number, of hot flashes and night sweats diminished significantly.