Soy?s Soiled Reputation

Soy protein, a once esteemed protein source for athletes, has been taking its share of knocks lately. As reported recently in this magazine, a study using pigs as subjects (real pigs, not humans) found that, compared to casein, using soy protein led to less protein synthesis and promoted muscle protein breakdown. Studies with humans, however, have not shown that effect’if anything, the opposite usually occurs.

Studies done by Romanian researchers in the early ’90s examined athletes from a wide range of sports who took in an average of 1.5 grams of soy protein per kilogram of bodyweight daily for eight to 16 weeks. Under hard-training conditions, those athletes showed increased or preserved muscular strength and maintenance of lean mass. Those who used a placebo lost muscle during the study.

Another study, involving five professional baseball players, showed significant losses in weight and fat mass after the subjects used supplemental soy protein. A study that compared diets based on animal proteins with soy-based protein diets found similar rates of protein synthesis and breakdown in the young male subjects in both groups.

The most recent study to examine the effects of supplemental soy protein on lean mass or muscle changes with exercise was presented at the ninth annual meeting of the Canadian Society for Exercise Physiology in November 2001. Forty-eight subjects, aged 18 to 35, were divided into groups and randomly assigned supplements of 60 grams of soy protein, 60 grams of whey protein or a placebo (maltodextrin, a carbohydrate). The subjects also participated in a six-week weight-training program.

Tests done before and after the program showed that the subjects who got either soy or whey protein had increases in lean mass that were 3 to 5 percent more than those who took the placebo. In addition, the men taking the protein supplements showed less muscle breakdown after training than the placebo group. A significant aspect of the study was that neither type of protein supplement’whey or soy’ showed any superiority in promoting muscular gains and decreased muscle breakdown.

Soy protein contains a higher percentage (35 percent) of several amino acids (glutamine, lysine and the branched-chain amino acids) than high-quality animal proteins, including whey, casein, eggs and beef. Soy is also rich in arginine, an amino acid associated with heightened immune response. While soy protein is relatively low in methionine, an essential amino acid, engineered soy proteins available today usually have added methionine, which several studies show makes such soy supplements comparable to milk and egg proteins in biological value.

Perhaps the main controversy concerning the effects of soy protein relates to its content of isoflavones, mainly genistein and daidzein. They impart many of the health benefits associated with soy foods, such as cardiovascular protection and decreased incidence of several types of cancer, especially breast and prostate cancers. Although the mechanism for cancer protection involves several aspects of isoflavone metabolism, many scientists attribute it to the structural resemblance between soy isoflavones and estrogen.

That similarity allows the isoflavones to displace estrogen in certain cellular estrogen receptors, thus blocking the hormone’s activity. In that sense soy isoflavones work like drugs such as Nolvadex or Tamoxifen, which many bodybuilders use to block anabolic steroids’ conversion into estrogens, a process that can result in gynecomastia, or male breast tissue increase.

Nevertheless, studies show that an intake of up to 40 milligrams a day of soy isoflavones has no adverse effects on male testosterone levels, nor will it increase estrogen levels in men. Few people realize that isoflavones must be activated by intestinal bacteria, which contain enzymes that convert isoflavones into their active forms. Soy isoflavones also have a finite absorption, reached when the ingested dose gets to 0.5 milligrams per kilogram of bodyweight. You absorb 30 to 50 percent of orally ingested isoflavones.

The most recent study suggesting problems with soy protein was presented at the 2001 Food and Nutrition Conference sponsored by the American Dietetic Association. It compared the effects of 60 grams of soy protein isolate with 60 grams of whey protein isolate in 27 adults who took one or the other protein for three months. The aim of the study was to determine if soy protein had any adverse effect on thyroid function. (Isolated-cell studies have shown that soy isoflavones interfere with an enzyme needed for thyroid hormone synthesis. Rat-based studies show that isoflavones inhibit this enzyme up to 60 percent, yet both the thyroid gland and its hormones were normal in the affected rats.)

Thyroid function was tested in the subjects at the beginning of the study and at the six- and 12-week points. Those taking in the soy protein showed a significant decline in production of T4, a thyroid hormone. Those taking the whey product showed no adverse changes in thyroid activity.

If soy truly blocks thyroid activity, why aren’t thyroid problems epidemic in Asian people, who eat soy daily for years? They average 50 milligrams of isoflavones a day derived from various soy-based foods. Each gram of soy protein averages two to five milligrams of soy isoflavones. In the study comparing soy to whey protein, the subjects taking soy got 60 grams of soy protein daily, equaling 120 to 300 milligrams a day of isoflavones. That far exceeded the average intake of Asian people.

Indeed, several studies show that soy protein may increase thyroid activity. In a 1996 study of elite female gymnasts some took a soy protein supplement while others got a placebo. The women on the soy protein showed higher levels of T4, while those on the placebo showed decreased levels (that’s the same thyroid hormone cited in the study reported above).

Studies published in 1999 and 2000 of older women who took soy isoflavones found no effect on the women’s thyroid function, even though the women took in 90 milligrams of soy isoflavones daily. Other data show that the soy isoflavones may only affect people predisposed to thyroid problems or those deficient in the mineral iodine, which is required for thyroid hormone synthesis. Other foods, including such cruciferous vegetables as broccoli, also contain substances that block iodine uptake in the thyroid gland. For that reason they could cause problems if consumed in huge amounts or when iodine isn’t also included in the diet’something that’s highly unlikely in the United States.