Soy

supplementation has been shown to lower cholesterol in humans. Soy is available in foods such as tofu, miso, and tempeh and as a supplemental protein powder. Soy contains isoflavones, naturally occurring plant components that are believed to be soy’s main cholesterol-lowering ingredients. A controlled trial showed that soy preparations containing high amounts of isoflavones effectively lowered total cholesterol and LDL (“bad”) cholesterol, whereas low-isoflavone preparations (less than 27 mg per day) did not. However, supplementation with either soy or non-soy isoflavones (from red clover) in pill form failed to reduce cholesterol levels in a group of healthy volunteers, suggesting that isoflavone may not be responsible for the cholesterol-lowering effects of soy. Further trials of isoflavone supplements in people with elevated cholesterol, are needed to resolve these conflicting results. In a study of people with high cholesterol levels, a soy preparation that contained soy protein, soy fiber, and soy phospholipids lowered cholesterol levels more effectively than isolated soy protein.

In a double-blind study of postmenopausal women, supplementing with 10 grams of twice a day for six weeks significantly reduced diastolic blood pressure by an average of 5 mm Hg, compared with a diet not containing soy protein. In another study, men and women with mild to moderate hypertension consumed 500 ml (approximately 16 ounces) of soy milk or cow's milk twice a day for three months. After three months, the average systolic blood pressure had decreased by 18.4 mm Hg in the soy group, compared with 1.4 mm Hg in the cow's milk group. The reductions in diastolic blood pressure were 15.9 mm Hg with soy milk and 3.7 mm Hg with cow's milk. In another study of people with hypertension who were consuming a low-protein, low-fiber diet, supplementing with a combination of soy protein and psyllium (a fiber source) lowered systolic blood pressure by an average of 5.9 mm Hg. The blood pressure reduction with soy protein alone or with fiber alone was less pronounced than that with combination treatment. Other research has also shown a blood pressure–lowering effect of soy protein.

Animal and human studies have suggested that when soy is used as a source of dietary protein, it may have several biological effects on the body that might help with weight loss. A preliminary study found that people trying to lose weight using a meal-replacement formula containing soy protein lost more weight than a group not using any formula. However, controlled studies comparing soy protein with other protein sources in weight-loss diets have not found any advantage of soy. When soy protein is used for other health benefits, typical daily intake is 20 grams per day or more.

In one preliminary study, elderly men participating in a 12-week strength training program took a liquid supplement containing 10 grams of protein (part of which was soy protein), 7 grams of carbohydrate, and 3 grams of fat either immediately following exercise or two hours later. Men taking the supplement immediately following exercise experienced significantly greater gains in muscle growth and lean body mass than those supplementing two hours later, but strength gains were no different between the two groups. A controlled study of female gymnasts found that adding 0.45 grams of soy protein (0.45 grams per pound of body weight per day) to a diet that was adequate in protein during a four-month training program did not improve lean body mass compared with a placebo. No research has compared different sources of protein to see whether one source, such as soy protein, has a better or more consistent effect on exercise recovery or the results of strength training.

A double-blind study of 78 postmenopausal women (average age, 49.5 years) were randomly assigned to receive 60 mg per day of soy isoflavones (as aglycone) or placebo for six months. Then after a one-month break, each woman received the alternate treatment for an additional six months. Thinking (cognitive) performance and mood were assessed by tests at the end of each treatment period. Compared with placebo, people treated with isoflavones experienced significant improvement on 6 of 17 measures of cognitive function and 7 of 8 measures of mood. In another double-blind study, 177 postmenopausal women (average age, 55 years) who were experiencing five or more hot flushes per day were randomly assigned to receive soy isoflavones (50 mg per day total of an approximately equal mixture of isoflavones genistin and daidzin) or placebo for 12 weeks. The average reduction in hot flash frequency after 6 weeks was significantly greater in the isoflavone group than in the placebo group; however, during the entire 12 weeks, the difference between the two groups was only marginally significant.

In a double-blind trial, supplementing with protein (providing 68 mg of isoflavones per day) for two menstrual cycles was significantly more effective than a placebo at relieving premenstrual swelling and cramping. The placebo used in this study was cow's milk protein. Some doctors believe that cow's milk, because of its estrogen content, can worsen premenstrual symptoms. If that is the case, then the beneficial effect of soy protein may have been overestimated in this study.