Soy has come a long way. Once viewed as an obscure, even mysterious, staple of the Asian diet, it became a darling of the American counterculture in the 1960s and ’70s. In the new millennium soy is emerging as a quintessential health food, a valuable source of protein that may help reduce the risk of heart disease, prostate cancer, and other malignancies. This dramatic transition raises two basic questions: Is soy right for you? And how can American men incorporate it into an enjoyable diet?

Tuesday, 01-10-2002

The Soy Story

Soybeans are legumes, members of the plant group that also contains lentils, peas, and other beans. Soy shares many attributes with other legumes, but it also has unique properties that may explain why it ranks first for health in this very healthful food group.

Protein. Soy is high in protein: One cup of soybeans provides approximately 20 grams of protein, about a third of the average man’s daily requirement. All proteins are made from building blocks called amino acids; the body uses just 20 individual amino acids to make the thousands of proteins that constitute the backbone of human life. The body can make 11 of these amino acids from raw materials (nitrogen, carbon, hydrogen, and oxygen), but 9 others — the essential amino acids — must be obtained from food. Soy provides all 9, making it a source of very high quality protein (see Harvard Men’s Health Watch, January 2001).

Fiber. Soy is also an excellent source of dietary fiber: One cup of the beans provides about 10 grams, or about 40% of the daily goal. Soy contains both insoluble fiber, which is particularly beneficial for intestinal health, and soluble fiber, which lowers cholesterol and blood sugar levels, thus reducing the risk of diabetes and heart disease (see HMHW, November 1996).

Fats. Like all plant food, soy has no cholesterol, and it’s also low in the saturated fats that contribute to heart disease. But soy does contain 2 grams per cup of the monounsaturated fatty acids and 5 grams per cup of the polyunsaturates that help promote cardiovascular health (see HMHW, May 2000).

Isoflavones. Although they are the least familiar nutrients in soy, the isoflavone proteins genestein, daidzein, and glycetein are most important; the first two are especially abundant. On average, a cup of soybeans contains about 100 mg of isoflavones.

Laboratory trials suggest that the isoflavones account for many of the unique properties of soy. But these humble beans also provide other important nutrients, including calcium, iron, potassium, and B vitamins.


Heart Disease

Heart disease is the leading killer of Americans, both men and women; about half of all deaths in the U.S. are caused by cardiovascular diseases. But heart disease is much less prevalent in Asia, where soy consumption is high. Does soy contribute to the difference?

Perhaps. Cross-cultural comparisons are tricky. In addition to soy, the Eastern diet also contains more fiber and fish but much less saturated fat and cholesterol than the Western diet. Still, soy has been a staple of the Asian diet for thousands of years, and it may help to protect Asians from heart disease. In fact, after reviewing more than 50 studies on soy and heart disease, the FDA has allowed food manufacturers to state that any food with at least 6–25 grams of soy protein per serving may help reduce the risk of heart disease when it’s part of a diet low in saturated fat and cholesterol. Soy may reduce cardiac risk in several ways:

Lowering cholesterol. It’s the primary cardiovascular benefit of soy. When researchers from the University of Kentucky analyzed 38 human trials of soy protein, they found that high-soy diets lowered total cholesterol by an average of 23 mg/dl (9%), LDL (“bad”) cholesterol by 22 mg/dl (13%), and triglycerides by 13 mg/dl (11%). More good news: People who needed the most help, those with the highest cholesterol levels, got the most benefit from soy. All in all, the improvement in cholesterol could reduce the risk of heart disease by 20%–30%.

Scientists don’t know just how soy protein lowers cholesterol levels, but they do know how much soy it takes to do the job. As little as 25 grams a day will help, but the optimal amount may be twice that, which is quite a lot.

Lowering cholesterol is crucial, but soy has other helpful properties as well.


Prostate Disease

Heart disease may be the greatest threat to American men, but for many, prostate cancer is the greatest worry.

Like heart disease, prostate cancer is much less prevalent in Asia than America. For example, a comparison of Chinese men living in China, Chinese men living in Australia, and Caucasian Australians found that the men living in China had the smallest prostates and the lowest prostate specific antigen (PSA) levels. But the explanation is not genetic, since the Chinese in Australia had results similar to those of non-Chinese Australians. Similarly, the prostate cancer death rate is 4.2 per 100,000 men in Japan versus 17.3 per 100,000 in America, but when Japanese men move to the U.S. and adopt a Western lifestyle, their risk of prostate cancer rises to the same levels that threaten other Americans.

The Asian and Western lifestyles are different in many respects; the much lower consumption of saturated fat in Asia may be the most important reason that the risk of prostate cancer is so much lower there (see HMHW, August 1996, May 1998, and March 2001). But the miso, tempeh, and other soy foods widely consumed in Asia may help.

Despite these interesting and hopeful observations, however, there is still no direct evidence that soy actually protects men against prostate cancer. Studies are in progress, but they will take years to complete.


Where’s the Soy?

In view of its many potential benefits, it’s no surprise that soy has moved from Asian restaurants and health food stores to mainstream supermarkets all over America. Whole soybeans can be boiled to serve as a vegetable or appetizer. Roasted soybeans can be eaten like nuts. Soy flour can be used in cooking and baking, where it can replace up to 25% of wheat flour. Soy milk is becoming popular as a substitute for cow’s milk, especially for cooking. Soy is now available in margarine and in soy butter, which resembles peanut butter; soy is also a common ingredient in artificial cheese. In addition, soy-based yogurt and nondairy desserts that taste like ice cream are increasingly popular. And soy protein is an excellent meat substitute, finding its way into prepared veggie burgers and franks as well as texturized vegetable protein and soy powder that can be used in home cooking.


Soy Powders and Pills

Ever eager for a quick fix, many Americans are turning to soy powders and isoflavone pills in the hope that they will prevent disease. This may not be a good idea. For one thing, whole foods have consistently outperformed extracts. In addition, dietary supplements are exempt from FDA standards that regulate the contents, purity, and efficacy of medications. Finally, some supplements provide enormous doses of isoflavones, often more than 10 times higher than the amounts found in the typical Asian diet. These megadoses could do more harm than good.


Is Soy Right for You?

It is. Even if the isoflavones in soy don’t live up to their promise in the fight against heart disease and prostate cancer, soy should help simply by replacing some of the animal protein and fat in the typical American diet.

Experiment with soy protein, soy flour, and soy milk at home. Look for prepared cereals, breads, frozen desserts, yogurt, and meat substitutes that contain soy protein. Once you find products that suit your taste, try to get up to 25 grams of soy protein a day. Although you may not say “soy-onara” to heart disease and prostate cancer, you will be taking a step in the right direction.