by John R. Lee, M.D.
If my mail is any indication, soy food products are becoming very popular in the US. And why wouldn't they? Soy contains the full complement of amino acids, can be made to fit with a low fat diet, and, as we all know, Japanese women who eat a lot of soy have less breast cancer than women in the U.S. Some proponents stretch the connection to credit soy with the renowned Japanese longevity advantage. Such claims, however, are grossly simplistic and probably misleading.What Do We Really Need to Know about Soybeans?
My experience as a U.S. Navy physician included 14 months in Japan and Okinawa back in the late 1950s. During that time I had extensive exposure to the traditional Japanese diet, which I found to be delicious. A typical meal consisted of wonderful vegetable or fish soup followed by five to six little dishes surrounding a bowl of rice along with green (un-roasted) tea, which is still my favorite tea. The little dishes contained small servings of seaweed, fish or shrimp, perhaps an egg, various vegetables (sometimes pickled), tasty noodles, and fermented soy products such as miso or tempeh, along with protein-rich soy curd, tofu. The alcohol consisted primarily of beer or tiny cups of sake. Vegetables, fish or shrimp were sauteed in a wok, using a small amount of nut oil. This was the traditional Japanese diet whose health benefits are widely extolled.
I visited Japan a few years ago and found that the traditional diet is a thing of the past. Young Japanese are now eating white bread instead of rice, drinking milk (with supplemental lactase, of course) and soda pop (there are vending machines for colas all over the place), whisky instead of sake, imported beef instead of fish, no seaweed, and a lot of processed food. Youngsters aged 10 are bigger and taller than their teachers, and the middle-aged men are getting prematurely bald, and dying of heart disease, hypertension, and cancer.
Beyond the Hype: A Close-Up on Soy
The bran (hulls) of all seeds and legumes contain substances called lignins, but none has the high lignin level found in soybeans. Soy lignins bind to minerals such as zinc and magnesium and prevent the body from absorbing them. This binding is so complete that scientists who wish to study the effects of low zinc concentrations in test animals merely add soy bran to the animals' diet. Zinc deficiency is known to impair the immune system and promote prostate disease. The traditional Japanese diet always included seaweed, which fortunately provides sufficient minerals to overcome the potential deficiency resulting from the effect of soy lignins. Our U.S. diet, on the other hand, lacks this potent source of minerals, so consuming too much soy in an American diet may lead to mineral deficiencies.
Soybeans also contain potentially healthful compounds called phytoestrogens (also known as isoflavones), such as genestein, diadzein and others that are weak-acting estrogens. Phytoestrogens occupy estrogen receptors and thereby reduce the effect of one's potentially more potent endogenous (made in the body) estrogens. This may well be the reason for the lower incidence of breast and prostate cancer in Japan. However, seen in context with the whole diet, large doses of these phytoestrogens are not as healthy as smaller doses combined with all the other factors found in the traditional Japanese diet.
Soybeans contain enzyme inhibitors that can block protein absorption as well as uptake of the enzyme trypsin, leading to thyroid deficiency and retarded growth. They also contain hemagglutinin that decreases the ability of red blood cells to properly absorb oxygen and distribute it throughout the body. These can be removed from soy products, but the usual attempts to do so (by pureeing the beans, soaking in an alkaline solution, and then cooking them in a pressure cooker) will, at the same time, make the beans proteins difficult to digest.
All Soy is Not the Same
In the U.S., the soybean industry is filling our shelves with soy derivatives such as soy flour, textured soy protein, partially hydrogenated soybean oil, and soy protein isolate — none of which were part of the traditional Japanese diet. They are found in soy cheese, milk, margarine, vegetable oils, burgers and hot dogs, baby formula, and flour, to name just a few. These soy derivatives have become a major (often-unrecognized) ingredient in fast foods and prepackaged frozen meals. They should not be confused with the natural and fermented soy components of the traditional Japanese diet.
Many people with wheat allergies will use soy products as a substitute. Unfortunately, when soy is a major component of the diet, soy allergy can develop.
I hope that it is obvious at this point that soy milk is not a good staple food for children. The last thing a small but rapidly growing body needs is a hefty dose of phytoestrogens and enzymes that block protein and mineral absorption.
Eat Soy Foods in Moderation
The traditional Japanese diet had, through centuries of trial and error, found ways to use soybeans in a healthy manner. They did not eat whole soybeans or soy protein isolate. They mainly ate fermented soy products. The fermentation process deactivates both trypsin inhibitors and hemagglutinin, while regular cooking does not. Soy was, in fact, only one component of a diet that included food rich with minerals, vitamins, and other essential nutrients.
If you plan to use soy, don t simply add the denatured soy products offered by American food mega-processors to the typically nutrition-deficient U.S. diet. Instead, my advice is to learn to prepare meals the Japanese way, with several components from their traditional diet. Single food items taken out of the context of the whole meal simply don't work. This, by the way, is true of many wonderful ethnic diets around the world.
Enjoy soy a few times a week but like all things, keep it in perspective. One of the simplest ways to eat soy is to stir fry tofu with fresh vegetables and some edible seaweed, put it on brown rice, and season with sesame seeds and tamari sauce. You can also add some aduki beans or fish for additional protein, and some freshly grated ginger to spice it up. Two recommended books on Japanese cooking are, Cooking with Japanese Foods: A Guide to the Traditional Natural Foods of Japan by John and Jan Bellame (Avery 1994), and The Folk Art of Japanese Country Cooking: A traditional diet for today s world, by Gaku and Gaki Homma and Emily Busch (North Atlantic Books, 1991).
This article was originally published in the John R. Lee, M.D. Medical Letter.