Asians who eat a lot of soy have lower incidence of certain cancers, most notably breast cancer and prostate cancer. Are these two factors causative or are they mere associations? In other words, I sing in the shower every morning and the sun rises. Does my singing cause the sun to rise or are these two factors merely happening at the same time? You get the picture. So scientists first looked to see if there was a causative factor. And they found two components of soybeans with anticancer activity. The first are phytoestrogens - a group of compounds (e.g. genistein and daidzein) that have estrogen-like activity in humans and animals.
Question. Why would a plant develop chemicals that affect the hormone levels of animals?
Answer: Protection. Remember the name of the game? (all together) survival. And if you're a soybean plant, survival depends on not getting eaten by bugs or animals. So soybeans evolved the ability to create estrogen-like hormones that mess up the reproductive cycle of such would-be soybean eaters.
Uh oh, I feel a digression coming on... Do I resist it, or go with it? Let's go with it. It's a short quiz. Now I get to ask you a question:
Q: What chemical prized by the majority of American adults, is produced by more than 80 plant species to dissuade predators?
Hint. This chemical gives the leaves a bitter taste. If the animal or insect persists in eating the plant, this chemical acts as a neurotoxin to kill or damage the predator.
But back to soybeans. The plant estrogens in soybeans have very weak activity in humans. But since they bind to estrogen receptors in a woman's breasts and reproductive organs, it is believed that they block the binding of more powerful estrogens that she may be producing which promote cancer. In men, a similar benefit may be achieved in the reduction of prostate cancer where estrogen plays a contributing role. Very interesting.
Scientists also found a group of protease inhibitors in soy and other beans. These biochemicals inhibit the digestion of protein by interfering with the activity of two important enzymes, trypsin and chymotrypsin. Once again, this is a survival mechanism. Insects and worms that eat protease inhibitors don't live very long. Protease inhibitors interfere with cell communication, protein metabolism and cell growth. Not surprisingly, this also produces an anticancer effect in humans by interfering with the growth and spread (metastisis) of tumors.
So we now have a documented cause and effect relationship between soybeans and two anticancer biochemicals. What do we do with this valuable information?
Option one: We promote soy as the perfect food for all human beings, including infants.
Option two: We carefully evaluate the effects of phytoestrogens on pregnancy and fetal health. We evaluate the effects of phytoestrogens and protease inhibitors on the growth of children, and we look for possible side effects that may appear elsewhere resulting from decreased estrogen binding. Estrogen, for example plays an important role in the maintenance and repair of the brain.
Right now, unfortunately, Americans (ya gotta love 'em) are pursuing option one with wild abandon. Fueled by the burgeoning soybean industry and supported by thousands of health-food enthusiasts (motto: where's the next panacea?) the soy frenzy has extended to the mass marketing of soy milk, soy protein powder, soy cheese, soy burgers, soy candy bars, soy butter (to replace peanut butter) and soy-enriched cereal, bread, pasta and chips. Concentrates of soy phytoestrogens are sold in capsules and the FDA now allows manufacturers to claim heart disease prevention benefits for any product providing 25 grams of soy protein per day.
This could be a big problem.
Excessive soy consumption may inhibit brain repair functions. The basis for this concern arose when researchers documented a dampening effect of phytoestrogens on brain repair in rats. Human studies appear to support this finding. A study with Japanese-Americans found a disturbing correlation between soy consumption and cognitive impairment. I am not suggesting that soy=brain degeneration, but the issue certainly needs further study.
What doesn't need further study is the association between high soy consumption and growth and development. Here, the concern is that soy phytoestrogens may cause early puberty in girls and delayed physical maturation in boys. Mary G. Enig, Ph.D., president of the Maryland Nutritionists Association states that the amount of phytoestrogens in a day's worth of soy infant formula has the same estrogenic effect as 5 birth-control pills to an adult. A study published in the British medical journal, Lancet found that infants fed soy formula had levels of phytoestrogens that were 13000-22000 times higher than natural estrogen concentrations in early life.
- The protease inhibitors found in soy foods are also a concern, in that they may inhibit normal growth and repair functions in children. At first the soy industry claimed that these inhibitors were destroyed by cooking, but that has been disproved. Protease inhibitors have been shown to survive cooking and processing to a small but significant degree. Certainly significant for a small child.
- The Thyroid Factor may also create problems for children and adults who consume high amounts of soy foods. High doses of the phytoestrogens genistein and daidzein have been found to interfere with thyroid function, and while this would have no effect on a person eating a varied diet, those using soy as their primary source of protein may suffer, even to the point of thyroid disease.
It is natural to jump on bandwagons. Nutrition is extremely complex and we all would love a simple solution to the deeply felt threats of heart disease and cancer. But I'd like to suggest that in regards to soy foods as well as in all things, the key is balance.
- If you eat a lot of meat, you can do yourself and the planet a huge favor by switching to vegetarian proteins at least a few days a week. That's not just soy foods, but any combination of beans, grains, low-fat dairy foods, nuts and seeds.
- If you are pregnant or nursing, it is probably not a good idea to make soy your sole source of protein.
- I would seriously reconsider using soy formula for babies. If you absolutely cannot breast feed, consult with a qualified health professional to work out a rotation strategy utilizing soy milk, goat's milk, almond milk and rice milk.
- If you have growing children, make sure they have a variety of proteins in their diet. Soy is fine, but if it is their sole protein, you may be affecting their future career choices by limiting their adult height. (So who wants to be a basketball star?)
- Regarding soy and brain health, you'll have to stay tuned. The recent reports are sure to spark follow-up research.