One of the most talked about foods in relation to health is the soybean; a little seed nestled inside of a hairy pod. It is packed with nutrients and is one of the few plant foods that contain all the essential amino acids of a complete protein crucial to sustaining human life. It is rich in omega-3 fats, polyunsaturated fat, B-vitamins, iron, zinc, antioxidants, phytochemicals and fiber. The soybean contains 40% protein and is considered an elegant rival to animal protein. It is highly valued source of protein for vegetarians. One cup of cooked soybeans provides half of the daily requirement of protein.
The other reason why soybeans have gained so much attention in the health community is their potential role in the prevention and control of disease, particularly cancer and heart disease. The idea partly came from observing other cultures around the world. For example, Asians who typically eat a diet rich in soy products, fish and fiber are known to have lower rates of breast cancer, high blood pressure and heart disease. Yet when they move and adopt a westernized diet stacked with meat and processed foods, their health profile is negatively impacted. This suggests that the health they enjoyed at home may not be due to genetics alone, but their diet as well.
This notion has set off a flurry of studies, thousands in fact, to see what health effects soy has on the human body. And what we have learned so far is soy can be a little two-faced: it can have conflicting positive and negative properties.
It's not clear why researchers are having trouble getting dependable findings. It could be inherent in the study; diet research is notoriously difficult to conduct and control. Or the fact that soy comes in many forms with varying components. Or the study population muddies the findings because people around the world have different personal and ethnic physiology.
Whatever the reasons, until science fully teases out the truth, medical and nutritional professionals find themselves choosing between the "soy harms vs. soy helps" camps, particularly when it comes to the effects on heart disease and breast cancer.
Soy became a popular approach to lowering lower LDL (bad) cholesterol, a risk factor for heart disease, when early research suggested that soy had an effect on cholesterol and other risk factors for heart disease. As the case with many initial studies, follow-up studies didn't cement the heart disease–reducing benefits of soy. In 2008, at the urging of the American Heart Association (AHA) and other health professionals, the Food and Drug Administration (FDA) was asked to reconsider the label they allowed manufacturers of soy products to display that touted soy as "heart healthy" pointing to the lack of strong and consistent scientific evidence.
Still, soy may lower LDL a few percentage points if people with unhealthy cholesterol levels (hyperlipidemia) eat more than half their daily intake of protein as soy to substitute for dairy and animal protein, which contains unhealthy saturated fat and cholesterol.
Among the many nutritional components in soy (and other legumes) are isoflavones, specifically genistein and daidzein. These plant-based compounds (phytoestrogens) are structurally similar to estrogen in humans, which means they can influence any tissue in the body that responds to estrogen.
The human body has had a long love-hate relationship with estrogen. Estrogen can keep bones and blood vessels strong and good cholesterol high, while waning estrogen has been associated with menopausal symptoms (hot flashes, memory problems), osteoporosis, cholesterol imbalances and heart disease. Combined estrogen and progestin hormone replacement therapy (HRT), once the universal remedy for postmenopausal women, has been demoted as a healthy option: Women who take HRT are at higher risk for heart disease, breast cancer and blood clots to the lungs. Estrogen can also fuel certain types of breast cancer.
How soy lands in the "harm" camp has to do with the protein receptors contained in and on the surface of many human cells, including cancer cells. Receptors have a particular affinity for certain molecules and are very finicky about what shaped molecule they will accept. It must be a fit, like a key fits a lock. The hook-up is important because it tells a cell how to behave. It can be a neurotransmitter chemical or a hormone such as estrogen, even a specially designed drug. Some "keys" will activate a cell's activity (agonist) and some will block one from occurring (antagonist).
Studies show soy can do both.
Cells can't distinguish soy molecules’ plant-derived estrogens from human estrogen because it has the same shaped key. Some cancer cells have estrogen receptors that fuel their growth. In fact, one treatment strategy for women with estrogen-positive breast cancer is to rid or block the body of any estrogen.
The theory holds that the phytoestrogens in soy may act similarly to human estrogen causing breast cancer cells to grow.
On the other side of the argument, some think that the plant estrogen could protect against breast cancer by hedging into the receptor slot in place of human estrogen, derailing estrogen's ability to fuel cancer growth.
A study published in the Journal of the American Medical Association (December 2009) tracked 5,000 women with breast cancer living in Shanghai, China and found that those who ate diets high in soy – more than 15 grams a day – had a 29% reduced risk of death and a 32% decrease in the risk of cancer recurrence.
The type of soy in Asian diets however is dissimilar to those of Americans, which tend to be processed soyfoods. Asians, mostly eat whole food tofu and fermented soy products that may contain many other important components. Asians eat 47 mg of isoflavones a day compared to the American's 1-6 mg.
Soy and Its Various Incarnations
There's the whole bean (edamame), and then there is a wide variety of soy products made from soybeans.
- Soymilk – the liquid residue of cooked soybeans
- Tofu – soft and firm blocks of coagulated soymilk
- Tempeh, miso, natto and soy sauce – fermented cooked soybeans
- Soy flour – defatted and finely ground flour
Some highly processed "frankensoy" products look and taste just like burgers, frankfurters, steak strips, cheese and other foods. Soy is also tucked into many products unannounced and unrecognizable as soy. It’s in baked goods, beverages, yogurt, nutrition bars, ice cream and crackers and other products.
Dr. Oz's Recommendation
Until the jury sets the record straight and says with confidence that dietary soy is indeed without risk, soy should be eaten in moderation as part of a healthy plant-based diet that also includes lots of fruits, vegetables, nuts, seeds and whole grains.
Soy protein is a still a great substitute for animal protein and dairy, which is high in saturated fat. As a general rule, women need to consume 46 grams of protein per day, while men need to 56 (roughly 0.80 grams of good quality protein/kg body weight/day).
So here's the bottom line…for now
- Limit soy to one serving a day (no more than 30 milligrams of isoflavones)
- Choose good quality soy such as tofu, tempeh and miso
- Skip the “frankensoy” processed soyfoods
- Avoid soy supplements made from isolated soy components such as isoflavones like genistein and daidzein