MD Anderson credits its protein and fiber. Mayo Clinic recommends its service to brain function. Yet plenty of nutritionists have concerns. As my friend said, “So true about soy. One day it’s ‘miracle food’ and the next it’s ‘too much will kill you.’”

Soy and its derivative foods. Why are they such a subject of uncertainty? What is soy and how should I consider its benefits and drawbacks?

Some backstory. The domestication of soybeans began 3,000 years ago in China, mainly as a fertilizer. Like beans, lentils and other legumes, they initially absorbed nitrogen in the air and transferred it to the soil. One thousand years later, soy became food when fermentation techniques were discovered, and then spread into Japan, Korea and other parts of Southeast Asia. Farmers appreciated the relative ease of soybean culture and the healthy property of the beans.

Today, soybeans and food derivatives are a staple of Asian cultures: soy sauce, edamame, miso, dry roasted soybeans, tempeh, tonya (soy milk), natto (which I rarely see) and perhaps the most common soy item to western home cooks, tofu.

Unsurprisingly, you can easily find research of soy foods providing positive health. According to a major Japanese study over 25 years, the elders of Japan’s Okinawa are among the healthiest and longest-lived people on the planet. The researchers conclude that high soy consumption is one of the main reasons that Okinawans are at extremely low risk for hormone-dependent cancers, including the breast, prostate, ovaries and colon. In fact, the Okinawans who consume the most soy deliver the world’s lowest cancer rates.

Occasionally I use soy sauce at home and eat soy foods in my neighborhood Japanese and Chinese restaurants, but my most common soy item is tofu. It is made from soybean liquid — called soy milk — that has been curdled and turned solid. Often used as a meat substitute, tofu is pretty plain and boring from a cooking perspective. It needs spices and sauces for taste, nearly always vegetables, and sometimes a little meat.

Health-wise, tofu and the soy from which it originates is often touted as an aid for reducing high cholesterol and the bone loss associated with osteoporosis, and improving the symptoms of menopause. Tofu is also high in protein and contains essential amino acids to help create protein.

soy-protein-cholesterol-dietIn fact, I’ve read researchers include soy (along with green tea and turmeric) as some of the highest levels of anticancer compounds found in nature. The technical discussion is this: soybeans are a rare source of isoflavones, and isoflavones are a class of polyphenols, a principal phytochemical compound — in other words, a dietary supplement. Polyphenols exist in many other plants and vegetables, but only in soybeans are they large enough to be of real nutritional value.

And yet I read no further than the second paragraphs of Wikipedia’s entry for isoflavones for an opposing view:

Isoflavones (and closely related phytoestrogens) have grown popular as dietary supplements, but there are few studies showing any benefits from these compounds, and their use is viewed within the scientific and medical community as pseudoscience. Some studies have also identified significant risks from isoflavones.

Specifically, this risk claim ties isoflavones and breast cancer. Isoflavones are thought to disrupt hormonal function and raise levels of estrogen in the blood. But others have noted that moderate amounts of soy foods (up to 1 – 2 servings a day) do not increase breast cancer risk or risk of recurrence.

A different and more critical view of soy-derived products notes that soybeans, wheat and corn dominate processed foods and shrink the biological diversity of the human diet. Their low cost and spread were driven by Richard Nixon in the 1970s in reaction to sharp food prices in other areas, such as animal fats. As a result we now find many, many listings of soybean ingredients on processed goods labels, all of which offend critics, including me, for their commonality. Response? Build the greatest diversity we can eat to ensure we cover all our nutritional bases.

My personal conclusion from this reading is that tofu and soy in general is a good protein source as part of a healthful diet when varied with other proteins — eggs, lean meat and poultry, fish, beans, etc. — so I’m not eating an overload of any one thing. Moderation is key. By the cleanest you can purchase — organically labeled if available. Many cultures have safely consumed tofu for centuries, but traditionally ate in small portions. Some modern vegetarians in particular may eat far more and perhaps too much.

In my view, while every cancer is different, we have little solid evidence that soy promotes or reduces cancer risk. But to be certain, don’t eat it every day. Unless you are an Okinawan.