Vegan for Life - Jack Norris [80]
There is no doubt about it: Both the traditional and the more modern soyfoods have made it easier than ever to be vegan. And aside from their practical benefits, soyfoods offer some unique health advantages—but there have also been questions about their safety.
It’s no small topic: Approximately 2,000 soy-related papers appear in medical and scientific journals every year. This chapter, which looks at both the potential benefits of soy as well as some of the more controversial issues, is meant to help clarify the findings so you can make an informed decision about how these foods fit into your diet.
SOY NUTRITION
Soybeans are unique among legumes. They’re higher in protein and fat and lower in carbohydrate than other beans. While much of the fat is the polyunsaturated omega-6 type, soybeans are one of the few good plant sources of omega-3 fatty acids. The carbohydrate in soybeans is composed largely of oligosaccharides, which are sugars that stimulate the growth of healthy bacteria.
The soybean’s claim to fame, though, is its protein content. Soy protein is highly digestible and its amino acid pattern closely matches human requirements. It is considered comparable to proteins from animal foods and, according to the protein rating system that ranks proteins based on their amino acid pattern and digestibility, is the most highly rated of all plant proteins.1 Soybeans are also good sources of iron, potassium, folate, and sometimes calcium. Although they contain absorption inhibitors like phytate and oxalate, iron2 and calcium 3,4 are both easily absorbed from soyfoods. In fact, the iron in soy is present in a form called ferritin, which makes these foods somewhat unique.5 Preliminary research suggests that ferritin iron may be very well absorbed. The calcium in fortified soymilk is absorbed as easily as calcium from cow’s milk.6,7
Soy Isoflavones
Soybeans are the only commonly consumed food that contains nutritionally relevant amounts of isoflavones. These are members of a larger group of compounds called phytoestrogens, or plant estrogens.
Isoflavones bind to the same receptors in the body—a necessary step for biological action—that bind the hormone estrogen. This has led to one of the biggest misconceptions about isoflavones—namely, that they are the same as estrogen. They’re not. Instead isoflavones are among a group of complex compounds called SERMS, or selective estrogen receptor modulators.8
It’s the word “selective” that describes how different isoflavones are from estrogen. There are two types of estrogen receptors in cells, and estrogen binds equally to both of them. But isoflavones preferentially bind to one type of estrogen receptor and, as a result, they can act very differently from estrogen in some parts of the body. Depending upon which type of receptor dominates in a given tissue, SERMs can have estrogen-like effects—or anti-estrogenic effects or no effects at all.
Isoflavones are natural SERMS, but some drugs used to treat cancer and osteoporosis are also SERMS. For example, the osteoporosis drug raloxifene has estrogen-like effects on bone and possibly on LDL-cholesterol, two areas in which estrogen is protective and beneficial. But raloxifene may have anti-estrogenic effects in the breast, thereby reducing breast cancer risk.9
One issue regarding isoflavones may be of particular interest to vegetarians and vegans. The way in which isoflavones are metabolized can differ significantly among individuals and this could impact their health effects. For example, one type of isoflavone is metabolized by intestinal bacteria to a compound called equol, which may be beneficial to health. But only around 25 percent of westerners have equol-producing bacteria in their intestines, compared to roughly 50 percent of Asians.10 Interestingly, a small study found that vegetarians are more likely than meat-eaters to be equol producers.11 Therefore, vegetarians and vegans may stand to gain more from consuming soyfoods than those eating a more typical American diet.
SOY