In the past, most Americans found tofu about as appetizingas, say, cardboard. Although traditional soy foods like tofu and miso are stillnot popular in the U.S., many new soy-based products are selling well, largelybecause of the perceived health benefits of soybeans. In fact, one-third ofAmericans now eat soy foods, spending more than $2.6 billion a year, mostly onsoymilk, soy burgers, soy dogs and other highly processed products.
Is all that soy doing us any good? More to the point, howmuch soy and what types of soy foods are best? And what about some not-so-rosysoy headlines that have cropped up lately? EN investigates what recentresearch says.
Soy Isoflavones vs. Soy Protein.Ironically, researchers still don’t know which of several soybean componentsprovide soy’s important health benefits, but the most studied ones includeisoflavones, protease inhibitors, phytates, phytosterols, saponins, phenolicacids, lecithin and omega-3 fatty acids.
“About 70% to 80% of soybeans? benefits can beattributed to isoflavones,” says James Anderson, M.D., a top soy researcherat the University of Kentucky in Lexington. As phytoestrogens (plant compoundswith estrogen-like activity), isoflavones are thought to block human estrogensthat may encourage the growth of hormone-sensitive cancers. However,phytoestrogens have been found to have both estrogenic and anti-estrogeniceffects. Researchers aren’t sure why, but it may depend on when in life youget them and in what quantity.
The naturally occurring isoflavones in soy includegenistein and daidzien?plus, to a lesser extent, glycitein. The amount ofthese isoflavones varies, depending on the variety of bean and the growingconditions.
Isoflavones seem to work in conjunction with soy proteinto reduce blood cholesterol, fight cancer and build bone, explains CherylSullivan, M.A., R.D., of the National Soybean Research Laboratory at theUniversity of Illinois in Urbana-Champaign. Recent research confirms that bothsoy protein and soy isoflavones provide greater benefits when combined than wheneither occurs alone. But just having both present at the party isn’t enough.Isoflavones attached to protein?as they are when they occur naturally, but notwhen they are added?Anderson explains, do the most good.
“Soy foods naturally contain about two milligrams ofisoflavones for every gram of soy protein,” says Anderson. What’sconfusing is that processed foods rich in soy protein are not always rich inisoflavones, because isoflavones are often lost during processing. Conversely,isoflavone supplements’sold to relieve menopausal symptoms or as bonesupplements?may not contain protein.
Soy and Heart Disease. In1999, the Food and Drug Administration approved a health claim that 25 grams ofsoy protein a day may reduce the risk of heart disease. But you?d have to eatabout four servings a day of foods rich in soy protein to meet this goal. Forsome, it may be worth the effort. Eating that much soy can lower yourcholesterol 5% to 6%, says Mark Messina, Ph.D., a soy expert at Loma LindaUniversity in California. Although soy protein alone will lower bloodcholesterol, soy protein plus isoflavones lowers it even more.
Soy may benefit the heart in other ways too, notesMessina, such as making blood vessels more flexible. He cites a study fromPennsylvania State University in which participants consumed 30 grams of soyprotein daily and saw a marked drop in blood pressure.
Soy and Cancer. In Asiancountries, where people consume a lot of soy, breast and prostate cancer aremuch less common than in places where they eat less soy. However, soyconsumption may coincide with other factors that influence differing cancerrates. In fact, several controlled studies in humans and animals showed littlereduction of cancer risk from soy.
Recent studies of soy and breast cancer have shown mixedresults. In studies where women were given soy isoflavone supplements, some datasuggest a decrease in breast density in postmenopausal women but not amongpremenopausal women. (Breast density is a marker for breast cancer risk.) Otherstudies suggest that exposure to soy during puberty may be what helps preventestrogen-dependent cancers later in life.
“By far, the most promising area for soy againstcancer is with prostate cancer,” says Messina. In a study of men withprostate cancer at the Cancer Institute at Wayne State University, those whoreceived 77 milligrams daily of soy isoflavone supplements appeared toexperience a stabilization of the cancer.
Soy and Bone Health. Somestudies have found a slight bone-building benefit from soy isoflavones amongpost-menopausal women, while other studies have found no effect. Soy may notreplace bone that’s lost, but it may slow or even halt further bone loss.
According to Messina, soy may offer bone protection simplybecause it is a vegetable protein. If you replace just 15 grams of animalprotein daily with 15 grams of soy protein, you will cut back on calciumexcretion by about 15 milligrams. This translates to needing 150 milligrams lesscalcium daily.
Soy and Menopause. Despitethe observation that women going through menopause in Asian countries?wheresoy is plentiful in the diet?report few hot flashes, clinical trials show soydoes not consistently alleviate symptoms like hot flashes, night sweats andvaginal dryness. Hormone replacement therapy (HRT) is far more effective forthat. Still, soy taken along with HRT may enhance the benefits of each.”Soy may especially benefit women with low circulating estrogen,” saysAnderson.
EN‘s Bottom Line.Despite all the good press, soy foods are not cure-alls. But they are healthyadditions to most people’s diets. That even includes people who have or havehad estrogen-dependent cancers, contends Messina, though EN cautions themto check with their doctors.
“But,” he adds, “there’s no reason to gooverboard and eat several servings of soy every day or load up on isoflavonesupplements.” Anderson advises striving to eat about two servings a day ofsoy foods (about 60 to 100 milligrams of isoflavones). There is probably nobenefit?and possibly a downside’to getting more than 100 milligrams ofisoflavones a day.
Of course, eating a varied diet rich in fruits,vegetables, whole grains?and soy?plus exercising regularly and maintainingideal weight, is the absolute best way to help prevent disease.
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