About Soy

REED MANGELS, PhD, RD
This issue's Nutrition Hotline reviews whether there are nutritional differences between canned and cooked beans and addresses the myths surrounding soy.

QUESTION: "I use canned beans most of the time because I just don't have time to cook beans and I never seem to remember to soak them ahead of time. What is the difference between canned and cooked beans, nutritionally speaking?"

ANSWER: There's not a big difference between canned and cooked beans. The effects of heating and cooking on nutrients would be similar whether the beans are cooked or canned. Folate appears to be lower in canned beans compared to cooked beans, although a cup of canned beans still provides close to 1/3 the folate RDA for adults. Certainly, sodium will be higher in canned beans compared to beans cooked without adding salt. You can reduce the sodium in canned beans by placing them in a strainer and rinsing them well with cold water. This has been shown to reduce the sodium by as much as 40 percent. Beans are a good source of fiber, protein, iron, and zinc. Don't avoid them just because you don't have time to cook them from scratch. Canned beans are an acceptable substitute.

QUESTION: "What's going on with soy? First I heard that eating soy would help with hot flashes and would be good for my heart, now I've heard that eating soy is dangerous. What should I believe?"

ANSWER: You're right to be confused. New results of scientific studies showing the benefits of soy products appear almost daily. Possible reasons to use soy products are to fight heart disease,1-4 promote stronger bones,5-7 reduce risk of some kinds of cancer,8-10 and to lose weight.11 However, a number of websites and brochures have appeared saying that soy isn't good for people at all. Reasons for avoiding soy are often loosely based on scientific studies, but a lot of times, the results have been twisted quite a bit to make the case that soy is harmful. In reality, soy is neither the cure for all of the chronic diseases that plague an affluent society, nor is it a food that should be avoided. Soy foods can certainly add variety to a vegetarian diet, and they do offer some health benefits, but they should be a part of the diet— not the foundation for it.

Soybeans contain relatively large amounts of isoflavones. Isoflavones are a type of phyto-estrogens, which are substances found in plants that have properties like the hormone estrogen. The levels of isoflavones in soy products are the most common reason for concerns about soy's effects on health. Recently a group of scientists from several different countries looked at more than 200 studies on soy safety and concluded that "the available scientific evidence supports the safety of isoflavones as typically consumed in diets based on soy, or containing soy products."8

If we look at the amount of soy isoflavones used in countries where soy is a regular part of the diet and where no harmful effects have been documented, perhaps this can give us some idea of a reasonable amount of soy. The average daily soy intake in Japan is about 65 grams per person,12 and the average isoflavone intake is about 20-32 milligrams per day.12-14 Higher intakes have been reported in China, where women's median isoflavone intake was 39 milligrams per day, and in Singapore, where the median intake was 35 milligrams per day.15-16 To find out the isoflavone level of your diet, use the USDA's isoflavone database,17 or look on packages of soy foods that you eat. Choosing 2-3 servings of soy per day will generally lead to an isoflavone intake similar to that seen in countries where soy is a regular part of the diet.

Here are some common areas of concern:

Soy and Infants, Children, Pregnancy
During pregnancy, isoflavones from the mother's diet appear to be passed on to the fetus. High levels of isoflavones were found in healthy Japanese infants whose mothers also had high blood levels of isoflavones, probably due to high intakes of soy.18 These levels of isoflavones have not been associated with any health problems in infants. One report has found that a birth defect of the penis called hypospadias occurred more frequently in infants whose mothers followed a vegetarian diet during pregnancy.19 Although some have attributed these results to use of soy, there was no significant association between use of soymilk and other soy products and development of hypospadias.19 Isoflavones from the mother's diet also appear in breast milk, although the daily isoflavone intake of breastfed infants remains negligible,20 even when breast milk levels are increased as much as tenfold by the mother's use of soy foods.21 At this point there is no scientific evidence of a need to avoid soy foods in pregnancy or during breastfeeding; 2-3 servings a day is a reasonable amount.

Ideally, all infants would be breast-fed. There are circumstances, however, where soy formula is the next-best alternative. Researchers have concluded that use of soy formula appears to have no effect on fertility, miscarriage rate, birth defects in offspring, or maturation.22 Based on the results of this study, and calculating isoflavone intake on a body weight basis, and assuming that older children absorb and metabolize soy isoflavones similarly to infants, a daily soy intake of 2-3 servings per day appears reasonable for children.

Children can use soy products as a part of a healthy diet. Sometimes, perhaps because of convenience, or perhaps because they look like what other children are eating, vegetarian children become over-dependent on soy products like veggie burgers, veggie dogs, and veggie deli slices. Using seitan or bean burgers, peanut butter sandwiches, or other foods can encourage some dietary variety rather than focusing strictly on soy.

Soy and Breast Cancer
Concerns have been raised about soy food consumption increasing the risk of breast cancer because of soy's phytoestrogen content. However, soy does not appear to have an estrogen-like effect on breast tissue, suggesting that it does not increase breast cancer risk.23 The results of a recent large study of hormone-replacement therapy (HRT) in menopause found that the combination of estrogen and progestin, rather than estrogen alone, increased risk of breast cancer.24 Since soy does not have progestin activity, it should not increase risk of breast cancer.25

Perhaps use of soy foods early in life is protective for breast cancer. One study has shown that Chinese women who had higher intakes of soy foods during adolescence had lower risk of breast cancer as adults.10 Several studies have suggested that the use of soy foods leads to a longer menstrual cycle, which may reduce risk of breast cancer,26-27 although not all studies show an effect on menstrual cycle length.28-31

Soy and Brain Function
Concerns have been raised about soy's effect on brain function based on one study that found an association between eating tofu regularly and dementia in Japanese-American men.32 Other research has contradicted these findings, and the original study's design has been questioned.33-35

Soy and Reproduction
Soy's phytoestrogen content has also led to concerns that soy will have a harmful effect on the ability to reproduce. Certainly, there have been no reports of widespread problems with reproduction in countries like China and Japan, where soy is a regular part of the diet. Other evidence that soy does not impair reproductive performance includes a study showing that two months of soy supplements had no effect on semen quality in men,36 several studies showing that ovulation occurs in women using soy products,8 and a study that found that soy formula had no effect on fertility. 22

Soy and Thyroid
What about thyroid function? As I write, I'm looking at a pamphlet that says, "Soy foods can cause thyroid problems." There's a bit of truth to this statement. Soy can be a problem for people whose diets don't contain enough iodine.8 By using iodized salt, sea vegetables, and other sources of iodine, any detrimental effects of soy on thyroid function can be minimized. In clinical trials where soy is added to people's diets, no harmful effects on thyroid function have been seen.8 One study has suggested that a diet containing higher levels of phytoestrogens is actually associated with a reduced risk of thyroid cancer in women.37

Soy Foods as a Protein Source
Some questions have been raised about the quality of soy protein and whether or not it supplies all of the amino acids that we need. The World Health Organization has identified soy as a high quality protein that can meet all of the essential amino acid requirements of humans.38 In addition, the newest protein recommendations state that soy is a high quality protein, equivalent to animal protein.39

Soy and Mineral Absorption
"Soy foods have high levels of phytic acid that reduce absorption of calcium, iron, and zinc," say soy's critics. Again, there is some truth to this statement. Soy products do contain high levels of phytic acid, which can reduce absorption of some minerals. However, this statement is not entirely true. Calcium is reasonably well-absorbed from soy products. For example, the calcium in fortified soymilk is absorbed about 75% as well as the calcium in cow's milk is.40 In addition, soy isoflavones have been shown to promote bone health.5-7 The phytates in soy products do appear to inhibit iron absorption, although this effect can be overcome, at least partially, by eating a good source of vitamin C along with the soy product.41-42 A recent study also suggests that soybeans can be a good source of iron for people who are marginally iron-deficient.43 However, zinc is poorly absorbed from soy foods. This alone is not a reason to avoid soy foods; rather, other sources of zinc, such as nuts, seeds, whole grains, and fortified foods, should be eaten daily.

The results of the most recent research suggest that it is all right to include soy as a part of a healthy diet, and that, in fact, there are some health advantages to using some soy products. A reasonable amount of soy for most people seems to be about 2-3 servings daily.

References
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The Vegetarian Journal published here is not the complete issue, but these are excerpts from the published magazine. Anyone who wishes to see everything should subscribe to the magazine.