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
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 dietó 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
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
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.
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metabolic products inhibit in vitro lipoprotein oxidation in serum. J Nutr
Scheiber MD, Liu JH, Subbiah MT, et al. 2001. Dietary inclusion of whole soy
foods results in significant reductions in clinical risk factors for
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Lichtenstein AH. 2001. Got soy? Am J Clin Nutr 73:667-8.
Lydeking-Olsen E, Jensen JB, Setchell KDR, et al. 2002. Isoflavone-rich soymilk
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Arjmandi BH, Khalil DA, Smith BJ, et al. 2003. Soy protein has a greater effect
on bone in postmenopausal women not on hormone replacement therapy, as evidenced
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Shu XO, Jin F, Dai Q, et al. 2001. Soyfood intake during adolescence and
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Nagata C. 2000. Ecological study of the association between soy protein intake
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Nagata C, Kabuto M, Kurisu Y, Shimizu H. 1997. Decreased serum estradiol
concentration associated with high dietary intake of soy products in
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North K, Golding J, The ALSPAC Study Team. 2000. A maternal vegetarian diet in
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Setchell KDR, Zimmer-Nechemias L, Cai J, Heubi J. 1997. Exposure of infants to
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Strom BL, Schinnar R, Ziegler EE, et al. 2001. Exposure to soy-based formula in
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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.