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VICUS.COM (27 June 2000) -- Isoflavones,
one of the families of phytoestrogens,
appear to make a positive contribution to health, and soy foods offer
the best source of isoflavones.
Soy also provides high-quality protein, as well as being low in saturated
fat, and is a good source of essential fatty acids.
Although there is no recommended daily intake for soy or
isoflavones, for those who want to increase their consumption of
soy-containing foods, here are some guidelines to consider.
How much is enough?
A summary of the daily amounts of soy and isoflavones that have or
are being studied to prevent a range of diseases is provided in Table
1.
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Table 1. Daily amounts of soy and
isoflavones studied or being studied to prevent a range of
diseases.
| Objective |
Daily dose |
| Lowering cholesterol |
25-50 grams of soy protein |
| Cancer (breast, prostate, colon)
prevention |
20-40 grams of soy protein |
| Hot flashes, reduction |
45 g of soy flour/day
80-160 mg of isoflavones |
| Osteoporosis,
post-menopausal women |
40 grams of soy protein per day
containing 90 mg total isoflavones/day for six months
had a positive effect on bone density |
Source: University of Illinois at
Urbana-Champaign, College of Agricultural, Consumer and
Environmental Sciences
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Sources of soy and isoflavones
Isoflavones are available in tablets, but these are not recommended
as the primary source of phytoestrogens. This is because it is not
known if the beneficial effects of isoflavones are due to the body's
response to isoflavones alone or a combined effect of isoflavones with
other nutrients.
For example, in addition to isoflavones, soybeans contain protease
inhibitors, omega-3
fatty acids and phytosterols, all of which are absent from
isoflavones tablets. In addition, soybeans contain other beneficial
nutrients such as iron, protein and calcium,
as well as non-nutrients such as fiber.
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What’s so special
about isoflavones?
Although scientists have known about the
existence of isoflavones for 50 years, only a handful of
scientific papers were published on these phytochemicals until
the early 1990s. In the past five years, more than 300 papers
have been published annually on isoflavones. The proliferation
is attributed to a 1990 National Cancer Institute decision to
allocate nearly $3 million to study the anti-cancer effects of
isoflavones. As interest in the potential anti-cancer effects
of isoflavones grew, researchers began to look at effects on
other diseases, including heart, disease, osteoporosis
and hypertension.
Source: Archer Daniels Midland
Company
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Most manufacturers do not list the isoflavone content of their
packaged foods, but here is a partial list (Table
2) developed through a collaborative effort between the U.S.
Department of Agriculture and the Department of Food Science and Human
Nutrition of Iowa State University (Ames, Iowa).
The USDA/Iowa State database reflects the isoflavone content of
foods produced in 1999. The year and place where the food was grown
affects the isoflavone content of soybeans as well as soy foods.
However, this list and the figures that follow provide guidelines
for selecting foods that are rich in isoflavones.
All soybeans are not equal ... in isoflavones
The average concentration of isoflavones varies according to the
source. For example, Taiwan soybeans have only about 40% of the
isoflavones found in soybeans from Korea, the richest source of
isoflavones from soybeans (figure1). Soy flour is even richer in isoflavones
(figure 2). Full fat, roasted soy flour has almost 200 mg of isoflavones per 100 grams compared with defatted soy flour, which has
about 40% fewer isoflavones for the same weight.
Eating soybean seeds as a snack, available in markets and health
food stores as soy nuts, is a convenient way to increase soy intake.
They can be eaten in place of peanuts (contain 0.26 mg
isoflavones/100g) or dried sunflower seed kernels (contain no
isoflavones). The concentration of isoflavones in soybean seeds varies
(from 12 to 153 mg/100g), depending on the maturity and processing of
the seed (figure4).
Other good sources of isoflavones are beans, although compared with
soybeans, their isoflavone content is rather low (figure
3).
Some food processors have attempted to capitalize on the growing
interest in soy and isoflavones. The five examples in (figure
5).Figure 5 reveal the wide differences in isoflavone content of
franks, links, patties, nuggets and adult formula.
A strategy for increasing dietary isoflavones
In order to reach the daily amounts of isoflavones that researchers
suggest are needed for a positive impact on health, it is logical to
concentrate on soy-based foods. This might include baking with soy
flour (up to 177.9 mg isoflavone/100g), drinking a soy beverage (109.5
mg/100g), or eating defatted soybean flakes (125.8 mg/100g). Other
good sources of isoflavones include tofu,
such as Mori-Nu silken firm (27.9 mg/100g) or dried-frozen (67.5
mg/100g), and tempeh, either cooked (53.0 mg/100g) or burgers (29.0
mg/100g).
Not all beans contain isoflavones. Those that do contain modest
concentrations of isoflavones and should be eaten to supplement an
isoflavone-rich diet. Navy, pinto, red, fava, garbanzo and small white
beans contain 0.1 to 0.7 mg/100g.
For people raised on meat and potatoes, it may require significant
(often pleasant) changes in cuisine to achieve the intake reported in
clinical studies, including eating miso, tofu and natto.
Contrary to popular belief, soya sauce (shoyu), the most important
Japanese condiment, is a relatively poor source of isoflavones (1.6
mg/100g).
John
Russo Jr., PharmD ,
is senior vice president of medical
communications at Vicus.com. He is a pharmacist and medical writer
with more than 20 years of experience in medical education.
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