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The Healthful Versatility of Soy

By Yousry Naguib, PhD.

Vitamin Retailer magazine, 12/7/2002


Soy has become a hot nutritional supplement following the
announcement in July 2002 by the National Institutes of
Health that it had called off a study of a hormone-replacement
therapy drug used by millions of menopausal women. This
drug was unexpectedly found to increase the risk of heart

disease, stroke and breast cancer. This news is prompting
women to seek natural alternative therapy, including soy
supplementation.

In 1999, the U.S. Food and Drug Administration (FDA)
announced that eating 25 grams of soy protein a day lowers
the risk of coronary heart disease, and ruled that any food
containing at least 6.25 grams of soy protein per serving can

carry a healthy-heart label.

Soy contains high levels of the isoflavones, also known as
phytoestrogens: genistein and daidzein, and their glycosides
(with a sugar moiety) genistin and daidzin. These
phytoestrogens are believed to be the active components that
ease the symptoms of menopause: night sweats, hot flashes,
and thinning bones. Soy has also been indicated in various

studies to combat cancer, diabetes, osteoporosis,
gastrointestinal disorders, and cardiovascular diseases.

Bioavailability
In one human study, ingestion of 40 grams of isoflavone -rich
vegetables for a period of five days raised the amount of
isoflavones in the urine 1,000 times.

In a recent study, researchers reported that the bioavailability
of isoflavones glycosides requires first the hydrolysis of the
sugar moiety by intestinal beta-glucosidase enzymes to the
corresponding sugar free form (aglycon) before uptake to the
peripheral circulation.

Fermented soy foods, such as miso and natto, contain mostly
the better-absorbed genistein, which is produced during

fermentation by the natural microbes that act as fermenting
agents and also cleave genistin's sugar moiety, converting it
to genistein.

Natto is produced by adding the bacteria Bacillus natto to
cooked soybeans, and contains five times more genistein than
either tofu or soy milk.

Cardiovascular Health

The National Cholesterol Education Program Adult Treatment
Panel recommends reductions in saturated fats and
cholesterol and therapeutic dietary options for lowering LDL -
cholesterol, with inclusion of plant stanols/sterols (2 g per day)
and increased soluble fiber (10 to 25 g daily), in addition to
foods with cardio-protective effects such as soy, nuts,
legumes, and garlic. It has long been recognized that rates of
coronary heart disease are lower in Japan, where soy

consumption is common, than in Western countries.

The health benefit of soy products in cardiovascular health
stems from their ability to lower total cholesterol and low-
density lipoprotein (bad cholesterol) levels in serum, reduce
their susceptibility to oxidation, and inhibit platelet
aggregation.

Compared with soy protein from which most of the phyto-
estrogens have been extracted, soy protein with intact phyto-
estrogens increases HDL -cholesterol but does not influence
LDL-cholesterol. In a randomized crossover trial, 22 healthy
young normal-lipidemic subjects (five men and 17 women)
consumed diets providing 56 or 2 mg isoflavones per day for
17 days. Plasma isoflavone concentrations were significantly
greater after the high isoflavones diet than after the low
isoflavones diet.


In another study, 41 hyper-lipidemic men and post-
menopausal women received either a low-fat control diet or a
soy diet high in isoflavones (73 mg) or a soy diet low in
isoflavones (10 mg) daily for one month. Compared with the
control diet, both soy diets resulted in significantly lower
cholesterol and cardiovascular disease risk, but no significant
differences were seen between the high- and low-isoflavone

soy diets.

Women's Health
Currently, there is an interest among postmenopausal women
to treat problems associated with menopause, such as hot
flashes, headaches, and joint pain, with natural products as
opposed to synthetic hormones. Indeed, clinical research
showed isoflavones reduced the incidence of hot flashes in

menopausal women. These compounds may be designated
as healthy alternatives to hormone replacement therapy.

Soy contains bioactive isoflavone compounds, called phyto-

estrogens, which mimic estrogen and are thus capable of
alleviating postmenopausal symptoms. The anti-estrogenic
effects of these isoflavones are ascribed to their ability to bind
estrogen receptor sites thereby making the receptors
unavailable for binding by the natural, more potent estrogens

of the body.

The anti-estrogenic effect of genistein in reducing menopausal
flashes was demonstrated in a 12-week, double-blind,
placebo-controlled, randomized study involving 104 women.
Those who received a daily diet containing 76 mg of soy
isoflavones providing 40 mg genistein experienced a 45
percent reduction of hot flashes. The study concluded that soy

protein when added to a daily diet can substantially reduce the
frequency of hot flashes in postmenopausal women.

A Japanese study of 284 women aged 40 to 59 years showed
that consumption of fermented soy products significantly
reduced hot flashes as compared to the control group.

In a double-blind, randomized study, postmenopausal women

given 400 mg per day of a standardized soy extract,
corresponding to 50 mg per day of isoflavones for six weeks,
showed a significant reduction in the number of hot flashes
per week. The study concluded that the particular soy
compound is safe and efficacious in relieving hot flashes.

Cancer Prevention
Epidemiological studies have indicated that high dietary intake
of soybeans by Chinese and Japanese is associated with

lower incidence of cancers, particularly breast and prostate.
The incidence rates of these types of cancers is higher in
Western countries, where consumption of soy foods is low as
compared to Eastern countries. Asians consume about 10 to
35 g of soy foods daily per capita, which translates to a daily
dose of 25 to 100 mg isoflavones.

On the other hand, Western populations ingest only a few

milligrams or less of isoflavones per day. A recent survey
showed the average intake of total isoflavones to be 15
mg/day, of which 7 mg is genistein, 6 mg is daidzein, and 2
mg is glycitein.

The consumption of soy has also been linked to the
prevention of colorectal, lung, and gastric cancers. The
compounds genistein, daidzein, and lignans in soy products

have been shown to exhibit anticancer effects. Genistein has
been recognized as the most potent of these compounds in
inhibiting various processes in the development of cancer
including cell proliferation and transformation, and to inhibit

the process of angiogenesis (the process of growth of new
blood vessels into tissue). Genistein has also been shown to
decrease the amount of reactive oxygen species produced by
tumor cells.

In a recent study, postmenopausal women received a soy diet
with either 73 mg or 10 mg isoflavones daily for one month.
The women on a high isoflavone soy diet showed increased
serum concentrations of pro-inflammatory cytokine interleukin-
6 (IL-6). This finding indicates that soy isoflavones may
enhance the immune response, and hence may explain the
lower incidence of certain cancers in population with high soy
intakes.


Because of the similar chemical structure of these isoflavones
and the hormone estradiol, it was hypothesized that these
compounds may be related to the risk of breast cancer.
However, a recent study showed that individuals with the
highest levels of daidzein, glycitein and total isoflavonoids in
their urine samples had about half the cancer risk than did
those with the lowest isoflavone levels. The study concluded

that high intake of soy foods may reduce the risk of breast
cancer.

Clinical studies also appear to support these epidemiological
observations. In a case-control study, women with newly
diagnosed early breast cancer with a high intake of
isoflavones had a substantial reduction in breast cancer risk.

A recent review published in Journal of Nutrition also reported

that isoflavone supplements do not affect breast tissue density
in pre-menopausal women and may decrease density in
postmenopausal women. These effects are opposite of those
of hormone replacement therapy. The study suggested that
breast cancer patients might enjoy soy products.

In a recent study, pre-menopausal women free of breast
cancer were randomly assigned to receive either a dietary

supplement of isoflavones (40 mg per day) or a placebo for 12
weeks. The hormone estradiol and its metabolite estrone
(anticipated to be a carcinogen) decreased significantly in the
isoflavone group as compared to the placebo. The study
suggested that increased isoflavone intake affects estrogen
metabolism, thereby demonstrating a potential to reduce the
risk of breast cancer.

Bone Health
Soy isoflavones have also been found to promote healthy
bones. Although calcium is the primary dietary factor that
dictates the course of osteoporosis, other variables, including

estrogen deficiency, exercise and protein composition of the
diet, influence the overall maintenance of healthy bones.
Diminished levels of estrogens due to menopause, lack of
exercise, and excessive protein intake all exert deleterious
effects on bone integrity.


To reduce these negative effects, the use of soy protein in
combination with calcium has been suggested as an
advantageous way to promote calcium uptake and bone
strength. An animal study concluded that supplementation
with genistein prevents the tearing down of bone, while
simultaneously encouraging bone mineralization.

Preliminary human studies show that isoflavones are able to
increase bone density in postmenopausal women.
Menopausal hormone decline contributes significantly to the
risk of osteoporosis. Hormone replacement therapy (HRT)
with estrogen or a combination of estrogen and progestins has
been recommended for treating osteoporosis.

In recent years, isoflavones have emerged as a natural

alternative to HRT, for treating osteoporosis. Genistein is the
main isoflavone present in soy that appears to bind to the
estrogen receptor, and in animal studies it appears to be
effective in preventing bone loss caused by estrogen
deficiency.

In a recent study, 23 healthy postmenopausal women were
randomly assigned to receive either 62 mg isoflavones or
placebo daily for four weeks. Both excretion of bone loss

markers, and total serum cholesterol and LDL -cholesterol (bad
cholesterol) were reduced significantly in the isoflavones
group. The study concluded that soy isoflavone
supplementation has the potential to reduce the risk of
postmenopausal osteoporosis and cardiovascular disease in
women.

Safety

The safety of purified isoflavones (genistein, daidzein,
glycitein) was demonstrated in a recent clinical study involving
30 healthy men. Ingestion of purified genistein, daidzein, and
glycitein in a single dose exceeding normal dietary intake
manifold resulted in rapid clearance from plasma and
excretion in urine. It is suggested to limit soy intake to 100 mg
of isoflavones daily.

Summary
Soy contains bioactive components that have been shown to
help reduce the risk of cardiovascular diseases, breast cancer,
prostate cancer, and to fight osteoporosis by stimulating bone

formation. Soy isoflavones may also relieve menopausal
symptoms such as night sweats, hot flashes, and thinning
bones. VR

 

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