| How did researchers
become interested in soy protein in regard to CVD? The
answer lies in epidemiology, or population studies. Japanese
women have a death rate from CVD that is less than half
the rate in American women-85 in Japan versus 192 (deaths
per 100,000 population) in the United States.
One difference between Japanese and American women
is the high intake of soyfoods in the traditional Japanese
diet, compared to a very low soyfood intake in the United
States. This has led researchers to investigate ways
in which soy may help lower the risk of CVD.
What you eat affects the levels of cholesterol and
triglycerides in your blood. This led scientists to
investigate differences in dietary habits between East
and West--one of these being dietary intake of soyfoods.
Soyfoods represent a major source of protein in the
traditional cuisines of Japan and many other Asian countries.
In the United States and other Western countries, however,
soyfoods are almost totally absent from the average
diet.
Soy Protein and Fats in Your
Blood
Significant research has been done to investigate soy
protein's effects on LDL-cholesterol and triglyceride
levels in your blood. The 1995 New England Journal of
Medicine meta-analysis mentioned earlier combined the
results of 38 clinical studies evaluating the effect
of consuming soy protein on LDL-cholesterol, triglyceride,
and HDL-cholesterol levels. This groundbreaking
research showed that consuming an average of 47 grams
of soy protein daily: decreased total cholesterol by
9.3% decreased LDL-cholesterol by 12.9% decreased triglycerides
by 10.5% increased HDL-cholesterol by 2.4%
The reductions in total cholesterol, LDL-cholesterol,
and triglycerides were statistically significant; the
increase in HDL-cholesterol was not large enough to
be of statistical significance. Although the meta-analysis
provided strong support for soy protein's effectiveness
in lowering cholesterol, the studies were not designed
to identify exactly what substance in soy protein had
this positive effect.
Looking at Soy Protein
Some research has focused on soy protein itself. For
example, the specific amino acids found in soy protein
may affect cholesterol levels. However, animals achieved
greater cholesterol lowering when they received intact
soy protein than when they were given a mixture of amino
acids identical to those in soy protein. This suggests
that although the specific amino acids in soy protein
may help lower cholesterol, something else is at least
partially responsible for the beneficial effect.
Researchers have also shown that particular storage
proteins found in soy protein can affect cholesterol
levels. These proteins, called 11S and 7S globulins,
helped lower cholesterol in animals. Other
researchers broke down soy protein into two fractions--one
high-molecular-weight fraction (HMF) and one with a
lower molecular weight. Giving HMF to animals resulted
in greater cholesterol lowering than was achieved by
giving intact soy protein or the lower-molecular-weight
fraction.
Introducing Soy Isoflavones
The components of soy protein currently receiving the
most attention are its bioactive components, in particular,
isoflavones--genistein, daidzein, and glycitein. Isoflavones
are one type of phytochemical--a substance naturally
occurring in plants.
Studies in animals and in humans
have shown that consuming soy protein with naturally
occurring isoflavones (and other bioactive components)
results in greater cholesterol lowering than consuming
either soy protein with the isoflavones removed or a
placebo, typically casein. However, isoflavones alone
don't appear to be the answer. A study in which menopausal
and perimenopausal women took isoflavones in a purified
form found that isoflavone pills didn't result in lower
cholesterol levels.
These results indicate that in order to achieve a decrease
in blood cholesterol, the type of soy protein consumed
is important. Research indicates that soy protein with
naturally occurring bioactives is what is needed.
References
View cardiovascular
disease references list.
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