| How did researchers
become interested in soy protein in regard to CVD? The
answer lies in epidemiology,
or population studies. Although CVD is a major cause of
death in most developed countries, death rates for CVD
are not uniform around the world. For example, CVD death
rates for American men and women are more than double
those for men and women in Japan.
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.
How Does Soy Protein Reduce Cholesterol?
The simple answer to this question is that researchers
don't know exactly how soy protein lowers blood cholesterol
levels. Many believe that soy protein's cholesterol-lowering
effect may be due to more than one factor.
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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