The benefits of soy protein depend on your age...
Soy protein is one of the most researched foods in science. But
even with this much information, studies still show conflicting
results.
Some women turn to soy as an alternative to traditional estrogen
therapy. However, researchers from the Netherlands find that soy
protein (in the form of daily soy supplements) has little effect
on bone density or cholesterol levels [1].
Why the sudden departure from previous findings that support a
beneficial effect of soy protein? The women studied were between
ages 60 and 75 when they started taking soy protein— when they
may have been too old to reap its reported benefits.
"Timing is certainly a reasonable explanation as to why we
didn't find an effect from soy in the women studied," says
researcher Dr. Sanne Kreijkamp-Kaspers.
"We know that immediately after menopause, there's a huge
decline in bone loss and that's when [bad] cholesterol levels tend
to increase," she says. "It could be that if you give
soy before a woman reaches menopause, it is effective at preventing
this."
But if supplementation begins a decade of so after menopause, it
may be too late, she explains.
In her study, published in the Journal of the American Medical
Association, 200 women received either 25 grams of soy protein
each day via a powder that could be mixed with food or drink or
a phony powder package. Each daily dose of soy protein contained
99 milligrams of isoflavones (52 milligrams of genistein, 41 milligrams
of daidzein, and 6 milligrams of glycetein).
None of the women had ever consumed soy supplements prior to the
study. Bone mineral density and cholesterol levels were measured
at the beginning of the study, and again one year later
The results show that test scores were virtually identical in both
groups of women.
Other studies show that soy protein supplements at similar can
keep bones strong — even after menopause. But many critics
say that these studies included only a small number of people and
their findings cannot be generalized to the population.
Kreijkamp-Kaspers also points out that many studies included premenopausal
women, as well as men, or tracked postmenopausal women who may have
started taking soy before they reached menopause.
Height and weight
Factors such as weight or calcium intake also play a role in the
extent to which soy isoflavones improve bone health among postmenopausal
women.
A study published in Menopause shows that postmenopausal
women taking 80 milligrams of isoflavones daily saw positive benefits
on bone mineral content, particularly if they were more than four
years into menopause, had lower body weight or had a lower calcium
intake.
The team from the Chinese University of Hong Kong enrolled 203
women, aged 48 to 62 years old, to receive either placebo with 500
milligrams of calcium and 125 IU of vitamin D, a mid-dose (40 milligrams
of isoflavones with 500 milligrams of calcium and 125 IU of vitamin
D), or a high-dose (80 milligrams of isoflavones with 500 milligrams
of calcium and 125 IU of vitamin D) supplement every day for one
year.
The most widely evaluated technique to measure bone mineral density
is is dual energy x-ray absorptiometry (also known as DEXA).
DEXA is based on measuring the attenuation of a beam of X-rays
when it passes through bone. The bone mineral density at
a measurement point corresponds to a particular path of the radiation
through the body. It is the mass of bone mineral in the path of
the beam divided by the cross sectional area of the beam.
It is also possible to scan the bone several times to obtain a
series of bone mineral density values which the computer can combine
to yield the total mass of bone mineral (in grams) in that region.
The total bone mineral content is known as bone mineral content.
The researchers found a significant benefit of high dose isoflavone
supplementation on bone mineral content at the total hip and trochanter
compared to those women on low-dose or placebo.
But there was no significant improvement in bone mineral density.
In addition, there was a significant benefit to women had been
postmenopausal for more than four years, as well as those with a
lower body weight.
In women with a high calcium intake, soy isoflavones seemed to
make no difference to their bone health. Soy isoflavones also appeared
to have little additional benefit to bone mass among women with
a high body weight.
It's important to remember that measuring bone mineral density
isn't necessarily the best way to assess bone health. It's often
hard to distinguish between healthy or osteoporotic bone by measuring
bone mineral density alone [3].
Bone mineral density can vary depending on the size of bone as
well as the concentration of minerals in the bone. A larger person,
for example, has larger bones than a small person. So, more minerals
will be found in a large person's bones than in a small person's
of the same density.
Estimating bone mineral density is more accurate than trying
to measure a change in bone mineral density [4].
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References
1. Kreijkamp-Kaspers, S., Kok, L., Grobbee, D.E., de Haan, E.H.,
Aleman, A., Lampe, J.W., & van der Schouw, Y.T. (2004).
Effect of soy protein containing isoflavones on cognitive function,
bone mineral density, and plasma lipids in postmenopausal women:
a randomized controlled trial. Journal
of the American Medical Association, 292, 65-74
2. Chen, Y.M., Ho, S.C., Lam, S.S., Ho, S.S., & Woo, J.L. (2004).
Beneficial effect of soy isoflavones on bone mineral content was
modified by years since menopause, body weight, and calcium intake:
a double-blind, randomized, controlled trial. Menopause,
11, 246-254
3. Ilich, J.Z., & Kerstetter, J.E. Nutrition in bone health
revisited: a story beyond calcium. Journal
of the American College of Nutrition, 19, 715-737
4. Heaney, R.P. (2001). Protein intake and bone health: the influence
of belief systems on the conduct of nutritional science. American
Journal of Clinical Nutrition, 73, 5-6
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