Soy
Protein & Bone Health
Since the prevalence
of osteoporosis isn't the same across cultures, researchers
are looking to other cultures for clues to possible ways
of reducing this disease. Surveys of women in Japan, for
example, have raised questions about factors affecting
bone health. Although Japanese women consume less calcium
than most women in Western countries and are unlikely
to use Estrogen
Replacement Therapy (ERT) or Hormone
replacement Therapy (HRT), they have a lower prevalence
of fractures.
Some researchers believe that part of the answer to
this puzzle may lie with the high intake of soyfoods
in the traditional Japanese diet. They first looked
at the effects of soy protein itself on calcium metabolism.
More recently, research has focused on the potential
effects of the isoflavones
found in soy.
Soy Protein & Calcium
Scientists have extended the scope of the research showing
that the amount of calcium excreted increases as protein
intake goes up. They discovered that not all types of
protein have the same effect-protein from animal sources
causes much greater calcium loss than vegetable protein.
This has been confirmed in a study of 755 Japanese men
and women. Researchers found that consuming animal protein
was associated with an increase in calcium excretion.
However, they found no significant relationship between
calcium excretion and the consumption of plant protein.
These findings become more meaningful when they are
translated into the effect on bone fractures. As part
of the large Nurses' Health Study, researchers looked
at usual dietary intake and fracture rates in 85,900
women. First, they compared women averaging less than
68 grams of protein per day with those consuming more
than 95 grams. The woman eating more protein had an
increased risk of fracturing their forearm.
Next, the researchers evaluated the type of protein
being consumed. The increased risk of forearm fracture
was seen in women consuming animal protein. Consumption
of vegetable protein, however, was not associated with
an increased risk.
The Isoflavone Story
Another area of intense study is the potential effect
of soy isoflavones on bone health. Isoflavones are a
type of phytochemical,
or "plant chemical." The isoflavones found
in soy and thus soy protein are genistein,
daidzein,
and glycitein.
Isoflavones can be classified as phytoestrogens,
which are substances in plants that have chemical structures
resembling estrogen. Isoflavones actually have several
potential modes of action in the body. Depending on
the circumstances, they may either act like a weak estrogen,
act as an antiestrogen
(blocking some effects of estrogen), or have effects
not related to estrogen.
Purified genistein has been shown to slow the activity
of cells that break down bone- osteoclasts- in the laboratory
and in animals. Other researchers also found that genistein
directly inhibited the breakdown of bone tissue in the
laboratory.
Some animal research has shown genistein's effects
on preventing bone loss to be similar to those of estrogen.
Where genistein and estrogen differed is that the isoflavone
had the benefit of not affecting the uterus.
Casein
and nonfat dry milk. In this short-term study the women
receiving isolated soy protein increased bone mineral
density in the lumbar
portion of their spine.
References
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bone health references list
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