Vitamin A and osteoporosis
Osteoporosis, a
disorder characterized by porous and weak bones, is a serious public
health problem for more than 10 million Americans, 80% of whom are
women. Another 18 million Americans have decreased bone density which
precedes the development of osteoporosis. Many factors increase the
risk for developing osteoporosis, including being female, thin,
inactive, at advanced age, and having a family history of osteoporosis.
An inadequate dietary intake of calcium, cigarette smoking, and
excessive intake of alcohol also increase the risk.
Researchers
are now examining a potential new risk factor for osteoporosis: an
excess intake of vitamin A. Animal, human, and laboratory research
suggests an association between greater vitamin A intake and weaker
bones. Worldwide, the highest incidence of osteoporosis occurs in northern Europe, a population with a high intake of vitamin A.
However, decreased biosynthesis of vitamin D associated with lower
levels of sun exposure in this population may also contribute to this
finding.
One small study of nine healthy individuals in Sweden
found that the amount of vitamin A in one serving of liver may impair
the ability of vitamin D to promote calcium absorption.
To further test the association between excess dietary intakes of
vitamin A and increased risk for hip fractures, researchers in Sweden
compared bone mineral density and retinol intake in approximately 250
women with a first hip fracture to 875 age-matched controls. They found
that a dietary retinol intake greater than 1,500 mcg/day (more than
twice the recommended intake for women) was associated with reduced
bone mineral density and increased risk of hip fracture as compared to
women who consumed less than 500 mcg/day.
This
issue was also examined by researchers with the Nurses Health Study,
who looked at the association between vitamin A intake and hip
fractures in over 72,000 postmenopausal women. Women who consumed the
most vitamin A in foods and supplements (greater than or equal to 3,000
mcg/day as retinol equivalents, which is over three times the
recommended intake) had a significantly increased risk of experiencing
a hip fracture as compared to those consuming the least amount (less
than 1,250 mcg/day). The effect was lessened by use of estrogens. These
observations raise questions about the effect of retinol because
retinol intakes greater than 2,000 mcg/day were associated with an
increased risk of hip fracture as compared to intakes less than 500 mcg.
A
longitudinal study in more than 2,000 Swedish men compared blood levels
of retinol to the incidence of fractures in men. The investigators
found that the risk of fractures was greatest in men with the highest
blood levels of retinol (greater than 75 mcg per deciliter). Men with blood retinol levels in the 99th
percentile (greater than 103 mcg per dL) had an overall risk of
fracture that exceeded the risk among men with lower levels of retinol
by a factor of seven .
However, high vitamin A intake does not necessarily equate to high
blood levels of retinol. Age, gender, hormones, and genetics also
influence these levels. Researchers did not find any association
between blood levels of beta-carotene and risk of hip fracture.
Researchers' findings, which are consistent with the results of animal,
in vitro (laboratory), and epidemiologic studies, suggest that intakes
above the UL, or approximately two times that of the RDA for vitamin A,
may pose subtle risks to bone health that require further investigation.
The
Centers for Disease Control and Prevention (CDC) reviewed data from
NHANES III (1988-94) to determine whether there was any association
between bone mineral density and fasting blood levels of retinyl
esters, a form of vitamin A. No significant associations between blood
levels of retinyl esters and bone mineral density in 5,800 subjects
were found.
There
is no evidence of an association between beta-carotene intake,
especially from fruits and vegetables, and increased risk of
osteoporosis. Current evidence points to a possible association with
vitamin A as retinol only. If you have specific questions regarding
your intake of vitamin A and risk of osteoporosis, discuss this
information with your physician or other qualified healthcare
practitioner to determine what's best for your personal health.
News
I have decided to start an active promotion of this website, so that we get more visitors soon. And it would be a waste of time if we didn't allow for discussion and sharing of informations. In light of that, discussion forum will be officially launched on June 21st.
I hope it will serve a positive function in opening alternative horizons in the area of general information about vitamins.