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Nutrition and the Skeleton
The Role of Calcium and
Other Nutrients
Nutrition is one of many factors that
influence bone mass. Dietary intake can also impact the
tendency to fall and plays a significant role in
maintaining a soft tissue cushion to protect the
skeleton from the impact of a fall.
Calcium and
Calcium Balance
Bone serves as the reservoir for 99% of the
body's total calcium. Calcium is an essential nutrient
for bone health. It is also needed for the heart,
muscles and nerves to function properly and for blood to
clot normally. The body loses calcium every day through
urine, feces, sweat and shed skin, hair and nails. The
lost calcium is normally replaced by calcium in the
diet. When the diet does not contain enough calcium to
offset such losses, the body breaks down bone to release
calcium needed to accommodate these physiologic demands.
Many other nutrients affect bone
health, too, because they impact the absorption or
excretion of calcium. Calcium balance-not just
intake-is necessary for healthy bones. This balance is
dependent on the absorption rate of calcium
consumed as well as the rate of (mostly urinary) calcium
excretion. For example, in a study of 560
healthy women, one researcher demonstrated that only
about 10 percent of the variance in calcium balance
among these women was explained by differences in their
calcium intakes and absorption only explained another 15
percent. Urinary losses explained slightly more then
50 percent.
Nutrients
Affecting Calcium Balance
Several nutrients significantly influence calcium
balance:
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Vitamin
D
has a positive impact on
calcium balance, as it increases calcium
absorption in the gastrointestinal tract.
The most readily available source of vitamin D
is exposure to direct sunlight. Vitamin D is
also found in fatty fish, eggs, liver, butter,
fortified foods such as milk and multivitamins.
Vitamin D deficiency may be a problem among some
elderly, those in institutional settings, and
some people with chronic neurological or
gastrointestinal diseases.
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Protein
is essential in our diets
to build tissue during growth and to repair and
replace tissue throughout the life cycle. It is
also needed for fracture healing and proper
function of the immune system. Protein
deficiency is an important factor contributing
to death, institutionalization, and loss of
independence among the elderly following hip
fracture.
However, protein also increases
the body's need for calcium by increasing
calcium excretion. Excess protein in
the diet is used by the body for energy, just
like fats and carbohydrates. However, as protein
is burned for energy, it produces a chemical
called sulfate, which the body excretes through
the kidneys. Sulfate increases the excretion of
calcium.
It is generally believed that
most Americans exceed the recommended intake for
protein--44 grams for women and 56 grams for
men.
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Sodium
(and chloride), the components of table
salt, increase the calcium requirement by
increasing urinary calcium excretion.
Individuals with low salt intakes may be able to
maintain calcium balance at low calcium intakes,
while those with more typical U.S. salt intakes
will have higher calcium requirements.
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Oxalate
is a nutrient that
increases the calcium requirement by interfering
with calcium absorption in the same
food (not in others). Spinach, for example,
is an extremely nutritious food, but its calcium
is not absorbed because it is chemically bound
to the oxalates that are present. However,
eating spinach with cheese would not affect the
absorption of calcium from the cheese. Foods
high in oxalates include spinach, rhubarb and
sweet potatoes.
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Phosphorous.
Some concern has been
expressed that there may be too much phosphorous
in the North American diet, particularly since
phosphorous (in the form of phosphoric acid) is
a constituent of cola beverages and because
phosphate is added to many processed foods.
Phosphorous may increase the body's need for
calcium by interfering with calcium
absorption. There is no scientific
consensus that current levels of phosphorous
intake are, by themselves, harmful. There is,
however, general agreement that today's higher
phosphorous intakes would be entirely safe in
individuals with normal kidney function if
calcium intakes were at recommended levels.
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Caffeine.
The amount of
caffeine in a cup of coffee can reduce calcium
absorption by a few milligrams, but
that loss can be easily offset by adding a
tablespoon or two of milk. Much of the
apparently harmful effect of caffeine appears to
be due not to the caffeine itself, but to the
fact that caffeine-containing beverages are
often substituted for milk in the diet.
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Fiber.
Adequate
fiberintake is necessary to aid digestion and
prevent several chronic diseases such as colon
cancer and heart disease. Fiber has a minimal
effect on the absorption of calcium.
The fiber in wheat bran is most likely to
interfere, but unless the intake level is
extreme, this is not a significant problem.
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The Effects of
Nutrition on Bone Density and Fracture Risk
Calcium and
Vitamin D
Many published studies have
demonstrated that low calcium intake is associated with
low bone mass and increased fracture risk. It has been
suggested that calcium deficiencies in youth can account
for a 5-10% difference in peak bone mass and can
significantly increase the risk for hip fracture in
later life.
Among postmenopausal women, studies
have shown that supplemental calcium
can decrease the rate of bone loss from the femoral
neck, the spine and the total body. Women who have been
postmenopausal for six or more years tend to reap a
greater benefit from calcium supplementation than those
who are within five years of menopause. Since calcium is
a nutrient, not a drug, the positive effects of
supplemental calcium are most pronounced among women
with low to moderate calcium intakes. Recent clinical
trials have suggested that supplementation with calcium
or calcium plus vitamin D, can reduce fracture incidence
by about 30-50% in subjects with low calcium intakes.
Vitamin D
deficiency is also a concern in bone health. The
nutrient is essential for calcium absorption and normal
bone mineralization. Studies have shown that low levels
of vitamin D can contribute to low bone density. Recent
evidence suggests that deficiencies in this nutrient may
additionally contribute to hip fractures in
postmenopausal women.
Protein and Bone
Health
It is known that high protein
intake increases the calcium requirement. However, the
association between protein and osteoporotic bone
fractures in individuals has not been fully explored.
Reduced protein intake has been
linked to low femoral neck bone density in elderly
hospitalized patients. In these individuals, clinical
outcomes following hip fracture were significantly
enhanced when protein intake was normalized through
nutritional supplementation.
Soy and
Isoflavones
The soybean plant, a legume,
contains specific phytochemicals known as isoflavones.
Phytochemicals are non-nutritive substances, in that
they contain no vitamins or minerals. Isoflavones are
also phytoestrogens. Phytoestrogens are compounds that
have mild estrogenic effects.
Chick peas and legumes are good
sources of isoflavones. The legume, soy, has the
greatest concentration of these chemicals.
Isoflavones have received a good deal
of attention for their possible cancer and heart
disease-preventive traits. Because of their
estrogen-like properties, many believe that isoflavones
and other phytoestrogens may one day play a role in
postmenopausal health.
One isoflavone derivative,
ipriflavone, is currently used outside of the United
States as an osteoporosis therapy. Ipriflavone has had a
bone-protective effect in several studies. Further
research is needed to understand the systemic effects of
this compound before it can be considered a therapeutic
approach for osteoporosis prevention and treatment.
Nutrition and
Weight
Nutritional
status and body weight are additional considerations in
skeletal health. Nutritional status can influence one's
tendency to fall and is a factor in the maintenance of
adequate soft tissue mass to protect the bones from a
fall- related fracture. Of particular concern are thin,
undernourished, elderly individuals who may lack
sufficient muscle and fat mass in the hip region.
Body weight, is an important
determinant of bone density. The skeleton of heavy
individuals tends to benefit from it's increased
load-carrying role. Studies have demonstrated that body
weight is positively correlated with bone mineral
density, and that weight loss is associated with bone
loss. (Increasing calcium intake appears to reduce the
bone loss that accompanies weight loss.)
Weight loss in older individuals has
been linked to an increase in fracture risk. Researchers
found that "extreme" weight loss (10% or more) beginning
at age fifty, increased the risk of hip fracture in
older women and men. Conversely, a weight gain of 10% or
more decreased hip fracture risk. Such studies suggest
that maintaining weight in later life may have a
protective effect on bone. |
