Facts
About Calcium
The body needs a constant level of
calcium in the blood for the normal action of nerves and muscles. A
hormone released from the parathyroid gland controls this level by
balancing calcium from the diet with stores in bones. If there is not
enough in the diet, the parathyroid hormone stimulates the release of
calcium from the bones. If there is more calcium than is needed to
keep the blood level constant, the excess can be deposited in bones.
Greater withdrawals than deposits result in the bones gradually
becoming less and less dense until they are so chalky that they
fracture from a slight fall, or just with the effort of standing. The
loss of calcium happens so slowly that it is many years before
weakness and chalkiness eventuate. There are no symptoms before the
final straw causes a fracture to occur.
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Calcium
Absorption into the body
A number of factors govern calcium
absorption into the bones. The positive factors which are essential
for its occurrence, include:-
- Calcium in the daily diet
.
- Normal levels of hormones (these
are vital for calcium to be retained by bones).
- Weight-bearing exercise (calcium
goes into bones when muscles exert a pull on the bone during
physical activity)
.
- Vitamin D. (Usually from sunlight
on the skin).
Some factors also work against
calcium retention. These include:
-
- Too much salt (increases loss of
calcium in urine)
.
- Too much protein (reduces
retention of calcium.
- Nicotine from cigarette and cigar
smoking.
- Very high levels of caffeine or
alcohol. (Small quantities are not a problem).
The chemical form of calcium may also
be important. For example, we know that the type of calcium in milk
and in dairy products is well absorbed. We do not yet know if the
calcium in products such as soy milks is absorbed to the same
extent.
Small, lightweight women have a lack
of body weight and always have less pull of muscle on bone with
weight-bearing exercise. They are more prone to weak, porous bones.
Blood tests are of little use in
determining calcium levels in the bones, because, the body keeps its
levels of blood calcium normal by withdrawing calcium from the bone.
Blood tests are therefore; normal, even when bones are dangerously
thin. A special type of X-ray machine is needed to measure bone
density. Such tests are expensive but are recommended for women with
a family history of fragile bones or spinal or hip problems.
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Osteoporosis
The condition of porous bones is
called osteoporosis. It is very common in older women and is related
to the fall in oestrogen levels that usually occurs at menopause,
accompanied with a low-calcium diet, a lack of weight-bearing exercise
and smoking. At the beginning of the 20th century, life
expectancy for women was about 50 years, so few women lived long
enough to develop osteoporosis. Over the past 20 or 30 years, many
women have stopped working hard physically, no longer make a daily
walk to the shops and carry home their food supplies and have less calcium
in their daily diet.
Among women, one in five can expect
to be hospitalized for a severe fracture to the hip or spine between
the ages of 50 and 70. This is a great expense for the community and
an enormous burden for the many women who must endure the pain of
such calamities. As many as 40 per cent of all women can also expect
to fracture wrists, ankles and other bones as a result of a slight
fall at some stage in their life. Osteoporosis is also the reason
why many older people become stooped, have back problems and appear to
shrink with age. “Dowager’s hump” is often the first sign of
osteoporosis.
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How Hormones Control Calcium In
Bones
In growing children, growth hormone
ensures that calcium is deposited into bones. In adults, the sex
hormones take over this role. At menopause, when the oestrogen levels
fall, calcium is no longer retained by the body. This may also occur
in younger women and in female athletes if they lose so much body fat
that their hormone levels change and their periods stop.
Osteoporosis also occurs in men,
although the much slower loss of hormones and shorter life expectancy
make it more rare. Women at greatest risk of osteoporosis are those
who:-
- Have had a low calcium intake
throughout their life.
- Are small and/or light.
- Have had little weight-bearing
exercise.
To maintain bone density in women,
most experts now recommend that those at risk of osteoporosis ask
their doctor for hormone replacement therapy (HRT). This will cause
their periods to continue past the usual menopause age but as they
will be slight and regular, many women find it is worth the
inconvenience to maintain their bone density.
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Types of bone
There are two types of bone to
consider in osteoporosis. Cortical bone in the long bones of the arms
and legs is lost when there is insufficient calcium in the blood and
the parathyroid hormone causes it to be withdrawn from the bone. Cortical
bone is also lost if we are lacking in vitamin D (from the sun).
Trabecular bone is spongy bone, which is found in areas such as the
spine. It is lost when hormone levels change and when the diet
contains too much salt. Over a lifetime, women lose half their
trabecular bone and one third of their cortical bone.
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Too thin or Too much exercise?
Dieting disturbs the hormone balance.
Losing body fat often causes periods to stop. When this occurs,
trabecular bone from the spine is lost. Strenuous exercise can have
the same effect.
This does not mean that women should
stay fat and not exercise. Sensible, moderate dieting and increased
exercise have a positive effect on health.
Girls and women with anorexia nervosa
lose substantial amounts of bone and are at high risk of crushed
vertebrae and other spinal problems and fractures. There is no
evidence that a change to a better diet can undo the damage caused by
periods of such intense dieting.
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