MineralsCalcium

Chapter 23
Minerals: Calcium
KARIN PIEHL AULIN
Introduction
Calcium is a micronutrient with great importance to many cellular events in different tissues
in the body, as well as forming the major structural component of bone. Athletes are often concerned that their normal diet will not provide
sufficient micronutrients, and the need for an
adequate dietary intake of calcium is as much a
concern for athletes as it is for the general population. Augmentation of the diet with specific
calcium supplements and with calcium-enriched
foods is common practice among athletes and
non-athletes alike, but there is limited information as to whether the need for calcium is
increased by physical activity, and whether such
supplementation is warranted.
The US Surgeon General’s Report (1988) states
that ‘inadequate dietary calcium consumption in
the first three to four decades of life may be associated with increased risk for osteoporosis in
later life due to a low peak bone mass’. Osteoporosis is a chronic disease characterized by a
progressive loss of bone mass: it affects women
more than men, partly because of the role played
by a falling oestrogen level after the menopause.
Bone loss is widely accepted as a normal part of
the ageing process, and occurs at a rate of about
0.5–1.0% per annum after the age of 40 (Cohn
et al. 1976): by age 90, one third of women and
one sixth of men will have suffered hip fractures
as a consequence. However, a number of nutritional and lifestyle factors have a major impact
on the rate of mineral loss from the skeleton:
318
these factors can be important in slowing
this process, and thus in delaying the point at
which the bone mineral density becomes so low
that the fracture threshold is easily exceeded.
Other important factors reported to be associated
with the maintenance of bone health are an
adequate level of physical activity and avoidance
or cessation of cigarette smoking and excess
alcohol intake. However, while there is a clear
role for physical activity in maintaining bone
mass, very high levels of exercise in women have
been associated with some degree of bone loss
(Drinkwater et al. 1990), so there are clearly a
number of issues of importance for women,
and perhaps also to a lesser extent for men,
involved in sports where high training loads are
involved.
Roles of calcium in the body
Calcium is an essential nutrient and a major component of mineralized tissue and is the mineral
found in the largest quantity in the body, representing about 1.5–2% of body mass in the
average adult: for men, total body calcium
content is about 1000–1100 g, and for women,
about 800 g (Cohn et al. 1976). Approximately
99% of the total body calcium is located in the
bones. The remaining 1% is accounted for by the
calcium found in the blood, muscle and nervous
tissue where calcium is necessary for blood coagulation, muscle contraction and nerve conduction: although the amounts are small, the role of
calcium is crucial for normal functioning.
minerals: calcium
Bone matrix is a mixture of tough fibres (made
of type I collagen), which resist pulling forces,
and solid particles (calcium phosphate as hydroxyapatite crystals), which resist compression.
Bone is by no means a permanent and immutable
tissue. There is a continuous turnover and
remodelling of the matrix with a concomitant
release and uptake of calcium: the cells involved
in bone breakdown are osteoclasts, while the
osteoblasts are involved in bone formation. The
regulators of calcium metabolism in bone tissue
are two hormones, parathyroid hormone (PTH)
and calcitonin. Excess PTH results in a rise in the
blood calcium with a corresponding fall in the
calcium content of the bones, and a loss of
calcium from the body by increased excretion in
the urine. PTH, and exposure to sunlight, also
stimulates the formation of the active form of
vitamin D, which governs the absorption of
calcium from the small intestine. The ability to
regulate the uptake of calcium is important, and
differences in bone mineral density can be
demonstrated in response to exercise, even
between groups with the same dietary calcium
intake. Calcitonin is released when plasma
calcium increases and stimulates bone formation. Calcium is excreted by the large intestine
and, to a lesser extent, by the kidney and by the
dermis.
In the overall function of skeletal muscle,
calcium plays two essential regulatory roles.
First, calcium is the link between excitation and
contraction. The concentration of free calcium in
the cytosol is low (about 10–8 m) in resting muscle
(Martonosi & Beeler 1983), whereas its concentration in the extracellular fluid and in the
endoplasmic reticulum (ER) is high. Calcium is
involved in a series of events which converts the
electrical signal of the action potential arriving at
the synaptic terminal into a chemical signal that
travels across the synapse where it is converted
back into an electrical signal in the postsynaptic
cell. Release of calcium from the terminal cisternae of the sarcoplasmic reticulum in response to
membrane depolarization upon the arrival of an
action potential allows the actin and myosin filaments to interact. The plasma membrane and the
319
ER membrane have mechanisms to regulate the
calcium concentration gradient during resting
conditions and to restore it after muscle and
nerve cell stimulation (Alberts et al. 1994). The
activation process involves the binding of
calcium to troponin C, one of the regulatory proteins associated with the actin filaments, and the
change in shape of these proteins allows interaction between actin and myosin to occur. Calcium
is then pumped back into the terminal cisternae
by an energy-dependent transporter in a process
that consumes adenosine triphosphate (ATP),
allowing relaxation of the muscle to occur. There
is good evidence that fatigue during highintensity exercise may involve a disruption of
the cell’s calcium-handling capability (Maughan
et al. 1997). A number of substances, including
caffeine, can alter the response of the muscle to a
single action potential, and the effects of some of
these compounds on exercise capacity may be
mediated by effects on calcium transport. These
processes are described in detail by Jones and
Round (1990).
A second key process requiring calcium is
the activation of numerous cellular enzymes
involved in energy production, and calcium is
important to both glycogenolysis and the glycolytic pathway in generating ATP (Tate et al.
1991; Clarkson & Haymes 1995). It seems sensible that the same process that allows the muscle
to do work is involved in the regulation of ATP
provision. The activity of phosphorylase, the key
enzyme involved in glycogen breakdown, is
stimulated by increasing cytosolic calcium concentration (Maughan et al. 1997) and this is
important for the activation of the glycolytic
pathway at the onset of exercise.
Calcium intake
An adequate calcium intake is needed to achieve
optimal peak bone mass in the first two or three
decades of life, to maintain bone mass throughout the middle years of life, and to minimize
bone loss in the later years (Andersson 1996). A
daily calcium intake that is sufficient to meet the
requirement may be achieved through diet alone,
320
nutrition and exercise
if some attention is paid to the composition of the
diet. Alternatively, calcium-fortified food or
calcium supplementation may be employed to
meet the need. The amount of calcium available
from the diet depends on the total dietary
calcium intake, the bioavailability, which
depends in turn on the amount of calcium in
solution and on the presence of other dietary
components, and on the activity of the intestinal
calcium transport systems. The bioavailability is
influenced by the presence of anions that form
insoluble compounds that cannot be absorbed:
these include oxalate (which is present in
rhubarb and spinach) and polyphosphate.
Vitamin D status will determine the activity of
the calcium transporters in the intestine. All
of these factors, in addition to the ongoing losses
of calcium from the body, will influence the
amount of calcium that the diet must supply to
meet the individual’s requirement.
When body mass is taken into account,
growing children require as much as two to four
times as much calcium as adults, and the United
States recommended dietary allowance (RDA)
for calcium is greatest during adolescence (11–18
years) and early adulthood (19–24 years), being
in the order of 1200 mg · day–1 (National Research
Council 1989). Males and females of all ages have
the same calcium requirement except when
females are pregnant or lactating. The RDA for
children (1–10 years) and adults 25 years and
older is 800 mg · day–1. The National Academy of
Science Food and Nutrition Board recently suggested new guidelines for calcium intake. They
recommend: during early childhood (1–3 years)
500 mg · day–1, 800 mg · day–1 between 4 and 8
years, 1300 mg · day–1 during adolescence (9–18
years) and 1000 mg · day–1 between the ages of 18
and 50 years.
In the general US population, it is estimated
that the average dietary calcium intake of men is
about 115% of the 1989 RDA, but for women the
figure is only 78%: for children, it is estimated
that the mean intake is about 105% of the RDA
(US Surgeon General 1988). Corresponding
figures for the UK indicate rather similar values,
with a daily mean intake of 940 mg for men and
717 mg for women (Gregory et al. 1990).
However, as the RDA for calcium in the UK is
only 500 mg for men and for women, the average
intake was well above the RDA. This discrepancy
between countries in recommendations for
dietary intake reflects the uncertainty as to
requirements: the dietary intake necessary to
maintain calcium balance has been reported to be
anything between 200 mg · day–1 and over
1000 mg · day–1 (Irwin & Kienholz 1973). The high
value recommended for the American population greatly exceeds the desirable intake recommended by the WHO/FAO, and reflects the high
dietary content of protein and phosphate in
that country: both protein and phosphate are
reported to increase calcium loss.
Surveys of dietary habits in female adolescent
athletes (gymnasts, ballet dancers and distance
runners) show their average calcium intake to be
well below RDA and often related to their lowenergy intake in order to maintain a low body
weight (Carroll et al. 1983). Low energy intake
together with a high weekly training load will
lead to a decreased percentage of body fat, and
insufficient levels of circulating oestrogen, resulting in menstrual dysfunctions such as oligomenorrhea or amenorrhea (Drinkwater et al. 1984,
1990). Several cross-sectional studies have
shown significant relationships between body
mass and bone mineral density and between
body mass and susceptibility to osteoporotic
fracture (Sowers et al. 1991; Lindsay et al. 1992).
Restriction of energy intake (which resulted in a
5% reduction in dietary calcium intake) for a
period as short as 6 months has been shown to
result in a significant reduction in bone mineral
density in healthy young women, even though
there was only a moderate (3.4 kg) loss of body
mass in these subjects (Ramsdale & Bassey 1994).
The combination of low body mass, low circulating oestrogen levels and low dietary calcium
intake clearly creates a high risk situation for
development of early osteoporosis, and the possibility of stress fractures due to overload of bone
tissue will then increase. Resumption of menses
by regain in body weight may restore some of the
lost bone tissue but not all is likely to be regained,
minerals: calcium
depending on the persistence of the amenorrhoea (Drinkwater et al. 1986).
Foods rich in calcium include dairy products,
some canned fish (especially if eaten with bones),
some vegetables, including broccoli, spinach and
collard greens, tofu, and some calcium-enriched
grain products. UK data for the general population indicate that milk and milk products provided about one half of the total calcium intake,
while cereal products provided about 25%: vegetables contributed only about 7%, and the use of
supplements was negligible (Gregory et al. 1990).
Where energy intake is a concern, as in weight
category sports, or when energy intake is otherwise restricted, the use of reduced-fat dairy
products should be encouraged: a wide range of
low to moderate fat varieties can be used to add
variety to the diet.
Calcium has been reported to inhibit the
absorption of iron from the food and it is therefore suggested that these two nutrients should
not be taken together in large amounts (Gleerup
et al. 1995). When both iron status and calcium
status are precarious, special attention must be
paid to the initiation of any supplementation
regimen. This may be particularly relevant to
female athletes, who may suffer from anaemia
due to both low energy intake and loss of iron
through the menses.
Calcium balance
Whole body net calcium balance reflects the relationship between the dietary calcium intake and
all routes of calcium loss. Positive calcium
balance occurs when calcium intake exceeds
calcium loss, and is necessary for bone growth
and peak bone mass to be achieved. Negative
calcium balance will lead to a decrease in bone
mass and density. Calcium loss is the sum of the
faecal, urinary, and dermal calcium losses. Faecal
calcium loss accounts for about 75–80% of the
dietary calcium ingested (Schroeder et al. 1972),
but about 20% of this is of endogenous origin
(Melvin et al. 1970). As discussed further below,
the urinary calcium loss may be influenced by a
number of factors, and the acidity of the urine,
321
which may in turn be influenced by the composition of the diet, appears to be an important factor
(Ball & Maughan 1997). The loss of calcium
through the skin is often estimated at 60 mg ·
day–1, but this may substantially underestimate
the actual calcium loss of individuals who
engaged in strenuous training programmes
(Matkovic 1991). Sweat calcium losses as high as
57 mg · h–1 have been reported during exercise
(Krebs et al. 1988). Sweat calcium concentration is
typically about 1 mmol · l–1 (40 mg · l–1), so losses
may be very much greater than this when sweat
rates are high or when prolonged exercise is performed, especially in hot environments (Shirreffs
& Maughan 1997).
Dietary factors other than calcium intake may
be of importance, and the association between
high protein diets and an increased urinary
calcium loss is widely accepted (Lutz 1984;
Kerstetter & Allen 1990); this effect appears to be
a consequence of the acid load that results from
protein metabolism. The effects of an acid load in
increasing urinary calcium output are well established, and the US Surgeon General’s Report on
Nutrition and Health (1988) concluded that
‘increased acidity induces calcium loss by
increasing renal excretion directly as well as by
increasing the dissolution of mineral from the
skeleton and impairing mineral deposition.’
A recent comparison of the dietary intake of
omnivorous women and a matched group of
vegetarians showed that the vegetarians had a
lower dietary protein intake and a lower 24-h
total urinary acid excretion than the omnivorous
women (Ball & Maughan 1997). Although there
were no differences between these groups in the
estimated (7-day weighed intake) dietary
calcium intake, the daily urinary calcium excretion of the omnivores was significantly higher
than that of the vegetarians. These results are
consistent with the suggestion that the acid/
alkaline characteristics of the habitual diet have
implications for calcium balance, and that this
may be amenable to manipulation by alteration
of specific dietary components.
There have been numerous recent reviews of
the current state of knowledge regarding nutri-
322
nutrition and exercise
Fig. 23.1 Most forms of exercise are good for bone
health and should be encouraged in youngsters. Only
a few athletes, most commonly young women in
sports where low body mass confers an advantage, are
likely to suffer accelerated bone mass. Photo ©
Allsport / M. Powell.
tion and bone metabolism and these have been
summarized in the US Surgeon General’s Report
on Nutrition and Health (1988) (see Chapter 7).
Exercise and calcium balance
Acute exercise results in a prompt increase in
serum calcium, both in its ionized and nonionized forms. This may be due in part to lactic
acidosis rather than to changes in PTH and calcitonin concentrations, and haemoconcentration is
also likely to be a significant factor (Vora et al.
1983; Cunningham et al. 1985). Marathon
running was found to be accompanied by a transient decrease in urinary calcium and serum
osteocalcin levels (Malm et al. 1992). Endurance
training has been reported to be associated with
increased serum levels of the active form of
vitamin D, leading to increased calcium absorption and a rise in total body calcium (Yeh & Aloia
1990). A few studies have demonstrated exerciserelated elevations in PTH (Ljunghall et al. 1985,
1986; Salvesen et al. 1994), but this has not been
confirmed in other studies (Aloia et al. 1985).
The influence of calcium intake and physical
activity on peak bone mass has been the subject
of much attention (Kanders et al. 1988; Mazess &
Barden 1991; Recker et al. 1992). There are both
cross-sectional and longitudinal studies that
favour a beneficial effect of calcium on the adult
skeleton and there are others that find no relationship between dietary calcium and bone mass
and rate of bone loss. Peak bone mass is achieved
during the third decade of life. Apart from
calcium intake, heredity is also an important
factor determining peak bone mass. A study on
identical twins, where one twin in each pair
received calcium supplementation and the other
a placebo, suggested that extra calcium in the
diet is beneficial to the achievement of peak bone
mass prior to puberty (Johnston et al. 1992).
The role of physical activity in optimizing
bone growth as well as maintaining bone mass is
well established (Torgerson et al. 1995): acute
reductions in weight-bearing activity are associated with a dramatic loss of calcium. Measurements of prolonged bed rest in healthy
volunteers and in patients, as well as in astronauts subjected to microgravity, have all shown
an increased calcium loss and a reduced skeletal
mass (Anonymous 1983). Increased physical
activity, and in particular running, has been
shown to be associated with an increased bone
density (Lane et al. 1986), and it seems clear that
the physical stress imposed on the bone is an
important factor (Lanyon 1992; Wolman 1994).
This is supported by a recent study showing
increases in bone area in adult male rats subjected to a resistance training programme
(Westerlind et al. 1998). There is, however, little
information on the type, frequency, duration and
intensity of exercise that will optimize bone mass
and minimize the age-related loss.
minerals: calcium
Apart from the implications for the development of osteoporosis in later life, a low bone
density may be detrimental in athletes. Myburgh
et al. (1990) found that athletes with lower
extremity stress fractures had significantly lower
femoral and lumbar mineral densities than
matched athletic control subjects. Athletes with
stress fractures had significantly lower calcium
intakes and had lower intakes of dairy products.
Injured female athletes were more likely to have
irregular menstrual cycles and less likely to use
oral contraceptives. This was not confirmed
among female ballet dancers but a great number
of dancers with stress fractures avoided dairy
products (Frusztajer et al. 1990). A lifetime
history of calcium intakes exceeding 800 mg ·
day–1 appears to reduce the risk for hip fractures
in older women compared with women with
lower calcium intake (< 450 mg · day–1) and this
also seems to hold for men (Matkovic et al. 1979).
Milk consumption during childhood and adolescence and as an adult was associated with a
greater bone mass in postmenopausal women,
but total calcium intake was not associated with
bone mass (Sandler et al. 1985; Bauer et al. 1993).
Exercise appears to be more important in preventing trabecular bone loss while calcium
intake may be a more important influence on cortical bone loss.
Conclusion
Intake of sufficient amounts of energy to balance
the expenditure, maintenance of circulating
hormone levels, and regular participation in
some form of weight-bearing exercise are of
greatest importance to achieve and preserve
skeletal health. This is important for the avoidance of stress fractures in young athletes and for
the preservation of bone health in later life. An
adequate dietary intake of calcium is also essential, and this can be achieved by consumption of
dairy products (which may be reduced-fat varieties) and other foods rich in calcium. Exercise
per se does not seem to lead to an increased
requirement for calcium by the body and there is
generally no need for calcium supplementation
323
for athletes provided that the amount of energy
consumed is sufficient. The RDA for calcium in
both men and women is different in different
countries, and is usually between 800 and 1200
mg · day–1. The recommended intake is the same
for males and females of all ages, except when
females are pregnant or lactating. In postmenopausal women, adequate hormone supplementation, physical exercise and dietary calcium
intake will prevent loss of bone tissue and delay
the development of osteoporosis.
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