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 sug-
gested new guidelines for calcium intake. They
recommend: during early childhood (1–3 years)
500 mg · day–1, 800 mg · day–1between 4 and 8
years, 1300 mg · day–1during adolescence (9–18
years) and 1000 mg · day–1between 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
320 nutrition and exercise
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 popula-
tion greatly exceeds the desirable intake recom-
mended 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 low-
energy 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, result-
ing in menstrual dysfunctions such as oligomen-
orrhea 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 circulat-
ing oestrogen levels and low dietary calcium
intake clearly creates a high risk situation for
development of early osteoporosis, and the pos-
sibility 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,