depending on the persistence of the amenor-
rhoea (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 popula-
tion indicate that milk and milk products pro-
vided about one half of the total calcium intake,
while cereal products provided about 25%: veg-
etables 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 other-
wise 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 there-
fore 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 rela-
tionship 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
(Melvinet al. 1970). As discussed further below,
the urinary calcium loss may be influenced by a
number of factors, and the acidity of the urine,
which may in turn be influenced by the composi-
tion 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–1have 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 per-
formed, 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 estab-
lished, 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 excre-
tion 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-