Introduction to Human Nutrition

194 Introduction to Human Nutrition

established very different recommendations for
calcium intake (Table 9.3). Much of this divergence
arises because of different interpretations of available
human calcium balance data. The higher recommen-
dations in the USA derive from defi ning calcium
requirements based on desirable calcium retention
estimated from human calcium balance studies, i.e.,
that which results in the maximum skeletal calcium
reserve.

Micronutrient interactions

There is considerable evidence from studies on exper-
imental animals that excessive calcium intake can
impair the nutritional status of other nutrients,
particularly iron, zinc, and magnesium, but data on
humans are not clear. While calcium interacts with
magnesium and phosphorus, and reduces their
absorption, there is no evidence that high calcium
intakes are associated with depletion of the affected
nutrient. Calcium inhibits the absorption of iron in a
dose-dependent and dose-saturable fashion. However,
the available human data fail to show cases of iron
defi ciency or even decreased iron stores as a result of
high calcium intake. There is some evidence that high
dietary calcium intakes reduce zinc absorption and

balance in humans and may increase the zinc require-

ment. Overall, the available data on the interaction

of calcium with these nutrients do not show any

clinically or functionally signifi cant depletion of the

affected nutrient in humans and, in the context of risk

assessment, these interactions should probably not be

considered adverse effects of calcium. However, such

interactions deserve further investigation. It is well

established that a defi ciency of vitamin D (arising

from a lack of exposure to sunlight, inadequate dietary

intake, or both) can result in a reduced effi ciency of

intestinal calcium absorption that, in turn, can lead

to a decrease in serum ionized calcium.

9.3 Magnesium

Like calcium, magnesium is an alkaline earth metal.

Magnesium is the eighth most abundant element in

the Earth’s crust. It does not occur uncombined, but

is found in large deposits in the form of magnesite,

dolomite, and other minerals. The metal is used in

fl ashlight photography, fl ares, and pyrotechnics. It

is one-third lighter than aluminum, and in alloys

is essential for airplane and missile construction.

Magnesium is used in producing nodular graphite in

cast iron and as an additive to conventional propel-

lants. The hydroxide (milk of magnesia), chloride,

sulfate (Epsom salts), and citrate are used in

medicine.

Magnesium was fi rst shown to be an essential

dietary component for rats in 1932 and later for

humans. This essentiality is a refl ection of the role

that magnesium plays in the stabilization of ATP and

other molecules. Since then, nutritionists have come

to realize that frank magnesium defi ciency is rare and

that it only occurs in clinical settings as a secondary

consequence of another disease. More recently, mod-

erate or marginal defi ciency has been proposed as a

risk factor for chronic diseases such as osteoporosis,

cardiovascular disease, and diabetes. These associa-

tions are controversial.

Absorption, transport and

tissue distribution

Magnesium is the second most common cation found

in the body (about 25 g). It is evenly distributed

between the skeleton (50–60% of total) and the soft

tissues (40–50% of total). In the skeleton, about one-

third of the magnesium is on the surface of bone. This

Table 9.3 Recommended calcium intakes in the USA and UK

UK RNI (1998)a US AI (1997)b

Age group (years) mg/day Age group (years) mg/day

0–1 525 0–0.5 210
1–3 350 0.5–1 270
4–6 450 1–3 500
7–10 550 4–8 800
11–14 M 1000 9–13 1300
15–18 M 1000 14–18 1300
11–14 F 800 19–30 1000
15–18 F 800 31–50 1000
19–50 700 51–70 1200

50 700 > 70 1200
Pregnancy
Pregnancy NI ≤ 18 1300
19–50 1000
Lactation + 550 Lactation
≤1 8 1300
19–50 1000

a Reference nutrient intake (RNI); UK Department of Health (1991).
b Adequate intake (AI); US Institute of Medicine (1997).
Estimates of Ca requirements refer to both males and females unless
stated otherwise.
M, requirements for males; F, requirements for females; NI, no
increment.