Chemistry Reference
In-Depth Information
mean intake exclusively from human milk. RDAs for zinc for all other
sex-age groups are listed in Table 10.2.
The adult human body contains between 1.5 and 2.5 g of zinc (Cousins,
2006), 85% is in skeletal muscle and bone (Food and Nutrition Board:
Institute of Medicine, 2001). About 95% of body zinc is intracellular, with
40% found in the nucleus. Plasma contains about 0.1% of the body's zinc,
which responds markedly to external stimuli including zinc intake, fasting
and acute stressors such as infection (Cousins, 2006). Thus, using plasma zinc
to asses zinc status should be done with caution. In humans, plasma zinc is
reduced in severe zinc deficiency, but not with moderate zinc deprivation
(Food and Nutrition Board: Institute of Medicine, 2001). Zinc concentra-
tions in human milk decline rapidly during the first 6 months of lactation
(Food and Nutrition Board: Institute of Medicine, 2001; Hunt et al., 2005).
They decline from approximately 4 mg/l at 2 weeks to 3 mg/l at 1 month, 2 mg/
l at 2 months, 1.5 mg/l at 3 months and 1.2 mg/l at 6 months postpartum. No
significant correlation was observed between dietary zinc intake and zinc
concentration in human milk, and zinc supplementation does not appreciably
affect its zinc concentration (Lonnerdal, 1997). In contrast to human milk,
the zinc concentration in mature bovine milk changes little after the first 3 d of
lactation when the concentration decreases by 50% in colostrum (de Maria,
1978). Mature bovine milk contains between 3 and 5 mg zinc/l (Lonnerdal
et al., 1981; Hunt and Meacham, 2001).
The reported mean distribution of zinc in human milk is 29% in the lipid
fraction (bound to the fat globule membrane), 14% associated with casein,
28% associated with whey proteins and 29% in the form of a low-molecular-
weight compound (probably as citrate) (Lonnerdal et al., 1982; Fransson and
Lonnerdal, 1983). The distribution of zinc in bovine milk is significantly
different; most is found in the skim milk fraction, with only 1-3% in the
lipid fraction (Fransson and Lonnerdal, 1983). Approximately 32% of the
zinc is bound to casein and most of the remaining zinc is bound to colloidal
calcium phosphate; only a small amount (
5%) is associated with a low-
molecular-weight compound, which was identified as citrate (Blakeborough
et al., 1983; Singh et al., 1989). Proteins in human milk, which bind most of
the zinc, are believed to be more easily digestible than casein, the major
protein in bovine milk. This difference may explain the higher zinc bioavail-
ability from human milk (Cousins, 2006). Pasteurization of human milk
decreases zinc bioavailability to the point of affecting zinc balance in preterm
infants, a situation probably caused by redistribution and alterations in the
zinc-binding pattern (Goes et al., 2002).
Because increasing zinc concentration from 1.8 to 5.8 mg/l in
infant formulae resulted in increased growth in male infants (Walravens
and Hambidge, 1976), zinc supplementation of formulae is practiced widely.
