1. Body content, biological role and childhood accretion rates

Table 1 provides details of the body content of Ca, P. Mg and Zn in a new-born baby born at term, a typical man and a typical woman. As can be seen, an adult contains approximately 1 kg and 0.5 kg of Ca and P respectively, while Mg and Zn are present in smaller quantities.

The body compartments of the four minerals are summarised in Table 1. Approximately 99% of calcium and 80% of phosphorus in the body are contained in the inorganic phase of bone and teeth, imparting structure and strength. The crystal structure of bone salt resembles hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂], which contains Ca and P in the proportion 2.15:1 g/g (Russell et al., 1986). However, crystallisation of bone salt occurs in several stages, proceeding from amorphous calcium phosphate through intermediate crystalline structures, such as octocalcium phosphate (Russell et al., 1986). These compounds have lower Ca:P ratios than hydroxyapatite and consequently the proportion of Ca to P in young bone is generally between 1.7:1 - 2.14:1 g/g (Specker & Tsang, 1987). The measured ratio in adult human bone ash is around 2.3:1 g/g (Mitchell et al., 1945).

Bone salt is not pure hydroxyapatite, since it contains anions, such as carbonate and citrate, and cations, such as Mg and Zn (Widdowson & Dickerson, 1964). These ions either substitute within the crystal lattice or are absorbed onto the crystal surface (Russell et al. 1986). Approximately 60% of body Mg and 30% of body Zn are present in the skeleton where their concentrations are higher than in other tissues in the body (Department of Health, 1991; Schwartz, 1990). Zn is associated with alkaline phosphatase at calcification sites and is also deposited within the inorganic matrix (Hambidge, Casey & Krebs, 1986). The function of Mg and Zn in bone is largely unknown. Mg may play a role in the control of crystal formation and in crystal stability (Schwartz, 1990) while Zn is thought to be involved in chondrogenesis, collagen synthesis, osteoblastic function and calcification (Hambidge, Casey & Krebs, 1986).

All four minerals have important functions outside the bone compartment and are widely distributed throughout the soft tissues and fluids (Department of Health, 1991). The remaining 1% of total body Ca is involved in processes such as nerve and muscle function, blood-clotting and intracellular signalling. Non-osseous P is a component of many essential compounds, such as phospholipids and those with high-energy phosphate bonds like ATP. Non-skeletal Mg is involved in DNA replication, RNA synthesis, and is a co-factor for enzymes requiring ATP. Zn is essential for cell division, nucleic acid and protein synthesis and is a component of many enzymes. Unlike the other three minerals, the major portion of total body Zn occurs not in bone but in the soft tissues, primarily in muscle (Table 1), although the concentration of Zn in bone is high.

Table 1. Whole body mineral content and compartments of calcium, phosphorus, magnesium and zinc in the human ^a

To convert mg to mmol divide by 40, 31, 24.3, 65.4 for Ca, P, Mg, Zn respectively.
^a Chemical data from Widdowson & Dickerson (1964).
^b Based on 3.5 kg full-term infant.
^c Based on 60 kg fat-free mass (e.g. man 70 kg body weight, 15% fat).
^d Based on 45 kg fat-free mass (e.g. woman 60 kg body weight, 25% fat).

Considerable quantities of all four minerals are deposited in the body between birth and maturity. The accretion of mineral is greater than the increase in body weight over the same period. For example, the amount of Ca expressed relative to body weight increases from approximately 8 g Ca/kg at birth to 19 g Ca/kg in a 70 kg man (Table 1; Widdowson & Dickerson, 1964). Table 2 gives estimated values, based largely on the compositional data in Table 1, for mineral accretion rates during childhood. The continuous rates have been obtained by assuming that maturity is reached by 18 years of age in both sexes and that the accretion rate is constant throughout childhood (British Nutrition Foundation, 1989). Such calculations indicate, for example, that a boy who at maturity has a fat-free mass of 60 kg (equivalent to a 70 kg man with 15% body fat) has to retain 200 mg Ca, 107 mg P, 4 mg Mg and 0.25 mg Zn every day for eighteen years in order to achieve the required mineral deposition (Table 2). In reality, of course, growth is not uniform and is greatest soon after birth and during adolescence. At these times accretion rates will be considerably higher than average, while rates will be somewhat less in the intervening years. In addition, mineral deposition during periods of catch-up growth, when children are recovering from illness or malnutrition, will be substantially above average. Estimates of likely accretion rates in infancy and in adolescence are given in Table 2, based on the arguments in British Nutrition Foundation (1989), Kanis & Passmore (1989), Fomon (1974) and Leitch & Aitken (1959). The values in infancy for Ca, P, Mg are somewhat lower than the continuous accretion rates due to the relatively low mineral content of young bone (Fomon, 1974).

In addition to the requirements for growth, losses of the four minerals occur in urine, sweat, gastrointestinal fluids, skin, hair and nails. Quantitative data on mineral losses in infants and children are very limited. There is evidence, however, that mineral losses are greatly reduced in individuals habituated to very low intakes (Widdowson & Dickerson, 1964; Begum & Pereira (1969); Nicholls & Nimalasuriya, 1939; Taylor et al. 1991) and it is unclear what figures should be used as estimates of obligatory losses. The British COMA Committee in their recent evaluation of dietary reference values assumed that there are no obligatory Ca losses in children and gave no figures for P or Mg (Department of Health, 1991). Endogenous Zn losses in infants have been estimated at 0.07 mg/kg/d (Ziegler et al. 1989) in faeces and 0.02 mg/kg/d in urine and sweat (Krebs & Hambidge, 1989), producing a total estimated requirement for accretion + losses of 0.9-1.2 mg/d (Department of Health, 1991; King & Turnlund, 1988).

The number of studies on the composition of the human body are very limited, involving the chemical analyses of only a small number of possibly atypical individuals (Widdowson & Dickerson, 1964; British Nutrition Foundation, 1989). In addition, differences in chemical composition may exist between the races; for example adult Blacks in the United States are known to have higher total body Ca and P content than Whites of the same height (Cohn et al. 1977). The figures in Tables 1 and 2, therefore, can be used only to provide an approximate assessment of mineral deposition during childhood. Similarly, the data on endogenous losses in children are insecure and there are likely to be considerable variations between individuals. However, these data provide a useful basis on which to discuss the likely adequacy of dietary supply for children in Third World countries, and for this purpose the following figures, based on the continuous accretion rate for boys (Table 1) + losses for Zn, are a useful rough guide: Ca 200 mg/d, P 100 mg/d, Mg 4 mg/d, Zn 1 mg/d. These figures will be referred to in the rest of the text as the 'biological requirement'.

Table 2. Estimated mineral accretion rates in childhood

a Average accretion rate in childhood based on assumption of continuous growth and maturity at eighteen years in both sexes (British Nutrition Foundation, 1989).

^b Accretion in infancy as calculated in Fomon (1974) and Krebs & Hambidge (1986).

^c Peak rate in adolescence based on calcium calculation of Kanis & Passmore (1989) and assuming proportions of Ca P, Mg, Zn are the same as during continuous growth.