maximal retention of magnesium. This is unlike calcium, for which maximal retention can be associated with benefit to bone mass accretion. Since the criterion of maximal retention could not be used for magnesium, magnesium balance data were used as the basis for establishing Estimated Average Requirements (EARs) for these age groups.
As mentioned previously, an adequate accretion rate (positive balance) for girls 10 to 12 years of age and weighing about 40 kg (88 lb) may be 8.5 mg (0.3 mmol)/day (Andon et al., 1996). It is probable that for older children, who are heavier and experiencing greater growth in lean and bony tissue, a positive balance in the range of 10 mg (0.4 mmol)/day of magnesium would be appropriate. This would allow for the greater need for magnesium during the specific periods of faster growth during older childhood. In the absence of more definitive goals, a daily positive balance of 8 to 10 mg (0.3 to 0.4 mmol) of magnesium seems to be a reasonable goal upon which to base an EAR for growing children and adolescents.
Balance Studies. For children under 10 years of age, there is only one report of a balance study published since 1960 (Schofield and Morrell, 1960). For children between 10 and 15 years of age, seven studies were available for consideration (see Table 6-1). The amount of magnesium lost via other routes (dermal, sweat, menses, and other losses) was not measured or estimated in the calculations of any of these studies. Other balance studies performed in children prior to 1960 (see review by Seelig ) were not considered because information regarding absorption over a range of intakes was not provided and results reported may not be reliable using the analytical methodology available at that time.
Given the information provided in the available balance studies, expression of magnesium requirements for children is probably more accurate on the basis of intake per day, rather than per unit of body weight or per amount of lean tissue. When expressed on a mg/kg/day basis, magnesium requirements determined by balance studies in subjects who were obese were much lower than those in subjects of normal weight (Jones et al., 1967), as fat contains less magnesium than other tissues and body fat increases with age. Expressing EARs and Recommended Dietary Allowances (RDAs) per kg ideal body weight or lean body mass would be more accurate than per kg total weight. However, since most reports of balance studies do not provide individual intake and body weight or height data, it is seldom possible to determine the response of individuals