tion near balance (Beaton, 1996). The intake associated with zero balance from each individual studied can then be grouped and the variability of requirements estimated (Beaton, 1996). Since magnesium intake is related to energy intake (Clarkson and Haymes, 1995; Niekamp and Baer, 1995), balance studies may report the findings in relation to estimated energy requirements if the data are to be applied to the general population. Magnesium may be obtained from food, water, nutrient supplements, or pharmalogical agents, but the bioavailability may differ.

Although numerous magnesium balance studies have been performed, not all met the requirements of a well-designed study. Some provided for a period of adaptation but did not include magnesium intakes, which would have allowed average requirements to be estimated. Balance studies performed prior to 1960 utilized less accurate means to measure magnesium as compared with atomic absorption spectrophotometry. The minimum criteria used here for inclusion of balance studies for the development of recommendations for magnesium requirements included either an adaptation period of at least 12 days or a determination of balance while the subjects consumed self-selected diets. The disadvantage of using self-selected diets is that only one level is being evaluated. If the individual is in balance or nearly so, it is not possible to discern if this is just an adaptation to that level or if it truly represents the minimal level of adequacy. Similarly, if only one level of intake is provided, no matter how accurate, it is not possible to get the dose response data necessary to estimate the requirement. At least two levels need to be evaluated: one below and one near the required level.

When a study is not carried out in a metabolic unit or under close supervision, data are generally lacking on magnesium intake from water. This omission precludes the use of many of the earlier studies conducted in free-living environments or current studies in which intakes were calculated rather than analyzed.

Estimates of Tissue Accretion During Growth

In the absence of data regarding specific requirements for magnesium for growth during various life stages, accretion rates of magnesium have been estimated (Nordin, 1976; Widdowson and Dickerson, 1964). The rates of tissue accretion during childhood are derived from analysis of cadavers, and the utility of these data is limited. In some cases using data from cadavers, the estimates of whole body mineral retention must be calculated based on measurements from regional sites (Fomon and Nelson, 1993). Fomon

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