tion to weight or height. For example, from Chapter 5, the RDA and thus the recommended daily intake of phosphorus for women aged 19 through 30 years is 700 mg (22.6 mmol)/day. This would be the case for a woman in this age range weighing 50 kg (110 lb), 55 kg (121 lb), or 70 kg (154 lb).
One would expect larger individuals to have larger skeletal mass and therefore a greater requirement for calcium, phosphorus, magnesium, vitamin D, and fluoride. However, given the variety of research designs and subject variability used in the studies that provided the data for deriving the DRIs, it would be somewhat misleading to express them per kilogram of body weight or per centimeter of height. This would imply a greater precision in the estimate than is possible, given the available data. Reference weights are provided (Table 1-3) to allow a calculation, when necessary, of the amount per unit of body weight for individuals who are outside the typical range of body size. For some nutrients (for example magnesium), requirements may be closely related to lean tissue; for others, the relationship is weak.
Needs for energy are not necessarily useful for adjusting nutrient needs for this group of nutrients. For example, the RDA for phosphorus would be the same for an 18-year-old long-distance runner whose energy needs were 4,000 kcal/day and an 18-year-old sedentary individual with energy needs of 2,000 kcal/day.
From Chapter 6, the RDA for magnesium is 130 mg (5.4 mmol)/day for healthy boys and girls ages 4 through 8 years. This recommended intake, consumed on a daily basis, on average, would allow essentially all children to achieve the positive magnesium balance needed for normal growth. Because the RDA was based on studies of requirements in children or adolescents with a normal range of body weights for their age, a reference weight and height for the age group are given. It is thus possible to determine the amount of the nutrient per kilogram of body weight that is recommended and to use this value for adjusting the RDA for individuals in the age category whose weights and heights deviate substantially from the reference. This might be done, for example, for small 4-year-old children or for large 8-year-old children.
The RDA is of limited use in assessing the adequacy of an individual 's nutrient intake. An individual's nutrient requirement is never known with certainty. If the individual 's intake, on average, meets or exceeds the RDA, there is good assurance that the intake is ade -