protein given the level of magnesium provided (258 mg [10.8 mmol]/day). Other studies in adolescents have shown improved magnesium absorption and retention when protein intakes were higher (93 versus 43 g protein/day) (Schwartz et al., 1973).
Physical Activity. Dietary magnesium intake in athletes has been reported to be at or above recommended intakes (Clarkson and Haymes, 1995; Kleiner et al., 1994; Niekamp and Baer, 1995), presumably due to their higher food intake. Plasma/serum magnesium concentrations have been reported to fall with chronic endurance exercise activity, while red blood cell values appear to rise (Deuster and Singh, 1993). Although the decrease in plasma magnesium has been suggested to reflect magnesium depletion in athletes (Clarkson and Haymes, 1995), no clear demonstration of magnesium depletion directly related to exercise has been shown. Magnesium supplements did not enhance performance in a study of marathon runners (Terblanche et al., 1992).
The U.S. Department of Agriculture Continuing Survey of Food Intakes by Individuals (CSFII) in 1994, adjusted by the method of Nusser et al. (1996), indicated that the mean daily magnesium intake in males aged 9 and older was 323 mg (13.5 mmol) (fifth percentile = 177 mg [7.4 mmol]; fiftieth percentile = 310 mg [12.9 mmol]; ninety-fifth percentile = 516 mg [21.5 mmol]) (Cleveland et al., 1996) (see Appendix D for data tables). The mean daily intake for females aged 9 and older was 228 mg [9.5 mmol] (fifth percentile = 134 mg [5.6 mmol]; fiftieth percentile = 222 mg [9.3 mmol]; ninety-fifth percentile = 342 mg [14.3 mmol]). In both sexes, intake decreased at age 70 and older. These intakes were similar to those found in the National Health and Nutrition Examination Survey (NHANES) III from 1988–1991 (Alaimo et al., 1994). National survey data for Canada are not currently available. Other surveys have reported lower intakes in both men and women (Hallfrisch and Muller, 1993).
The NHANES III study demonstrated ethnic differences in intake. In that report, non-Hispanic black subjects were found to consume less than either non-Hispanic white or Hispanic subjects. Another study demonstrated that elderly Hispanic males consumed a