Potassium, magnesium, and calcium balance in obese adolescents on a protein-sparing modified fast.

Ten obese adolescents consumed the protein-sparing modified fast (PSMF), a high-protein, low-carbohydrate diet, for 92 +/- 19 d and lost 14.7 +/- 5.3 kg. The effect of weight loss using the PSMF on potassium, magnesium, and calcium was evaluated using balance method at days 2-4 and 12-20, RBC-Mg, RBC-K and total body K (TBK). The vitamin- and mineral-supplemented PSMF allowed positive Ca and K balance and improved Mg balance (p less than 0.005). TBK decreased significantly, 118.7 +/- 13.7 to 97.4 +/- 1.0 g (3.04 +/- 0.35 to 2.50 +/- 0.03 mol) from baseline to 90 d (p less than 0.001), with no change in the RBC-K concentration. An unexpected finding was a significant decrease in RBC-Mg, 3.2 +/- 1.1 to 1.8 +/- 0.3 mmol/L (p less than 0.001) after 60-90 d on the diet despite maintenance of normal serum Mg level.