Responses of plasma magnesium and other cations to fluid replacement during exercise.

Changes in plasma magnesium (Mg) and other cations in response to exercise have been previously reported, but whether ingestion of fluids providing carbohydrate and electrolytes modifies the changes has not been determined. We examined patterns of change in plasma Mg, calcium (Ca), sodium (Na), and potassium (K) in 10 men [age 30 +/- 2 years; maximal oxygen uptake (VO2max) 57.4 +/- 3.2 ml.kg-1.min-1] who ran on a treadmill for 2 hours at 60-65% of their VO2max. Subjects drank 200 mL of water (W) or a 7% glucose polymer/fructose/electrolyte solution (GPFE) at 0 time and every 30 minutes while running (30, 60 and 90 minutes). Plasma Mg continued to decline throughout exercise and was lowest when exercise was terminated at 120 minutes (87.9 +/- 1.2 and 88.9 +/- 2.2% of pre-exercise for W and GPFE, respectively). In contrast, a significant increase in serum K was noted (105.2 +/- 3.6 and 112.8 +/- 3.0% of pre-exercise at time 120 for W and GPFE). Serum Na increased slightly, but no changes in plasma Ca were observed. Serum glucose and lactate increased transiently for all running conditions, but no differences across fluid treatments were noted. Serum free fatty acids (FFA) increased during exercise with W ingestion, whereas no rise in serum FFA occurred with GPFE until the end of exercise. That patterns of change in Mg and K during exercise were not altered by providing a fluid replacement beverage containing glucose polymer, fructose, Mg, K, Ca, and Na, as compared to water, suggests that these events may be requisite to maintenance of homeostasis. Mechanisms must be sought to explain the decline in plasma Mg that occurs with exercise.