Body weight reduction by exercise increases the urinary 3-methylhistidine excretion level with relatively positive nitrogen, sodium, and potassium balances when compared to dietary restriction.

Background: Regarding changes in muscle mass, differences due to types of exercise and/or nutritional interventions, and associations with nutrient balances are still unclear. Methods: To estimate changes in muscle mass during a body weight loss program using either a diet or exercise regimen, we investigated levels of muscle mass-related indices, and body contents of nitrogen, sodium, and potassium as measured by the balance method. Six healthy young adult male volunteers participated in two 10-day crossover experiments (20 days total). The first 5 days comprised an adjustment period (energy intake, 2656 ± 367 kcal/day (mean ± standard deviation)). During the second 5-day period, the participants either reduced their energy intake to 1770 ± 244 kcal/day (diet regimen) or exercised on a bicycle ergometer to expend 886 ± 122 kcal/day (exercise regimen). Results: The nitrogen, sodium, and potassium balances were significantly more positive during the exercise regimen than during the diet regimen. The urinary excretion levels of creatinine, 3-methylhistidine (3-MH), aldosterone, and catecholamines, and the 3-MH/creatinine ratio were significantly increased only during the exercise regimen. Conclusions: The exercise regimen suppresses the decrease in muscle mass-related indices during body weight loss compared to the diet regimen with a relatively positive state of whole-body protein, sodium, and potassium balances, accompanied by an increase in sympathetic/adrenal cortical functions.