High sodium chloride intake might contribute to muscle wasting via low-grade metabolic acidosis

Kurzfassung

Osteoporosis is a multifactoral musculoskeletal issue: factors that influence muscle likewise affect bone metabolism. Anyway, muscle degradation is caused by mechanical unloading but also by several other environmental stimuli, e.g. in renal failure metabolic acidosis has been linked to muscle wasting. We have recently shown that a high sodium chloride (NaCl) intake induces low grade metabolic acidosis. We therefore hypothesized that high NaCl intake exacerbates disuse-induced protein losses. Eight healthy male subjects (mean age 26.25 ± 3.5; mean body weight: 78.5 ± 4.1 kg) participated in a 14-day crossover bed rest study in the metabolic ward of the DLR – Institute of Aerospace Medicine, Cologne, Germany. Each of the study phases lasted 22 days (5 days adaptation, 14 days bed rest and 3 days recovery). During the bed rest phases every subject received a low NaCl diet (0.7 mEq/kg/day) in one phase and a high NaCl diet (7.7 mEq/kg/day) in the other one. The diet was individually tailored and weight-maintaining with a constant protein intake of 1.3 g protein/kg/day. Blood gases were taken on predefined times. Nitrogen balance was calculated from the difference between nitrogen intake and excretion in 24h urine. The high NaCl intake during bed rest induced a low grade metabolic acidosis (pH: high NaCl: 7.419 ± 0.017; low NaCl: 7.426 ± 0.018, (p = 0.061); bicarbonate: high NaCl: 25.21 mmol/L ± 1.14; low NaCl: 26.4 mmol/L ± 1.36, (p<0.001)) and a more negative nitrogen balance (high NaCl: -1.26g/d ± 2.3; low NaCl: -0.47g/d ± 2.3, (p<0.001)). Our results indicate that high NaCl intake exacerbates disuse-induced protein losses. We conclude that a high NaCl intake is a risk factor for osteoporosis not only due to its influence on bone itself but also because it induces protein wasting, most likely from skeletal muscle. Currently we are investigating if the observed low grade metabolic acidosis with high NaCl intake might be the reason for increased protein losses.