Gender and age differences in plasma carnitine, muscle strength, and exercise tolerance in haemodialysis patients.

BACKGROUND Haemodialysis patients have impaired exercise tolerance that may be related to impaired carnitine availability and/or alterations in carnitine metabolism. As carnitine metabolism is sex- and age-related, we examined plasma free and esterified carnitine concentrations, muscle strength, and estimated exercise tolerance in female (n=51) and male (n=63) patients of different ages (18-86 years). METHODS Concentrations of free carnitine and acetylcarnitine were determined in the plasma of patients. Isometric handgrip strength was measured using a dynamometer while exercise tolerance was predicted from scores for self-reported walking distance on the level and on an incline. RESULTS Plasma total carnitine concentration in the females was significantly lower than that of males (35.4+/-1.3 and 42.4+/-1.4 micro mol/l, respectively, P<0.01). This was almost entirely due to the lower plasma free carnitine concentration of females when compared with the males (20.6+/-0.9 and 26.3+/-1.1 micro mol/l, respectively, P<0.05). Furthermore, plasma free carnitine concentrations were negatively correlated with age in females (r=-0.45, P<0.001) even when the linear effects of haemoglobin, albumin, body weight, time on dialysis, and muscle strength were removed from the regression analysis (partial correlation coefficient; pcc=-0.43, P=0.018), but not in the males (r=0.03, P>0.05). Isometric handgrip strength and estimated exercise tolerance on the level were lower in females than males (138.9+/-10.9 and 259.0+/-15.2 N, P<0.001; and 3.5+/-0.2 and 4.3+/-0.2, P<0.05, respectively). Isometric handgrip strength and estimated exercise tolerance (walking distance on the level and on an incline) were positively correlated with plasma free carnitine concentrations in females, but not in the males. CONCLUSIONS There was a strong relationship between plasma free carnitine concentrations and age in female patients, but not in males. It is unlikely that the reduction in plasma free carnitine in females was a direct causative factor in their reduced exercise tolerance, but probably reflects greater muscle de-conditioning/atrophy with age in female patients.