Effect of dietary macronutrient content on carnitine excretion and efficiency of carnitine reabsorption.

We examined the effect of macronutrient content on glomerular filtration rate (GFR), and excretion, reabsorption, and filtered load of carnitine. Ten subjects consumed five diets [high protein (HP), low protein (LP), control, high fat (HF), and high carbohydrate (HC)] of equal energy and carnitine content for 6 d each, in a randomized crossover manner. The rate of carnitine excretion was lower after the LP diet than after the HP diet because of lower GFR after the LP diet. The rate of carnitine reabsorption was lower after the LP diet than after the HP diet, also because of the lower GFR after the LP diet. The rate of carnitine reabsorption was not different after the HF and HC diets, nor was GFR. The filtered load of carnitine, however, was greater after the HF diet, resulting in a higher rate of carnitine excretion.