Effect of aging on lipid and carnitine metabolism.

In healthy subjects aged 20 to 50 years, the urinary excretion of carnitine and its serum concentration increased rapidly and markedly after synthetic beta 1-24 ACTH-Z was injected. Their serum triglyceride levels changed inversely. In contrast, healthy subjects older than 70 and patients aged 45 to 50 with atherosclerosis exhibited lower and delayed changes of carnitine excretion and serum concentrations of carnitine and lipid after ACTH injection. When the aged subjects and the atherosclerotic patients were administered with triiodothyronine prior to ACTH, their metabolic responses to ACTH improved towards normal. The results suggest that the aging process impairs the homeostatic regulation of serum lipid through a "lipid-carnitine" system and that thyroid hormone plays a contributory role in the activation of this system.