L-carnitine is a quaternary ammonium compound required for the transport of fatty acids into mitochondria for oxidation. Carnitine is synthesised endogenously from lysine and methionine with total synthesis producing roughly ≤1µmol L-carnitine per kg body mass per day. However, 2 to 13µmol L-carnitine is obtained daily from the diet, mainly from meat, due to the high L-carnitine content of skeletal muscle. Total body content of L-carnitine in meat eaters is around 2mmol. However, vegetarians have a negligible carnitine intake (0.004 to 0.41µmol L-carnitine per kg body mass per day) and plasma levels can be 20 to 30% lower than non-vegetarians. Therefore it is possible that they have lower total body stores of L-carnitine compared to meat eaters, 95% of which is found in skeletal muscle. Supplemental carnitine in vegetarians results in increases in body concentrations, but it appears that this does not result in increased transport into skeletal muscle.
Researchers1 have investigated the effects of L-carnitine supplementation on 41 healthy vegetarian and non-vegetarian subjects (mean age 22 years). Subjects receives an intravenous infusion of L-carnitine while plasma insulin was raised to the supraphysiological level of 170mU/L in order to stimulate muscle carnitine uptake. In a second experiment, subjects received 3g of oral L-carnitine at a fasting insulin concentration of 6mU/L. Basal levels of plasma carnitine were 16% lower in vegetarians compared to non-vegetarian subjects. In addition, vegetarians had lower levels of carnitine excretion (58%), lower levels of muscle carnitine content (17%), lower levels of muscle carnitine transporter (organic cation transporter 2) messenger mRNA (33%) and a lower muscle carnitine muscle transporter protein expression (37%). When carnitine was infused, non-vegetarians had a 15% increase in carnitine muscle content, but vegetarians showed no significant muscle uptake.
These results suggest that vegetarian muscle tissue has a decreased ability to uptake carnitine and that the increase in carnitine total body content seen in vegetarians after infusion, was not located in skeletal muscle. Vegetarian skeletal muscle appears insensitive to the uptake of carnitine and the carnitine retained following infusion (as measured by excretion rates) must therefore reside in other tissues. The lack of uptake of carnitine into muscle tissue is perhaps due to lack of suitable transporter, as evidenced by a lack of organic cation transporter 2 messenger mRNA and protein. It could be speculated therefore, that those tissues that have a high carnitine requirement such as the heart may have upregulated organic cation transporter 2 activity in vegetarians (to compensate for lower plasma carnitine levels) and are the store of excess carnitine when provided in the diet or via infusion.
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