Vegetarians have a reduced skeletal muscle carnitine transport capacity

BACKGROUND: Ninety-five percent of the body carnitine pool resides in skeletal muscle where it plays a vital role in fuel metabolism. However, vegetarians obtain negligible amounts of carnitine from their diet. OBJECTIVE: We tested the hypothesis that muscle carnitine uptake is elevated in vegetarians compared with that in nonvegetarians to maintain a normal tissue carnitine content. DESIGN: Forty-one young (aged ∼22 y) vegetarian and nonvegetarian volunteers participated in 2 studies. The first study consisted of a 5-h intravenous infusion of L-carnitine while circulating insulin was maintained at a physiologically high concentration (∼170 mU/L; to stimulate muscle carnitine uptake) or at a fasting concentration (∼6 mU/L). The second study consisted of oral ingestion of 3 g L-carnitine. RESULTS: Basal plasma total carnitine (TC) concentration, 24-h urinary TC excretion, muscle TC content, and muscle carnitine transporter [organic cation transporter 2 (OCTN2)] messenger RNA and protein expressions were 16% (P < 0.01), 58% (P < 0.01), 17% (P < 0.05), 33% (P < 0.05), and 37% (P = 0.09) lower, respectively, in vegetarian volunteers. However, although nonvegetarians showed a 15% increase (P < 0.05) in muscle TC during L-carnitine infusion with hyperinsulinemia, L-carnitine infusion in the presence or absence of hyperinsulinemia had no effect on muscle TC content in vegetarians. Nevertheless, 24-h urinary TC excretion was 55% less in vegetarians after L-carnitine ingestion. CONCLUSIONS: Vegetarians have a lower muscle TC and reduced capacity to transport carnitine into muscle than do nonvegetarians, possibly because of reduced muscle OCTN2 content. Thus, the greater whole-body carnitine retention observed after a single dose of L-carnitine in vegetarians was not attributable to increased muscle carnitine storage.