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.