Dietary carnitine intake and carnitine status in endurance-trained males

Nutrition Research Newsletter, Dec, 2006

Carnitine is an amino acid derivative whose primary roles in the human body are in transporting long-chain fatty acids into the mitochondria for use as a fuel and buffering excess acyl-CoA accumulation within mitochondria. Carnitine is predominantly found in skeletal and cardiac muscle. Carnitine homeostasis is maintained from two sources: first, endogenous synthesis from the amino acid precursors methionine and lysine, and second, from exogenous dietary carnitine intake (primarily meat products), reported to be 2 mg to 135 mg daily. The daily requirement of exogenous carnitine in humans is unknown.

Athletes have been considered by some to be at risk of carnitine insufficiency, especially as endurance training over several months has been observed to reduce muscle carnitine concentrations. Athletes also have higher iron requirements than the general population, and a proportion are vegetarians, which could result in lower dietary protein, carnitine and vitamin B6 intakes, and lower iron status. Although the suggestion has been made that exercising humans may achieve performance benefits from ingesting supplemental carnitine, their habitual dietary intake of carnitine has not been well documented nor is it known whether endogenous carnitine synthesis, urinary carnitine excretion, or dietary requirements for carnitine are altered by regular exercise. The aim of the present study was to report the habitual dietary carnitine intakes of endurance-trained adult males, and to determine the relationship between habitual dietary intake and baseline plasma and urinary carnitine concentrations.

Fourteen nonvegetarian endurance-trained males completed a seven day weighed food record and exercise logs to determine habitual dietary carnitine intake. Resting venous blood samples and 24 h urine collections were used to determine plasma carnitine concentration and urinary carnitine excretion.

The mean dietary carnitine intake was 64 (range 21 to 110) mg/day. Mean [ or -] SD resting plasma total carnitine was 44 [ or -] 7 mmol/L and acyl: free carnitine ratio was 0.28 [ or -] 0.11, which were within normal ranges. Urinary carnitine excretion was 437 [ or -] 236 mmol/day. There was no correlation between dietary carnitine intake or dietary macro- and micronutrients and plasma carnitine or urinary carnitine excretion.

The main finding of the present study was that the dietary intake of these endurance-trained subjects provided carnitine intakes comparable to other adults, and the subjects were not deficient in any dietary precursors to carnitine biosynthesis as assessed by seven-day food diaries. Circulating carnitine concentration and urinary carnitine excretion were within reported normal ranges and no subjects had an acyl carnitine:free carnitine (AC:FC) ratio greater than 0.4. No correlation was found between dietary carnitine intake, dietary macro- or micronutrients, and plasma carnitine or urinary carnitine excretion. Therefore, the trained individuals in the present study showed no indication that they were at risk of carnitine insufficiency.

E. Broad, C. Bolger, S. Galloway. Dietary Carnitine Intake and Carnitine Status in Endurance-Trained Males. JADA; Australia. (September 2006). [Correspondence: E. Broad, 44 Tallebudgera Creek Road, Tallebudgera, Qld 4228, Australia. E-mail: lizbroad@aapt.net.au.]