Significance of Dietary Carnitine for Growth and Carnitine Turnover in Rats

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Abstract
A series of experiments was conducted to investigate the effect of dietary carnitine supplementation on growth rate of rats. Nutritional and environmental factors, such as high fat feeding, low dietary choline and cold exposure that have been suggested to increase the need for carnitine in rats were employed to examine the possible methionine-sparing action of carnitine. Basal diets low in methionine (0.22%) and cysteine (0.04%) were used. Dietary carnitine supplementation (0.2% DL-carnitine) did not exert a growth-promoting effect under the conditions tested. Liver and muscle carnitine concentrations were increased by dietary carnitine or dietary choline supplementation. High fat feeding decreased muscle carnitine levels but increased liver carnitine concentrations. Methionine supplementation greatly enhanced growth but decreased tissue carnitine concentration. Body carnitine content was about 31 µmoles per 100 g body weight for rats fed a low methionine, low carnitine, high fat, choline-deficient diet (diet 4) under normal room temperature conditions and about 44 µmoles per 100 g body weight for rats fed the same diet in cold-exposed conditions. DL-(14C-methyl) carnitine was injected into rats fed diet 4 under normal room temperature conditions to determine the turnover of carnitine. The results suggested that there are two carnitine pools with turnover times of 4.6 and 24.5 days. Based on these results these rats would have to synthesize no more than 1.5 µmoles carnitine per 100 g body weight per day. This would amount to less than 5% of the methionine intake in rats consuming 7 to 8 g of a diet containing 0.22% methionine. These results suggest that even under conditions that have been indicated to increase the demand for carnitine, only a small fraction of the ditary methionine is needed to satisfy this need in rats. A growth-promoting effect by supplemental carnitine, therefore, would be difficult to demonstrate.