Comparison of the Activity of the Tryptophan-NAD Pathway between Rats Fed a Fat-free Diet and a Fat Diet

The effect of dietary fat on tryptophan-NAD metabolism was investigated. Weanling male rats of the Sprague Dawley strain were fed a 40% casein diet (nicotinic acid-free) with or without 20% fat for 13 days. Although the food intake in 13 days was significantly higher in the fat-free group than in the fat group, the gains in body weight in the two groups were almost the same, because of the same energy intakes. The urinary excretion of tryptophan metabolites such as quinolinic acid, niacin and N¹-methylnicotinamide was greatly increased in the fat group in comparison with that in the fat-free group. The urinary excretion of xanthurenic acid was almost the same in the two groups. The blood NAD level of the fat group was significantly increased. The activities of liver amino-carboxymuconate-semialdehyde decarboxylase and liver nicotinamide methyltransferase in the fat group were significantly reduced, and that of liver NMN adenylyltransferase was significantly increased. The changes of these three enzymes could be advantageous for the increased formation of NAD from tryptophan. As a result, the feeding of a high fat diet to rats increased the formation of niacin and niacin-related compounds.