The purpose of this study was to determine the effects of a high-fat/high-sucrose diet on lipid metabolism in rats kept in constant darkness. Four-week-old rats (F344 strain) were kept in constant darkness (D) or normal lighting (12-h light-dark cycle) for 4 weeks. The experimental diets were the AIN-93G diet as a control (C) and high-fat/high-sucrose diet (H). Thirty rats were divided into the following three groups: normal lighting and the control diet (C group), normal lighting and the high-fat/high-sucrose diet (H group), and constant darkness and the high-fat/high-sucrose diet (DH group). Five rats in each group were dissected in the fourth and seventh weeks of experimental treatment. Body weight gain in the H group was greater than that in the DH group in the fourth week and greater than that in the DH and C groups in the seventh week. Liver weight (g/100 g BW) in the DH group was lower than that in the H group in both the fourth and seventh weeks. The weights of perirenal and epididymal fats (g/100 g BW) in the H group and DH group were higher than those in the C group. In addition, perirenal fat weight in the DH group tended to be higher than that in the H group. There were no differences among the three groups in serum total cholesterol concentration in the fourth and seventh weeks. HDL cholesterol concentration in the DH group was lower than that in the C and H groups in the seventh week. The expression of Apo-a1 mRNA in the liver did not differ significantly among the three groups. However, the expression of Abc a-1 in the liver in the DH group was lower than that in the C group and tended to be lower than that in the H group.

　From the above results, it was suggested that the low serum HDL cholesterol concentration in the rats kept in constant darkness and fed a high-fat/high-sucrose diet was induced by the low expression of Abc a-1 mRNA in the liver.