Downregulation of plasma insulin levels and hepatic PPARγ expression during the first week of caloric restriction in mice

Calorie restriction extends lifespan by decreasing the rate of tumor formation, an effect occurring within 8 weeks of initiating a restricted diet. Our goal was to define how the first weeks of a calorie restricted diet (60% of ad libitum calories) affects putative mediators of the calorie restriction phenotype, focusing on regulators of fatty acid biosynthesis. In C57Bl/6 mice, insulin decreased over 50% (p < 0.05) during the first week of calorie restriction whereas IGF-1 was unaffected. In the liver, PPARγ mRNA fell to 13% of baseline after 1 week of calorie restriction (p < 0.05), whereas hepatic SREBP-1c and SIRT1 mRNA levels were unaffected. No changes in abdominal or subcutaneous adipose tissue were observed until after 4 weeks of caloric restriction. We conclude that calorie restriction-induced decreases in insulin and hepatic PPARγ are rapid enough to support a role for these molecules in triggering the initial phase of the calorie restriction phenotype.