Caloric Restriction Paradoxically Increases Adiposity in Mice With Genetically Reduced Insulin

Antiadiposity effects of caloric restriction (CR) are associated with reduced insulin/IGF-1 signaling, but it is unclear whether the effects of CR would be additive to genetically reducing circulating insulin. To address this question, we examined female Ins1(+/-):Ins2(-/-) mice and Ins1(+/+):Ins2(-/-) littermate controls on either an ad libitum or 60% CR diet. Although Igf1 levels declined as expected, CR was unable to reduce plasma insulin levels in either genotype below their ad libitum-fed littermate controls. In fact, 53-week-old Ins1(+/-):Ins2(-/-) mice exhibited a paradoxical increase in circulating insulin in the CR group compared with the ad libitum-fed Ins1(+/-):Ins2(-/-) mice. Regardless of insulin gene dosage, CR mice had lower fasting glucose and improved glucose tolerance. Although body mass and lean mass predictably fell after CR initiation, we observed a significant and unexpected increase in fat mass in the CR Ins1(+/-):Ins2(-/-) mice. Specifically, inguinal fat was significantly increased by CR at 66 weeks and 106 weeks. By 106 weeks, brown adipose tissue mass was also significantly increased by CR in both Ins1(+/-):Ins2(-/-) and Ins1(+/+):Ins2(-/-) mice. Interestingly, we observed a clear whitening of brown adipose tissue in the CR groups. Mice in the CR group had altered daily energy expenditure and respiratory exchange ratio circadian rhythms in both genotypes. Multiplexed analysis of circulating hormones revealed that CR was associated with increased fasting and fed levels of the obesogenic hormone, glucose-dependent insulinotropic polypeptide. Collectively these data demonstrate CR has paradoxical effects on adipose tissue growth in the context of genetically reduced insulin.