The C-174G Promoter Polymorphism of the IL-6 Gene Affects Energy Expenditure and Insulin Sensitivity

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Abstract
Interleukin-6 (IL-6) is a pleiotropic cytokine expressed in many tissues. IL-6 null mice show low energy expenditure, but the effect of the variants of the IL-6 gene on energy expenditure has not been previously studied in humans. Therefore, we investigated the effect of the C-174G promoter polymorphism of the IL-6 gene on energy expenditure, measured by indirect calorimetry in healthy Finnish subjects (n = 124). We also measured insulin sensitivity by the hyperinsulinemic-euglycemic clamp. Subjects with the C-174C genotype of the IL-6 gene had significantly lower energy expenditure than subjects with the G-174C or G-174G genotypes both in fasting (CC 13.68 ± 1.98, CG 14.73 ± 1.57, GG 14.81 ± 2.01 kcal · kg−1 · min−1; P = 0.012) and during the euglycemic-hyperinsulinemic clamp (CC 15.24 ± 2.05, CG 16.62 ± 2.06, GG 16.66 ± 2.50 kcal · kg−1 · min−1; P = 0.007). Moreover, subjects homozygous for the C allele had lower rates of whole-body glucose uptake than carriers of the G allele (CC 50.95 ± 13.91, CG 59.40 ± 14.17, GG 59.21 ± 15.93 μmol · kg−1 · min−1; P = 0.016). The rates of both oxidative (P = 0.013) and nonoxidative (P = 0.016) glucose disposal were significantly affected by the IL-6 promoter polymorphism. In conclusion, the C-174C promoter polymorphism of the IL-6 gene influences energy expenditure and insulin sensitivity in healthy normoglycemic subjects. Whether this polymorphism is a risk factor for obesity or type 2 diabetes can be estimated only in prospective population-based studies.

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