Adiposity-Independent Effects of Aging on Insulin Sensitivity and Clearance in Mice and Humans

Abstract

Objective: Aging is associated with impaired insulin sensitivity and increased prevalence of type 2 diabetes. However, it remains unclear whether aging-associated insulin resistance is due to increased adiposity or other age-related factors. To address this question, the impact of aging on insulin sensitivity was investigated independently of changes in body composition.

Methods: Cohorts of mice aged 4 to 8 months ("young") and 18 to 27 months ("aged") exhibiting similar body composition were characterized for glucose metabolism on chow and high-fat diets. Insulin sensitivity was assessed by hyperinsulinemic-euglycemic clamp analyses. The relationship between aging and insulin resistance in humans was investigated in 1,250 nondiabetic Mexican Americans who underwent hyperinsulinemic-euglycemic clamps.

Results: In mice with similar body composition, age had no detrimental effect on plasma glucose and insulin levels. While aging did not diminish glucose tolerance, hyperinsulinemic-euglycemic clamps demonstrated impaired insulin sensitivity and reduced insulin clearance in aged mice on chow and high-fat diets. Consistent with results in the mouse, age remained an independent determinant of insulin resistance after adjustment for body composition in Mexican American males.

Conclusions: This study demonstrates that in addition to altered body composition, adiposity-independent mechanisms also contribute to aging-associated insulin resistance in mice and humans.

Figure 1 –

(A) Body weight (BW) and composition of young (4–5 months) and aged (18–21 months) mice (n = 9 mice per group). Vertical axis shows mass in grams for BW, Lean and Fat, or body composition as percentage of fat for Fat%. (B) Blood glucose levels in overnight fasted and ad libitum fed mice (n = 9 mice per group). (C) Plasma insulin levels in overnight fasted and ad libitum fed mice (horizontal bars represent median, n = 7 fasted mice per group and 9–10 fed mice per group). (D) Glucose tolerance test (n = 9 mice per group). *, p < 0.05; comparison by 2-way repeated measures ANOVA (left panel) or Student’s t test (right panel). (E) Plasma insulin levels 15 minutes after glucose injection (horizontal bars represent mean, n = 6 young + 7 aged mice). *, p < 0.05; comparison by Student’s t-test. (F) Plasma C-peptide levels 15 minutes after glucose injection (horizontal bars indicate mean, n = 6 young + 7 aged mice). *, p < 0.05; comparison by Student’s t-test. (G) Insulin tolerance test (n = 9 mice per group). Data are presented as mean ± SEM unless stated otherwise.

Figure 2 –

(A-G) Hyperinsulinemic-euglycemic clamp results in young (n = 12; 8 months of age) and aged (n = 11; 27 months of age) male C57BL/6J mice on chow diet. (H-N) Hyperinsulinemic-euglycemic clamp results in young (n = 7; 6 months of age) and aged (n = 5; 24 months of age) diet-induced obese male C57BL/6J mice. (A and H) Blood glucose levels. (B and I) Glucose infusion rate (GIR) during the clamp (left panel) and at steady state (80–120 min; right panel). P value corresponds to the effect of ‘Age’ factor in 2-way repeated measures ANOVA. (C and J) Suppression of hepatic glucose production. (D and K) Skeletal muscle glucose uptake. (E and L) Adipose tissue glucose uptake. (F and M) Plasma insulin levels during the final 20 min (100–120 min) of the clamp. (G and N) Metabolic clearance rate of insulin (MCRI) during the final 20 min of the clamp. (A-E and H-L) Data are presented as mean ± SEM. *, p < 0.05; comparison by Student’s t-test. (F-G and M-N) Horizontal bars represent median. *, p < 0.05; comparison by Mann-Whitney U test.

Figure 3 –

(A) Body weight (BW) and composition of young (6 months) and aged (24 months) mice (n = 8 mice per group) after 9 weeks of HF diet feeding. Vertical axis shows mass in grams for BW, Lean and Fat, or body composition as percentage of fat for Fat%. (B) Blood glucose levels in 5-hour fasted (n = 8 young + 6 aged mice) and ad libitum fed (n = 8 mice per group) mice. *, p < 0.05; comparison by Student’s t-test. (C) Plasma insulin levels in 5-h fasted (n = 8 young + 7 aged mice) and ad libitum fed mice (n = 7 young + 6 aged mice). Horizontal bars represent median. (D) Glucose tolerance test (n = 8 mice per group). *, p < 0.05; comparison by 2-way repeated measures ANOVA (left panel) or Student’s t test (right panel). (E) Insulin tolerance test (n = 8 mice per group). *, p < 0.05; comparison by 2-way repeated measures ANOVA.