Vitamin E and vitamin C do not reduce insulin sensitivity but inhibit mitochondrial protein expression in exercising obese rats

Abstract

Controversy exists as to whether supplementation with the antioxidants vitamin E and vitamin C blocks adaptation to exercise. Exercise is a first-line means to treat obesity and its complications. While diet-induced obesity alters mitochondrial function and induces insulin resistance (IR), no data exist as to whether supplementation with vitamin E and vitamin C modify responses to exercise in pre-existing obesity. We tested the hypothesis that dietary supplementation with vitamin E (0.4 g α-tocopherol acetate/kg) and vitamin C (0.5 g/kg) blocks exercise-induced improvements on IR and mitochondrial content in obese rats maintained on a high-fat (45% fat energy (en)) diet. Diet-induced obese, sedentary rats had a 2-fold higher homeostasis model assessment of insulin resistance and larger insulin area under the curve following glucose tolerances test than rats fed a low-fat (10% fat en) diet. Exercising (12 weeks at 5 times per week in a motorized wheel) of obese rats normalized IR indices, an effect not modified by vitamin E and vitamin C. Vitamin E and vitamin C supplementation with exercise elevated mtDNA content in adipose and skeletal muscle to a greater extent (20%) than exercise alone in a depot-specific manner. On the other hand, vitamin C and vitamin E decreased exercise-induced increases in mitochondrial protein content for complex I (40%) and nicotinamide nucleotide transhydrogenase (35%) in a muscle-dependent manner. These data indicate that vitamin E and vitamin C supplementation in obese rodents does not modify exercise-induced improvements in insulin sensitivity but that changes in mitochondrial biogenesis and mitochondrial protein expression may be modified by antioxidant supplementation.

Keywords: acide ascorbique; antioxidants; antioxydants; ascorbic acid; biogenesis; biogenèse; glucose tolerance; insulin resistance; insulinorésistance; mitochondria; mitochondrie; tocopherol; tocophérol; tolérance au glucose.

Fig. 1

Diagram of study timeline. en, energy; HF, high fat; LF, low fat; OGTT, oral glucose tolerance test; Vit, vitamin.

Fig. 2

Supplementation with vitamin C and vitamin E does not block restoration of insulin sensitivity. Following 12 weeks of exercise or sedentary activity on the respective diets, rat underwent an OGTT. (A) Blood glucose levels over time. Note that the glucose over time curves for all HF-fed groups were significantly different than the LF control group. (B) Glucose AUC determinations derived from the glucose responses. Note that exercise while consuming the HF diet did not reduce the glucose AUC. (C) Plasma insulin levels during the OGTT. The insulin versus time curves for the HF sedentary groups was significantly different from those of the other groups. The LF and the exercising groups (HF+Ex and HF+Ex+CE) did not differ. (D) HOMA-IR scores derived from time zero data of the glucose and insulin curves. For (A) and (C), curves were compared using a repeated measures ANOVA with Tukey contrasts. For (B) and (D), data were compared using a 1-way ANOVA with Tukey contrasts and bars with different letters are significantly different (p < 0.05). Data in all graphs are the mean ± SD (n = 13 or 14). *, HF different from LF; †, HF+Ex different from LF; and ‡, HF+Ex+CE different from LF points. AUC, area under the curve; HF, high fat; HF+Ex, high fat with exercise; HF+Ex+CE, high fat with exercise with vitamin E and vitamin C; HOMA-IR, homeostasis model assessment of insulin resistance; LF, low fat; OGTT, oral glucose tolerance test.

Fig. 3

Supplementation with vitamin C and vitamin E modifies mitochondrial changes in response to exercise in skeletal muscle. mtDNA content (A) and mitochondrial protein content (B, C) of multiple muscles were determined following the end of the exercise period for sedentary (LF and HF) and exercising rats (HF+Ex and HF+Ex+CE). The high-fat diet in sedentary rats significantly suppressed mtDNA and mitochondrial protein content. Antioxidant supplementation blocked the ability of exercise to increase mitochondrial protein content. Two-way repeated-measures ANOVA was used to compare the effects on muscle group, dietary treatment, and their interactions. Horizontal bars denote the experimental groups that are significantly different (p < 0.05) within a specific muscle type. HF, high fat; HF+Ex, high fat with exercise; HF+Ex+CE, high fat with exercise with vitamin E and vitamin C; LF, low fat. Data are the mean ± SD (n = 13 or 14).

Fig. 4

Supplementation with vitamin C and vitamin E modifies mtDNA changes in visceral adipose. mtDNA content (A) and mitochondrial protein content (B, C) of visceral, epididymal adipose depots were determined following the end of the exercise period for sedentary (LF and HF) and exercising rats (HF+Ex and HF+Ex+CE). Horizontal bars denote the specific groups that are significantly different (p < 0.05) within a specific muscle type. HF, high fat; HF+Ex, high fat with exercise; HF+Ex+CE, high fat with exercise with vitamin E and vitamin C; LF, low fat. Data are the mean ± SD (n = 13 or 14). Data were compared using a 1-way ANOVA with Tukey contrasts.