Reactive oxygen species (ROS) generated by oxidative stress induced by physical activity are important mediators responsible for adaptive processes in skeletal muscle (1). ROS induce the production of myokines (muscle-derived cytokines) that mediate systemic metabolic effects (1) and induce an increase in the expression of genes involved in mitochondrial biogenesis (2). Therefore, the oxidative stress induced by exercise may play an important role in the cellular responses occurring within skeletal muscle after training.
Supplementation with the antioxidant vitamin C pharmacologically inhibits positive effects of exercise on the parameters of glucose metabolism (3). However, these data conflict with the finding that high levels of vitamin C also reduce the risk of type 2 diabetes (4). Therefore, we aimed to examine whether vitamin C levels are associated with a negative outcome of lifestyle intervention (LI). We analyzed the effect of baseline plasma vitamin C levels on changes in postprandial glucose and insulin sensitivity during 9 months of an exercise and dietary LI, the Tuebingen Lifestyle Intervention Program (TULIP), in 86 nondiabetic subjects at increased risk for type 2 diabetes. At least one of the following risk factors was present in every subject: BMI >27 kg/m2, first-degree relationship to a patient with type 2 diabetes, impaired glucose tolerance, or a history of gestational diabetes.
Vitamin C levels were measured at baseline by reversed-phase high-performance liquid chromatography (BioTeSys, Esslingen, Germany). The study was approved by the local ethics committee. The statistical software package JMP (SAS Institute, Cary, NC) was used. Baseline vitamin C levels were found to be 51.9 ± 1.7 μmol/l (range 26.2–93.1). BMI (29.9 ± 0.7 vs. 29.1 ± 0.7 kg/m2, P < 0.0001) was reduced by LI, and 2-h glucose decreased during oral glucose tolerance tests (6.9 ± 0.2 vs. 6.6 ± 0.2 mmol/l, P < 0.01). This was accompanied by an increase in insulin sensitivity estimated from oral glucose tolerance tests using a validated index (5) (13.9 ± 0.8 vs. 15.7 ± 0.8 arbitrary units, P < 0.01). In multivariate regression models, baseline vitamin C levels were negatively associated with BMI at baseline (P < 0.001) independent of sex and age, but not with the change in BMI (P = 0.6) independent of sex, age, and traits at baseline. High levels of vitamin C were associated with favorable changes in postprandial glucose (P = 0.04) and insulin sensitivity (P = 0.04) independent of sex, age, and traits at baseline. The effects of vitamin C levels lost significance after adjusting for changes in adiposity (P = 0.12 and P = 0.13, respectively).
We analyzed the effects of vitamin C levels on changes in metabolic traits related to glucose metabolism during LI. We cannot demonstrate any negative effects of high vitamin C levels on the outcome of LI with respect to the studied phenotypes. In contrast, antioxidants supplemented in pharmacological doses have recently been shown to prevent the positive effects of exercise on glucose metabolism (3). In this context it is important to note that the results from the present study were not obtained in a laboratory setting but close to reality. In summary, we conclude that vitamin C ingested with a healthy diet containing fruits and vegetables does not reduce the positive effects of LI.
Acknowledgments
No potential conflicts of interest relevant to this article were reported.
