The combined effect of high intensity intermittent training and vitamin D supplementation on insulin sensitivity in overweight and obese males and females

High intensity intermittent training (HIIT) is a time efficient mode of exercise, which has been shown to induce metabolic benefits, such as an improvement in insulin sensitivity (IS) and thus type 2 diabetes (T2D). Vitamin D₃ (25(OH)D₃) supplementation has also been shown to influence the pathogenesis of IS. The aim of this study was to investigate if there is a combined effect of HIIT and 25(OH)D₃ supplementation on IS.Twelve inactive and overweight adults (9 male, 3 female; age: 32 ± 18-45 y, BMI: 31.9 ± 2.8 kg·m²) performed HIIT 3 times/week for 6 weeks, with oral glucose tolerance tests and peak oxygen consumption (V̇O₂peak) tests done at baseline and post-intervention. The HIIT protocol consisted of 10 x 1 min intervals cycling at 97 ± 8 % V̇O₂peak separated by 1 min active recovery. Participants were randomised to ingest 4000 IU/day 25(OH)D₃ (n=6) or a placebo (n=6). Plasma glucose, insulin, 25(OH)D₃, adiponectin, leptin, and the lipid profile were analysed pre and post training.Peak V̇O₂ and power output was increased in all participants (P<0.01). Systolic blood pressure (BP) was reduced in all participants and the vitamin D group (P<0.05) but not the placebo group. Insulin area under the curve (AUC) was significantly reduced by 16.6% in the placebo group (P<0.05) but not overall for all participants or the vitamin D group. There was no change in fasting glucose or glucose AUC. Insulin sensitivity index (ISI) and homeostatic model assessment of insulin resistance (HOMA-IR) remained unchanged across all groups. All participants were 25(OH)D₃ deficient (<20 ng·ml-¹ ) at baseline, with an increase in 25(OH)D₃ in the vitamin D group after 6 weeks (P<0.05). Adiponectin decreased in the placebo group but was unaltered in the vitamin D group. Leptin remained unaltered in all groups. Plasma triglycerides were reduced in all participants (P<0.05).In conclusion, 6 weeks of HIIT increased physical capacity but had no effect on the ISI and HOMA-IR in overweight and obese males and females. The findings showed that 6 weeks of supplementation resulted in an increase in 25(OH)D₃ and an improvement in systolic BP