This RCT found that in type 2 diabetes patients, vitamin D combined with exercise intervention had a potentially beneficial effect on glycemic control, although the effect was not statistically significant. Exercise significantly decreased total and trunk body fat percentage, and vitamin D supplementation significantly reduced bone loss.
In addition to the traditional role in maintaining bone health, higher serum 25(OH)D levels have been found to be associated with a low risk of developing type 2 diabetes 7–10. However, to date, the findings in previous studies are inconsistent 12–17, and these studies have mainly been conducted in non-Asian populations. Additionally, accumulating evidence indicates that vitamin D supplementation combined with exercise intervention may have a synergistic effect on the improvement of type 2 diabetes via different glucose utilization pathways 21–24. However, few studies have investigated the synergistic effect of vitamin D supplementation and exercise training intervention on metabolic profiles in type 2 diabetes patients.
Glycemic control is an important metabolic aspect in the improvement and control of type 2 diabetes and has been identified as a risk factor for the development of diabetic complications 32. Although there were no vitamin D and exercise main effects or combined effects on fasting blood glucose, insulin, or HbA1c levels or HOMA-IR, a reduction in glucose levels in the VEG and VDG and an increase in the insulinogenic index in the VDG were observed in this study. However, the effects were not statistically significant, probably due to the relatively small sample size and the relatively short follow-up period. Our results are consistent with those in a study conducted in 42 type 2 diabetes elderly women 33 but inconsistent with those in a study conducted in type 2 diabetes model rats, in which rats were fed with alfacalcidol instead of vitamin D 34. Further studies with larger samples and prolonged follow-up periods are warranted to corroborate the current findings.
Exercise is recommended for both prevention and treatment of type 2 diabetes 20. Consistent with previous findings 35, we found the main effects of exercise on the reduction of body fat and blood triglyceride, but not on glucose profiles. In addition, previous studies showed that improved glucose profile, for example HbA1c levels, were not achieved by aerobic or exercise training alone but were observed in the combination of aerobic and resistance training in type 2 diabetes patients 36, 37. In the present study, exercise training was mainly aerobic training, which probably led to a limited effect on glucose profiles. Moreover, HbA1c is a measure of how well blood glucose has been controlled over a period of about 3 months 38 thus, long-term interventions may be needed to improve HbA1c levels in type 2 diabetes patients with non-insulin dependence.
Weight loss during type 2 diabetes management may lead to a high rate of bone loss, which would increase the risk of future fractures in type 2 diabetes patients 4, 39. In the present study, body weight, BMI, and body fat%, in addition to total and trunk BMC, were significantly decreased in the exercise alone group, but those results were not observed in the other three groups, suggesting that exercise training alone may increase fracture risk in type 2 diabetes patients accompanied with weight loss. Vitamin D is well known for its beneficial effect in maintaining bone health and reducing the risk of osteoporosis 5. Consistently, we found that vitamin D supplementation could efficiently maintain total BMC, trunk BMC, and spine BMD. Thus, a combination of vitamin D and exercise intervention in type 2 diabetes management may help mitigate the adverse effects of exercise training on bone health.
China is facing a rising epidemic of NCDs, with no sign of abating. Prevention of NCDs, including diabetes, by promoting healthy eating and lifestyles has become a national public policy priority 2, 40. In October 2016, China issued the “Healthy China 2030,” thereby bringing the NCDs issue into a sharper and more concrete focus 41. The findings in this study of vitamin D supplementation and exercise training in Chinese type 2 diabetes patients could provide a comprehensive understanding of the development of therapeutic strategies for type 2 diabetes.
The strengths of this study include the 2 × 2 factorial RCT design, high participant retention and adherence to the intervention, and objective and high-accuracy assessment of glycemic control, body fat, and bone mass. Our study provides a novel finding that combined vitamin D and exercise training could not only mitigate potential adverse effects during type 2 diabetes management but also have possible benefits in glycemic control. Some limitations of this study should be noted. First, the study duration was relatively short, although it has been demonstrated that body fat and blood metabolic profile changes can be detected during 12 weeks 42. Second, this study did not comprehensively assess dietary intake, although participants were instructed not to alter their diet; these factors could have affected the results. Thus, future studies should attempt to thoroughly control the diet. Third, the intensity and frequency of exercise training in our study were supervised, and the sample size was relatively small. However, the pre-power analysis showed that the sample size in our study would provide a more than 85% chance to demonstrate the effect.