The effect of vitamin D supplementation on mental and functional health outcomes in African Americans with type 2 diabetes

In patients with type 2 diabetes (T2DM), depression increases the risk of poor glycemic control and decreases adherence to medications, exercise, and diet. Studies have shown an inverse relationship between plasma vitamin D (VD) level and depression risk. However, there are few interventional trials of African Americans (AAs), a demographic with higher prevalence of diabetes, depression, and VD deficiency. This randomized controlled trial evaluated the efficacy of vitamin D₃ supplementation [4000 vs. 600 international units (IU)/day] for one year on mental and functional health outcomes in 75 adult AAs with T2DM with serum 25-hydroxy vitamin D (25(OH)D) level < 25 ng/mL. PHQ-9 and PROMIS questionnaires evaluated mental health outcomes, and 6-minute walk test estimated the ability to perform daily activities. At baseline, groups had similar levels of 25(OH)D, calcium, parathyroid hormone, hemoglobin A1c, and body mass index, and 25(OH)D levels correlated positively with a 6-minute walk distance. Surprisingly, both supplementation strategies increased 25(OH)D to > 30 ng/mL by 6 months with a plateau thereafter. Vitamin D₃ 4000 IU/day in AAs with T2DM did not produce significant difference in mental and functional health scores compared to 600 IU/day. Post-hoc analysis of those with baseline VD deficiency [25(OH)D < 20 ng/mL] demonstrated trends towards worsening pain interference and higher depression and fatigue scores throughout the study, plus consistently shorter 6-minute walk distances, most of which were independent of vitamin D supplementation group. These results suggest that VD deficient AAs with T2DM may be refractory to supplementation for improvement in mental and functional health outcomes.