Vitamin D and metabolic health with special reference to the effect of vitamin D on serum lipids.

Considering that the vitamin D receptor as well as the 1-α-hydroxylase enzyme that converts 25-hydroxyvitamin D (25(OH)D) to its active form 1,25-dihydroxyvitamin D have been found in tissues throughout the body, it is likely that vitamin D is important for more than the calcium balance. Accordingly, low serum levels of 25(OH)D have been associated with mortality, cardiovascular disease, type 2 diabetes, hypertension and obesity. Low serum levels of 25(OH)D have also been associated with an unfavourable lipid profile, which could possible explain the relation with cardiovascular disease and mortality. However, the relation between vitamin D and lipids have so far received little attention and is therefore the main focus of the present review. A PubMed search identified 22 cross-sectional studies where serum levels of 25(OH)D and lipids were related and that included a minimum of 500 subjects, and 10 placebo-controlled double-blind intervention studies with vitamin D where more than 50 subjects were included. In all the cross-sectional studies serum 25(OH)D was positively associated with high-density lipoprotein cholesterol (HDL-C) resulting in a favourable low-density lipoprotein cholesterol (LDL-C) (or total cholesterol) to HDL-C ratio. There was also a uniform agreement between studies on a negative relation between serum 25(OH)D and triglycerides (TG). On the other hand, the intervention studies gave divergent results, with some showing a positive and some a negative effect of vitamin D supplementation. However, none of the intervention studies were specifically designed for evaluating the relation between vitamin D and lipids, none had hyperlipemia as an inclusion criterion, and none were sufficiently powered. In only one study was a significant effect seen with an 8% (0.28 mmol/L) increase in serum LDL-C and a 16% (0.22 mmol/L) decrease in serum TG in those given vitamin D as compared to the placebo group. Accordingly, the effect of vitamin D supplementation on serum lipids is at present uncertain. Considering the numerous other promising vitamins and minerals that when properly tested have been disappointing, one should wait for the results of forthcoming vitamin D intervention studies before drawing conclusions on potential beneficial effects of vitamin D.