Let there be light! And vitamin D pills.

Vitamin D is the ‘sunshine vitamin’ that we make in our skin when exposed to ultraviolet radiation, traditionally from sunlight but also from modern technology like sunbeds^[1]. As depicted in Figure 1, vitamin D is made when UVB light ranging between 280-320 nanometers in the light spectrum transforms a cholesterol-like precursor in the skin. Vitamin D is then released into the bloodstream, where it travels to the liver and kidney for further metabolism into its active forms, 25(OH)D and 1,25(OH)₂D. Vitamin D can also be obtained from the diet in the same form that is released by skin into the bloodstream.

Figure 1: How vitamin D is synthesized using UV light

The metabolite, 25(OH)D, is the most commonly used blood marker of vitamin D status. Although levels for optimal health remain debated, the Endocrine Society states that 25(OH)D levels should be above 30 nanograms per milliliter (ng/ml) for general health, with less than 20 ng/ml considered a deficiency and 20-29 ng/ml considered an insufficiency. Observational evidence^[2][3] has suggested that American adults with 25(OH)D concentrations less than 20 ng/ml have significantly higher LDL-c, lower HDL-c, and higher triglycerides than people above 30 ng/ml. However, a meta-analysis^[4] of randomized controlled trials suggests that vitamin D repletion through oral vitamin D supplementation does not significantly influence these blood lipids.

Although untested, it is possible that oral vitamin D supplements and sunlight-derived vitamin D have different metabolic effects. This would explain the uncoupling between observational evidence and controlled trials regarding vitamin D’s effect on blood lipids. Accordingly, the study under review sought to compare the effect of vitamin D repletion on blood lipids, achieved with the use of either oral supplements or UVB light exposure treatments.

Vitamin D deficiency (less than 20 ng/mL) is associated with a more harmful blood lipid profile than seen in people with healthy levels (more than 30 ng/mL), but interventions correcting deficiency with vitamin D supplementation have shown that blood lipids remain unchanged. Since vitamin D is naturally made in the skin upon exposure to UVB radiation, the study under review sought to test if blood lipids would be changed when vitamin D deficiency was corrected with the use of UVB light treatments, compared to oral vitamin D supplementation.