Vitamin D supplementation: less controversy, more guidance needed [version 1; referees: 3 approved]

Vitamin D is a secosteroid hormone with multiple functions that extend beyond the regulation of intestinal calcium absorption. In recent years, the publication of research articles investigating associations between vitamin D status and health has reached an all-time high, and an increase in supplementation studies has followed. Given the pleiotropic effects of vitamin D, the scientific focus has gone beyond its known classic benefits on skeletal health to include diabetes and cardiovascular, neurological, respiratory, renal, and liver diseases, yet numerous conflicting findings continue to emerge. This review presents some examples of recent work within the context of controversies surrounding vitamin D and highlights key factors that should be considered when designing vitamin D supplementation regimens.