One striking observation is that a large portion of the inter-person variation in serum 25-hydroxyvitamin D (25(OH)D) levels is unexplained. In vitro and in vivo studies indicate vitamin D synthesizing and metabolizing enzymes are Mg-dependent. Magnesium (Mg) supplementation substantially reversed the resistance to vitamin D treatment in patients with magnesium-dependent vitamin-D-resistant rickets. The investigators reported in 2013 from observational studies conducted in the general US population that Mg intake significantly interacted with vitamin D intake in affecting vitamin D status as well as interacted with serum 25(OH)D in risk of cardiovascular disease mortality and, maybe, colorectal cancer mortality. The potential interaction between Mg and vitamin D was supported by two subsequent studies, including a Finnish cohort study and a mouse study.