Vitamin D: introduction
Vitamin D is well known for its essential role in maintaining healthy bones. Deficiency leads to rickets in children and osteomalacia in adults. The majority of vitamin D is produced via skin synthesis under the action of sunlight, with smaller amounts from dietary sources such as oily fish, meat and eggs. Adequate exposure to ultraviolet (UV) light is therefore necessary to maintain levels.
Other physiological roles of vitamin D in maintaining health are now recognised. Some evidence suggests that vitamin D deficiency is associated with increased risk of cardiovascular disease, some cancers, type 2 diabetes and multiple sclerosis. In addition, there is also some evidence that low levels are associated with increased risk of all cause mortality. However, most of the studies relating low vitamin D with higher mortality is derived from observational studies and it is not clear if the association represents a causal relationship. For example, it has been suggested that vitamin D levels are modified by disease processes, rather than the cause of disease processes.
Rickets, the most obvious manifestation of vitamin D deficiency in children, was in the past common in Scotland but its incidence declined rapidly over the 20th century such that it is now uncommon. The majority of cases seen recently in Scotland have been in individuals of Asian or African ethnicity. Vitamin D production is in part dependent on skin pigmentation with paler skinned individuals able to synthesise vitamin D more efficiently under UV exposure; therefore, darker skinned individuals who move to a region experiencing low sunlight are at particular risk of deficiency.
It has been suggested that vitamin D deficiency, resulting from low sunlight exposure, may be in part responsible for the excess mortality seen in Scotland, and particularly in Glasgow, compared to other parts of the UK. However, the excess mortality in Scotland is most marked in younger age groups, i.e. those under 65 years. A systematic review and meta-analysis investigating the association between vitamin D deficiency and premature mortality concluded that although a significant increase in mortality risk was seen with low compared to high concentrations of vitamin D, the risk was lower in the studies that contained younger participants and there was potential for substantial residual confounding. It seems unlikely, therefore, that vitamin D deficiency plays a major role in Scotland’s ‘excess’ levels of mortality.