Vitamin D: introduction

Vitamin D is well known for its essential role in maintaining healthy bones. Deficiency causes bones to become soft and weak: a condition known as 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 evidence 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, as vitamin D levels are reduced by disease processes, low levels may be the result rather than the cause of various illnesses.   It is also possible that the relationship is mediated through the association of vitamin D deficiency with other risk factors, such as lack of physical activity and obesity.

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.

The Scientific Advisory Committee on Nutrition (2016) recommends that individuals should have serum concentrations of vitamin D above a ‘population protective level’ of 25 nmol/L throughout the year.  This concentration is considered sufficient to prevent rickets in children and osteomalacia in adults.  Elsewhere, serum concentrations below 50 nmol/L are considered to be deficient (Bischoff-Ferrari et al. 2006), and higher concentrations such as 75 nmol/L have been recommended for optimal bone health (Hypponen and Power, 2007). 

The effect of having higher than average vitamin D is unclear, with one meta-analysis of observational studies suggesting that pancreatic cancer is more common in individuals with the highest levels. Some of the available observational evidence suggests that all-cause mortality is higher in individuals with both the lowest and highest levels of vitamin D and that health benefits are greatest at more moderate concentrations. There have been few well-conducted randomised controlled trials of vitamin D supplementation in healthy adults and those that have been performed have not shown clear evidence of a benefit from taking supplements. The potential risks of excessive sun exposure must also be considered in the context of attempts to increase vitamin D levels. Clearer evidence is required regarding the health effects, safety and practicality of increasing population vitamin D levels.