Bone, muscle and immune function: the roles of vitamin D

Vitamin D can be obtained from dietary sources or synthesised by the body due to the exposure to UV radiation from the sun. Although it is more appropriate to refer to vitamin D as a hormone, vitamin D is similar to true vitamins as humans deprived of sun exposure are dependent on a food source. The synthesis of vitamin D in the skin by the action of sunlight is insufficient to meet the needs of northern European countries, especially during winter when there is little exposure to sunlight.¹

The main physiological function of vitamin D in all vertebrates, including human beings, is to maintain blood calcium and phosphorus concentrations at a level that supports cellular processes, the neuromuscular function and bone formation. Vitamin D achieves this goal by increasing the efficiency of the small intestine to absorb calcium and phosphorus from the diet, and by mobilising calcium and phosphorus from bones.¹

An adequate intake of vitamin D is needed to achieve a sufficient amount for the normal mineralisation of bones and teeth during childhood and adolescence, and for bone maintenance in adults and the elderly. Vitamin D deficiency has been shown to reduce the accumulation of minerals in the bones of children and adolescents, and to accelerate bone loss in adults and the elderly.¹

From a public health perspective, the benefits of a higher vitamin D concentration in bone mineral density (BMD) is of interest to different age groups. Higher BMD protects against fractures in older adults, while in younger adults a positive association is related to the concept that higher blood vitamin D levels can contribute to peak bone mass. Accordingly, maintaining high levels of vitamin D in younger adults might further protect against fractures at older ages.²

Apart from the effects on calcium metabolism and bones, vitamin D, and especially its biologically active metabolite, act as powerful immune-regulators. The discovery that vitamin D exerts activity in monocytes, macrophages and immune organs such as the thymus suggests a specific role for vitamin D and its metabolites in the immune system. Most cells of the immune system, except B cells, have got an affinity for vitamin D.³