Vitamin D functions more as a hormone than a vitamin. It regulates the expression of over 1,000 genes across virtually every tissue type in the body, including immune cells, muscle tissue, brain tissue, and cardiovascular tissue. Deficiency, defined as serum 25-hydroxyvitamin D below 50 nmol/L, is extremely prevalent in northern European populations including those living on the Isle of Man, where UVB radiation sufficient for skin synthesis is available for fewer than four months of the year.
Vitamin D and Immune Regulation
Vitamin D receptors are present on virtually every immune cell type. Activated vitamin D enhances the innate immune response by upregulating the production of antimicrobial peptides including cathelicidin and defensins, while simultaneously modulating the adaptive immune system to prevent excessive inflammatory responses. Adults with vitamin D deficiency show higher rates of respiratory infections, slower wound healing, and higher circulating inflammatory markers.
Vitamin D and Muscle Function
Vitamin D receptors are present in skeletal muscle, and deficiency is independently associated with reduced muscle strength, impaired muscle protein synthesis, and higher rates of falls in older adults. Research from Harvard found that adults with the lowest vitamin D levels had a 60 percent higher risk of muscle weakness compared to those with optimal levels. Supplementing vitamin D in deficient adults consistently improves grip strength and physical performance.
Dietary Sources and Supplementation
- Oily fish: salmon, mackerel, and sardines provide 200 to 600 IU of vitamin D3 per portion
- Egg yolks: approximately 40 IU each, with pastured hens producing significantly more
- Fortified dairy or plant milks: typically provide 100 to 120 IU per serving
- Liver: very high in vitamin D3 but should not be eaten daily due to high vitamin A content
- The UK government recommends 400 IU daily supplementation for all adults from October to March; adults with limited sun exposure may benefit from 1,000 to 2,000 IU year-round