Vitamin D:
The UK deficiency
nobody talks about

Public Health England estimates approximately one in five UK adults and one in six children have low vitamin D levels. The UK Biobank analysis confirms this pattern at scale across nearly half a million adults. Deficiency is defined as serum 25-hydroxyvitamin D (25(OH)D) below 25 nmol/L. Insufficiency, defined as below 50 nmol/L, affects a considerably larger proportion of the population and is associated with meaningful functional impairment even without clinical deficiency symptoms. Risk is higher in darker-skinned individuals (reduced UVB synthesis efficiency), older adults (declining skin synthesis capacity with age), those working long hours indoors, and anyone living above approximately 52 degrees latitude, which includes most of the UK north of Peterborough.

The NHS recommends 400 IU (10 micrograms) of vitamin D daily for all adults during autumn and winter. This recommendation is based on the dose required to prevent clinical deficiency, specifically rickets in children and osteomalacia in adults. It is not sufficient to raise serum 25(OH)D to levels associated with optimal health outcomes in most UK adults, particularly those starting from a deficient or insufficient baseline. A dose-response meta-analysis found each 100 IU of daily vitamin D raises blood levels by approximately 1–2 nmol/L. Starting from a winter baseline of 25 nmol/L and supplementing at 400 IU, you would expect to reach roughly 29–33 nmol/L, still in the insufficient range. The Endocrine Society clinical practice guideline and multiple expert consensus documents support doses of 1,500–2,000 IU daily for adults, with higher doses for those with documented insufficiency.

Vitamin supplements contain either D3 (cholecalciferol, the form produced in human skin) or D2 (ergocalciferol, found in some fungi and plants). D3 is more effective at raising and maintaining serum 25(OH)D levels than D2 at equivalent doses, this is well-established in head-to-head comparison studies and confirmed by expert consensus. D2 has a shorter half-life and lower binding affinity for the vitamin D-binding protein. The practical consequence is that some cheaper supplements use D2, which performs less well. Look for cholecalciferol (D3) on the label. Most products now use D3, but checking remains worthwhile, particularly for multivitamins and some plant-based formulations where D2 is used to maintain vegan status.

Vitamin D enhances calcium absorption from the gut. This is its primary bone health mechanism. However, absorbed calcium needs to be directed appropriately, to bones and teeth rather than to soft tissue and arteries. Vitamin K2 (specifically the menaquinone-7 form, MK-7) activates osteocalcin and matrix Gla protein, two proteins that direct calcium to bone and prevent its deposition in arterial walls. At higher supplemental vitamin D doses, the case for co-supplementing with K2 is reasonable, as increased calcium absorption without adequate K2 may theoretically increase vascular calcification risk. The evidence for K2 alone is primarily from observational studies and smaller trials; the strongest data comes from bone mineral density outcomes in postmenopausal women. K2 at 90–200 micrograms daily (MK-7 form) is the dose used in most trials.

Vitamin D receptors are present in skeletal muscle, and deficiency is associated with measurable impairment in muscle strength, power output, and exercise recovery. Observational data consistently shows an association between low vitamin D status and reduced grip strength, reduced lower limb power, and increased fall risk in older adults. Intervention trials in deficient individuals demonstrate improvements in muscle function with vitamin D repletion, though the effect sizes are modest and confounding is difficult to eliminate entirely. For athletes and people training seriously, the practical concern is that training in a state of vitamin D deficiency represents an avoidable performance and recovery limitation, particularly relevant during winter months when most UK adults are suboptimal.

Vitamin D receptors are present on virtually all immune cells, including T cells, B cells, macrophages, and dendritic cells. Vitamin D acts as an immunomodulator, promoting innate immune responses to pathogens while modulating adaptive immune responses to reduce excessive inflammation. Deficiency is associated with increased susceptibility to respiratory tract infections, a relationship that became prominent during COVID-19 research but was established in the pre-pandemic literature. A 2017 meta-analysis by Martineau et al. covering 25 RCTs and 11,321 participants found that daily or weekly vitamin D supplementation reduced the risk of acute respiratory tract infection, with the greatest benefit in those who were deficient at baseline. The immune benefit applies to supplementation that corrects deficiency; supplementing on top of adequate levels shows minimal additional benefit.

Serum 25(OH)D testing is available via NHS GP referral (for high-risk groups) or privately via at-home finger-prick tests from several UK providers (typically £25–40). A baseline test before supplementing is ideal but not essential, for the majority of UK adults, particularly those not regularly eating oily fish or spending extended time outdoors in summer, some level of deficiency or insufficiency is likely and supplementation at 1,000–2,000 IU daily is safe, cheap, and appropriate without testing. If you have darker skin, are over 65, work indoors year-round, or have bone health concerns, testing provides useful information. Toxicity from vitamin D supplementation requires sustained high doses, typically above 10,000 IU daily for prolonged periods. At 2,000–4,000 IU, risk in healthy adults is negligible.