A third of Britons are deficient in vitamin D during the winter months, when sunlight is too weak for the skin to manufacture the nutrient, prompting the NHS to recommend that almost everyone consider taking a daily supplement. The advice, backed by the Department of Health and Social Care, applies year-round for those at particular risk and is especially important from autumn through spring, when around 57% of UK adults have been recorded with low levels.
NHS recommendations
The standard NHS recommendation is 10 micrograms (mcg) of vitamin D per day for most adults and children aged four and over. Babies under one year need 8.5 to 10 mcg daily unless they consume more than 500 ml of infant formula. Pregnant and breastfeeding women, as well as children aged one to four, are also advised to supplement. People with darker skin — including those of African, African-Caribbean or South Asian backgrounds — are at higher risk of not making enough vitamin D from sunlight because of higher melanin levels, and are advised to take supplements throughout the year. The same advice applies to those who are housebound, spend little time outdoors, or are obese, a condition associated with lower vitamin D levels. Smoking also correlates with lower levels, though the exact reason remains unclear. Regional variations exist: Scotland, the West Midlands and Northern Ireland report the lowest average levels in the UK (73 nmol/L), slightly below the optimal threshold of 75 nmol/L, while the South West of England has the highest (77 nmol/L). Ethnic disparities are marked: a 2016 health report found average vitamin D levels were 45.8 nmol/L in white adults, compared with 20.5 nmol/L in Asian adults and 27.7 nmol/L in Black adults.
Cognitive benefits: the potential for children
The most extensively discussed area of emerging research concerns the effect of vitamin D on the developing brain. Vitamin D receptors are present in several areas of the brain, and lower blood levels have been associated with a higher risk of dementia. Research has also found that higher vitamin D concentrations in brain tissue correlate with better cognitive function, and a meta-analysis indicated that a 10 nmol/L increase in 25(OH)D levels could reduce dementia risk by 5% and Alzheimer’s disease risk by 7%.
But the most prominent new evidence comes from a study published this week in JAMA Network Open, which examined whether high-dose vitamin D supplementation during pregnancy could improve children’s cognitive outcomes. Standard prenatal vitamins already contain 10 mcg (400 IU) of vitamin D per day. The trial compared that dose with a four-times-higher tablet — roughly 2,800 IU per day — in about 500 women. By the time the children reached ten years of age, those whose mothers had taken the higher dose performed better in some verbal and visual memory tests. The findings could carry significant public health potential, according to Professor Andrew Shennan, an obstetrician at King’s College London who was not involved in the study.
However, Professor Shennan cautioned that the results might be a statistical fluke, as the children were given several types of intelligence tests and the high-dose group did not score better across all of them. The analysis was secondary, meaning it was not the original primary outcome of the trial, and the improvements were modest. No significant gains were seen in overall intelligence or most other cognitive functions. Experts stress that these findings do not justify changing current clinical recommendations for pregnant women, and any higher-dose supplementation should be discussed with a healthcare professional.
Additional research has linked higher serum vitamin D levels to increased levels of brain-derived neurotrophic factor (BDNF), a protein crucial for brain health, and better cognitive performance in older adults.
Fighting infections
Vitamin D receptors are found in many types of immune cells, and observational studies have indicated that people with lower vitamin D levels are more susceptible to respiratory infections such as colds and flu. During the Covid-19 pandemic, some researchers argued that supplements could help the UK population fight the virus, and clinically vulnerable people were offered free tablets. However, randomised controlled trials — the gold standard of medical evidence — have given mixed results on whether vitamin D is effective as a treatment for infections. The evidence for a causal link between vitamin D status and Covid-19 severity or mortality remains lacking.
Dampening pain
Vitamin D appears to selectively modulate the immune system rather than simply revving it up, and this balancing act may influence pain. Chronic inflammation, driven by overactive immune cells, contributes to many medical conditions including persistent pain. Some previous studies have found that people with low vitamin D are more predisposed to long-term pain. A study published this week in Regional Anesthesia & Pain Medicine examined women after surgery for breast cancer. Among 184 women, those who were vitamin D deficient were three times more likely to report moderate or severe pain after the operation and required more opioid medication. “Vitamin D supplementation in breast cancer patients with [low] vitamin D levels may have a role in modulating postoperative pain,” said Dr Mohamed Hasan Ragab, an anaesthetist at Fayoum University in Egypt and an author of the study.
However, the picture for chronic pain is more complex. While some research indicates that severe deficiency is linked to chronic widespread pain, a Cochrane review found no consistent pattern that vitamin D treatment was better than placebo for any chronic painful condition, citing low-quality evidence. Certain medications used for chronic pain and rheumatic conditions, such as anticonvulsants and steroids, can also reduce vitamin D levels.
Fighting multiple sclerosis and rheumatoid arthritis
Autoimmune diseases — in which the immune system attacks the body’s own tissues — are another area where vitamin D appears to play a protective role. Low vitamin D status has long been considered a significant risk factor for conditions such as multiple sclerosis (MS), which involves nerve damage, and rheumatoid arthritis, which causes painful joints. Vitamin D influences key immune cells, including dendritic cells and T cells, potentially by hindering the activation of T cells that in autoimmune conditions attack healthy tissue.
A major trial in 26,000 people — the VITAL study — found that taking vitamin D supplements for five years reduced the incidence of autoimmune conditions as a group by 22%. The effect was more pronounced in individuals with a body mass index (BMI) under 25. Research also suggests significant effects on autoimmune thyroiditis, connective tissue disease, and autoimmune neuromuscular diseases. Genetically predicted vitamin D levels have been linked to a lower risk of psoriasis and suggestively with systemic lupus erythematosus.
How to know if you have low vitamin D
GPs will offer a blood test for vitamin D levels if patients have symptoms such as bone pain or muscle weakness, or if they belong to certain high-risk groups, including those aged 65 or over. However, NICE guidelines advise against routine testing for the general population, even for those at high risk, unless symptoms are present. Testing is reserved mainly for individuals with established bone disease, suspected osteomalacia, or specific medical conditions that predispose to deficiency.
Supplements are available over the counter in pharmacies and supermarkets. Women and children eligible for the Healthy Start scheme can obtain free vitamin D supplements. But excessive intake can be dangerous. Prolonged high-dose supplementation can cause hypervitaminosis D, leading to hypercalcaemia — dangerously high blood calcium levels that can weaken bones and damage the kidneys and heart. Symptoms include nausea, vomiting, confusion, weakness, fatigue and kidney problems. The safe upper limit for daily intake in adults is generally 100 mcg (4,000 IU). Children aged one to ten should not exceed 50 mcg (2,000 IU) daily, and infants under 12 months should not exceed 25 mcg (1,000 IU) daily. People should follow their physician’s advice and not take more than the recommended amount.