Vitamin D in psychiatry — why it is worth checking your level

A vitamin that behaves like a hormone

Vitamin D was discovered as the cure for rickets. Cod liver oil worked, and because it is rich in vitamin A that was assumed to be the active ingredient; only experiments that removed vitamin A showed a separate nutrient was doing the work. For decades it was regarded as a vitamin needed for calcium absorption and bone health, and little else.

Towards the end of the last century, vitamin D receptors were found in most tissues of the body, including the brain. The receptor turned out to influence gene expression and immune function, and vitamin D itself regulates neurotransmitter levels, serotonin among them. That is why it is now better described as a steroid prohormone than as a vitamin.

How does it act on the brain?

• Serotonin: vitamin D takes part in regulating the synthesis of serotonin, a neurotransmitter central to mood.
• Inflammation: it reduces neuroinflammation, which accompanies many mental health conditions.
• Plasticity: it supports the brain's ability to form and rebuild connections.

At Poland's latitude the skin makes almost no vitamin D from roughly October to March. Deficiency is common here regardless of mental health.

Which conditions has it been studied in?

Depression

This is where the evidence is strongest. The starting point was seasonal affective disorder: vitamin D falls in winter, so deficiency was suspected as one cause. Early trials on mood in winter were promising, though not all were positive. Benefit was later shown in a larger trial in people with overweight and depression, and in a trial that added vitamin D to standard antidepressant treatment. Recent meta-analyses confirm that low vitamin D correlates with depression and that supplementation improves depressive symptoms.

The limits of this research are worth knowing. Many trials never measured the starting vitamin D level, or gave everyone the same dose regardless of how deficient they were. Some used doses too small to expect an effect. That is one reason the results are inconsistent.

Suicide risk

Low vitamin D is associated with higher suicide risk. A 2013 study among active duty service members found the lowest levels corresponded to the highest risk. People with depression who had attempted suicide had lower vitamin D and higher markers of inflammation than people with depression who had not. A 2023 meta-analysis confirmed the association with suicidal behaviour.

In a study of more than 300,000 veterans, vitamin D supplementation was associated with roughly half the risk of suicide, and among people with dark skin or with deficiency the risk was about 64% lower. That study was retrospective, built on data collected earlier, so it shows an association rather than proving that vitamin D itself accounts for the difference.

ADHD

Vitamin D appears to be necessary for normal brain development in childhood. A 2019 meta-analysis found significantly lower vitamin D in children with ADHD. A second meta-analysis that year found that vitamin D used as an add-on treatment reduced ADHD symptoms, with no serious adverse effects. Correcting deficiency in pregnancy is associated with a lower risk of ADHD and autism in the child.

Anxiety

In a small trial in people with generalised anxiety disorder and vitamin D deficiency, adding vitamin D to standard treatment lowered anxiety by almost a third, while the group on standard treatment alone did not change significantly. In women with prediabetes and low vitamin D, anxiety fell by 19% on vitamin D alone and by 30% when fish oil was added. In adolescents with premenstrual syndrome and severe deficiency, anxiety scores fell by 61%, against no appreciable change on placebo.

Not every trial was positive. A trial in people aged 60 to 80 found no benefit for anxiety symptoms, and neither did an earlier trial that selected participants only for a low vitamin D level. In that one, symptoms were mild to begin with, leaving little room for improvement.

Other diagnoses

Lower vitamin D has also been described in obsessive-compulsive disorder and in post-traumatic stress disorder. These are observations of level rather than treatment trials, so for now they speak to co-occurrence, not to a benefit from supplementation.

Testing and supplementation

Vitamin D is checked with a single blood test (25(OH)D). Some psychiatrists argue it is worth doing in everyone under psychiatric care. The 2024 endocrine guidelines are more cautious and advise against routine testing in most adults. Opinion is therefore divided, which is why the decision about testing is best made together during a visit.

1. The test: a single 25(OH)D blood measurement, with no special preparation.
2. The dose: set against the result. Severe deficiency may need more than a preventive dose.
3. Follow-up: when treating a deficiency it is worth repeating the test, because some people absorb vitamin D poorly and do not reach a normal level despite supplementing.

Safety

Vitamin D is fat-soluble and accumulates in the body, so very large doses taken for a long time without monitoring can raise blood calcium. Trials using it as an add-on treatment reported no serious adverse effects. Even so, it is not worth taking high doses on your own, without a test result and without advice.

Vitamin D is not a treatment for mental illness and will not replace medication or psychotherapy. It is, however, cheap, easy to check and often low, and correcting it may support the rest of your treatment. Discuss your result and the dose at your visit.