How do I know whether I am getting enough vitamin D, and should I be supplementing?

A blood test measuring your 25-OHD level is the only reliable way to know your vitamin D status. Keep in mind that some of the vitamin D in your blood is bound to a protein; only the free fraction is biologically active. During pregnancy or with liver problems, that ratio can become skewed. If your result is around a borderline value, it is worth discussing with your doctor.

In children, the link between vitamin D deficiency and bone problems such as rickets is well established and accepted as causal. Getting enough vitamin D through sunlight, diet or supplements is therefore clearly important for children. Older people are also at greater risk: their skin produces less vitamin D when exposed to sunlight, they tend to go outside less, and their diet is sometimes less varied. For older adults, a blood test is therefore especially worthwhile.

Beyond bone and muscle health, the picture is far less clear. Observational studies do identify associations between low vitamin D levels and cardiovascular disease, but that is not evidence of causality. Mendelian randomisation studies, which use genetic variants to test for causality, do not support that association. Large randomised trials, the highest standard in medical research, likewise find no evidence that vitamin D supplements reduce the risk of a heart attack or stroke.

Taking more is not without risk either. Prospective research shows that high doses, especially in combination with calcium supplements, increase the risk of kidney stones and can lead to hypercalciuria (too much calcium in the urine). If you are already predisposed to kidney stones, your risk is even greater. Never take high doses without medical supervision.