DOCTORS may soon check weather reports before setting drug doses, now that it seems sunlight can affect how fast we metabolise drugs.

Previous studies suggested that vitamin D, which is produced by the skin on sunny days, boosts the expression of a gene for the liver enzyme CYP3A4, which is involved in drug metabolism.

To find out whether drug metabolism varies throughout the year, Erik Eliasson and his colleagues at the Karolinska Institute in Stockholm, Sweden, investigated whether the concentration of immunosuppressant drugs in the blood varied between the summer and winter months.

Over 10 years the team collected 70,000 blood samples from 6000 people who’d been taking one of three drugs broken down by CYP3A4, or another immunosuppressant metabolised in a different way. They compared the ratio of drug dose to blood concentration during the three-month periods associated with the highest and lowest vitamin D levels.


Eliasson’s team found that levels of two of the drugs metabolised by CYP3A4 were 7 per cent and 17 per cent lower during the summer months, suggesting that the body had broken them down more rapidly. In contrast, the drug metabolised by a separate mechanism showed no change (Drug Metabolism and Disposition, DOI: 10.1124/dmd.111038125). The implication is that during sunnier weather, more of certain drugs would be needed to have the same effect. The team failed to see a difference in the third CYP3A4-metabolised drug, though they put this down to weaknesses in the method, and expect to see differences in follow-up studies.

The findings could have implications for tailoring drug doses in countries where sunlight levels vary dramatically throughout the year. “It’s a neat idea,” says Robert Branch, director of the Center for Clinical Pharmacology at the University of Pittsburgh, Pennsylvania. If you live in northern England and then go and sit on a Mediterranean beach for a month, that could be enough to have a relevant effect, he says.

Immunosuppressants are unlikely to be the only drug affected since CYP3A4 metabolises over half of the drugs we take. What’s more, other subtypes of the CYP family might be vulnerable to changes in vitamin D levels. Jonatan Lindh, lead author of the study, is now investigating whether blood levels of anticoagulant drug warfarin – metabolised by another subtype – are influenced by levels of vitamin D. “The mechanism could be of great clinical importance for warfarin,” he says, as even small changes in drug concentration can trigger life-threatening bleeding.

Eliasson reckons our ability to metabolise drugs and other toxins evolved to become more efficient in summer when animals would have been easier to hunt and humans would have eaten more bacteria-laden meat.