Sunlight Affects Medicine Absorption

A new study shows that your vitamin D level may affect what dosage your doctor will prescribe for certain medications. This study, published in Drug Metabolism & Disposition, was the first of its kind to show that the body’s vitamin D level can affect the liver’s ability to break down and metabolize medication. This may give physicians and researchers insight into a puzzling question: Why do patients with otherwise similar medical histories respond differently to the same dosage of medicines?

The study was conducted at the Karolinska Institutet, a medical university in Sweden. Researchers evaluated data on 70,000 patients who were taking immune-suppressant drugs, including tacrolimus and sirolimus, to prevent transplant rejection.

These patients had their blood monitored regularly, so it was easy for researchers to compare blood samples drawn in the winter (January – March) with those taken in late summer (July – September). The comparison showed that blood concentrations levels of tacrolimus and sirolimus were lower in the summer months and higher in the winter months. This means that these patients required more of the medicine in the summer to achieve the same amount of protection they could receive in the winter from a lower dosage.

Sweden has long winter nights and long summer days, so one’s exposure to sunlight—and its ability to produce vitamin D—is significantly different in winter and summer. Sunlight and vitamin D activate a liver enzyme called CYP3A4, which aids in the breaking down of certain medicines, including tacrolimus and sirolimus.

Researchers also took blood samples from patients taking cyclosporine, a drug used for the same purpose. No significant difference was seen in blood concentration levels between winter or summer, probably because the enzyme CYP3A4 is not required for metabolizing cyclosporine.

Although more research is necessary, this study suggests that the enzyme CYP3A4 may provide an important piece of information to doctors prescribing medications. Given that vitamin D is the sunshine vitamin, and vitamin D levels increase with sun exposure, it’s likely that researchers will discover additional medications that can be lowered in the summer while still conferring the needed benefits. 

This may call for closer monitoring of certain drug blood levels depending on the season. In the case of these immunosuppressant transplant drugs correct dosage is critical to prevent organ transplants from being rejected. As more and more people realize the benefits of taking Vitamin D this also will require doctors to monitor blood levels of drugs.

Although more research is required, it seems that Vitamin D dosages of patients will probably affect the metabolism of some drugs. Prospective studies need to be done to prove a causal relationship.

Lindh, J.D., Seasonal variation in blood drug concentrations and a potential relationship to vitamin D, Drug Metab Dispos, 2011 May;39(5):933-7. Epub 2011 Feb 24.


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