There are two reports this month on the interaction between taking AEDs for long periods and vitamin D levels in the body. Vitamin D is required for normal growth of teeth and bones. Not enough vitamin D in a growing animal causes the disease rickets, where the bones do not harden.

Vitamin D is found in large amounts in fish liver oils, and but most of it is produced naturally in the skin by exposure to sunlight. Vitamin D levels are often already lower-than-healthy in parts of the general population who get limited sun-exposure and have deficient diets.

Men's bone health is affected by enzyme-inducing AEDs
The first study investigated vitamin D levels in men taking AEDs, in particular those of the "enzyme inducing" type. These AEDs are broken down in the liver in a process that increases the presence of an enzyme called 3A4. Enzyme 3A4 degrades vitamin D, and therefore reduces the amount of it present in the body. Enzyme-inducing AEDs include phenytoin, phenobarbital, carbamazepine, oxcarbazepine, and felbamate (not licensed in the UK).

The study included 210 men (average age 58 years), taking a variety of AEDs. It found that vitamin D levels in patients taking enzyme-inducing AEDs were significantly lower than those in patients taking other types of AED: 19.2 ng/mL, compared with 23.8 ng/mL. Presented at the American Epilepsy Society 60th Annual Meeting in San Diego, California, USA, last month, results also showed deterioration in other markers of bone health affected by vitamin D, including blood calcium levels.

Taking AEDs is known to increase the chances of osteoporosis, especially in women at the menopause, but these effects had not previously been studied in men. The principal researcher, Dr Gina Jetter, from the Texas Health Science Center in San Antonio, said it's not yet clear whether taking additional vitamin D could help improve bone health in this group.

Giving extra vitamin D to children and adults: does dose matter?
A second report, in the journal Neurology in December 2006, described a pair of parallel studies from Beirut in Lebanon, one in children aged 10 to 18 and the other in adults, on the effects of vitamin D dosing on bone mineral density.

All patients included (78 in the children's study and 72 in the adults' study) had epilepsy and were taking AEDs. Researchers compared the effects of two different doses of vitamin D: a low dose of 400 international units (IU) per day and a high dose of 2000 IU per day in children or 4000 IU per day in adults. Bone mineral density was measured in the lower spine, hip and forearm in all patients at the beginning and end of the study year, and was compared to that of control patients, matched for age and gender.

In the children's study, initial bone mineral density in patients with epilepsy and controls was about the same. In the epilepsy patient group, it improved significantly over the year of the study, but there was no difference in improvement between the dose groups.

At the beginning of the adults' study, patients with epilepsy had significantly lower bone mineral density levels than the control group. After a year of dosing, bone mineral density had improved in the high-dose group (though it was still below normal) and had not improved in the low-dose group.

The study author, Dr Ghada Fuleihan from the American University of Beirut Medical Center, said that vitamin D dosing can improve bone mineral density in both adults and children with epilepsy taking AEDs. High doses are better in adults, but in children lower doses may be enough.