Looking north to south

The study, published in Neurology, included Australians living from latitudes 27S to 43S. These latitudes receive about the same amount of annual sunlight as a region extending from Tampa, Florida, to Concord, New Hampshire, and from Baja California, Mexico, to Eugene, Oregon.

Study participants were 216 people who had recently been found to have the type of nerve damage seen in multiple sclerosis, known as demyelination. They were compared with 395 otherwise similar people without demyelination. All participants answered questions about their past, recent, and total leisure-time sun exposure. They also had their skin examined and rated for sun damage, and blood levels of vitamin D (25-hydroxy) measured.

How sun and vitamin D relate to nerve damage

Similar to previous studies, this one found that more people with the demyelination type of nerve damage lived in the far southern latitudes than the middle latitudes. Moreover:

• People who reported getting more leisure-time sun in the past three years were less likely to have nerve damage.
• People with higher sun exposure since age six were also at lower risk of nerve damage.
• Sun damage to skin, which is considered to be an objective measure of lifetime sun exposure, was associated with a lower risk of nerve damage.
• People with high vitamin D levels appeared to benefit from sun exposure.
• Having fair skin and lots of freckles increased the likelihood of nerve damage.
• People with higher vitamin D levels had a lower risk of nerve damage, regardless of sun exposure.

"Our findings suggest that sun exposure and vitamin D status are independently important factors in the development of multiple sclerosis," said study lead author Associate Professor Robyn Lucas of the Australian National University. "If this is true, vitamin D supplementation alone may be less effective as a preventive strategy than previously thought."

Take care of your nerves

While many people may be out enjoying the summer sun while they have it, it's worth a discussion with your doctor on the relative risks of premature aging and skin cancer compared with multiple sclerosis risks. In any case, if you live in a place where summers are short, you might want to take extra steps in the hope of preventing nerve damage:

• Take a teaspoonful of cod liver oil every day. In addition to being a good source of vitamin D, the omega-3 fats from cod liver oil might prevent or slow the inflammation that causes demyelination.
• Cut down on animal fat. Several studies have linked a high-saturated fat diet with increased risk of multiple sclerosis, and some have found that a low-fat diet supplemented with fish oil can help people who already have multiple sclerosis.
• Consider a vitamin D supplement. As the findings from this study point out again, regardless of how much sun you get, taking extra D may be helpful.

(Neurology 2011;76:540-8)