New Zealand researchers are testing the hypothesis that sunlight can cure multiple sclerosis, a debilitating condition on the increase around the world, especially in colder climates.
MS is three times more prevalent in Otago and Southland than in the North Island, with a rate of 134 incidence per 100,000 people as opposed to 73 per 100,000 nationally.
This latitudinal trend is reflected throughout the world.
A world-first trial will test the long-held theory that a lack of vitamin D contributes to the prevalence of MS. It will track up to 300 people from Australasia, including up to 100 from New Zealand, who display early symptoms of MS, for two years.
Deborah Mason, a Canterbury-based neurologist who is running the New Zealand side of the study, said some of the subjects would be given vitamin D and others a placebo.
"There are probably a whole lot of things, both environmental and genetic, that come together to cause MS.
But we think maybe if we can supplement people's vitamin D, improving one factor, then maybe the threshold won't be reached."
People of European descent are far more likely to get MS, and women are three times more likely to get it than men.
Dr Mason said the increased incidence rate of MS over the past 30 or 40 years could be because the "slip, slop, slap" message about sun avoidance was be getting through, which was good for the skin, but not so for the bones.
She said the majority of people with MS suffer intermittent attacks where they may experience loss of vision, balance disturbance or irregular limb function. Over time they can develop debilitating disabilities.
"It can be a tragic thing to live with because these people are usually young, they have young families and they can acquire huge amounts of disability.
"In New Zealand we are way behind the eight ball in terms of treating people with this condition. In Australia, America, England - all western countries - they have powerful drugs that they use early in the disease to stop disability. In New Zealand those drugs are not being funded. It's a Pharmac issue."
Pamela von Hurst, a Massey University lecturer in human nutrition, said about 50 per cent of New Zealand's the population are below adequate vitamin D levels.
She said modern-day lifestyles were probably the biggest contributor, with most people spending too much time inside.
Low levels of vitamin D have also been linked to conditions such as rickets in children and osteoporosis and osteomalacia in adults, as well as non-skeletal health conditions cardiovascular disease.
The Ministry of Health and Cancer Society recommend sun protection between September and April especially between 10am and 4pm.
"A daily walk or some other form of outdoor activity in the early morning or late afternoon is recommended."
Between May and August, outdoor activity around noon with face, arms and hands exposed is recommended.
For vitamin D synthesis, exposure must be to direct sunlight as UVB does not pass through glass.