High doses of vitamin D can dampen down the hyperactive immune response linked to multiple sclerosis (MS), a study has found.
The US discovery could point the way towards a simple and cheap treatment for people with the disease, which is caused by the body's own immune system damaging nerve fibres.
Low levels of vitamin D in the blood are known to be associated with an increased risk of developing MS. Patients with the disease are also likely to have a more active condition and greater disability if they lack the vitamin.
For the new study, 40 patients with relapsing-remitting MS - a form of the disorder characterised by active and passive periods - received either 10,400 or 800 international units (IU) of vitamin D3 supplements every day for six months.
The first dose was significantly higher than the recommended daily allowance for vitamin D of 600 IU.
Patients taking the high dose experienced a reduction in the percentage of specific immune system T-cells related to MS activity.
Above a certain threshold, every five nanograms per millilitre increase in vitamin D blood levels led to a 1% reduction of the T-cells, the researchers reported in the journal Neurology.
No such change was seen in those patients taking the lower dose supplements.
Lead scientist Dr Peter Calabresi, from Johns Hopkins University School of Medicine in Baltimore, said: "These results are exciting, as vitamin D has the potential to be an inexpensive, safe and convenient treatment for people with MS.
"More research is needed to confirm these findings with larger groups of people and to help us understand the mechanisms for these effects, but the results are promising."
Side effects from the vitamin supplements were minor and did not differ between patients taking the higher and lower doses.
One person in each group had a relapse of disease activity, but while the study tested the effect of vitamin D supplements on the immune system it did not look closely at the clinical impact of the treatment. This will have to be the subject of future research.
MS affects around 100,000 people in the UK, most of whom are diagnosed between the ages of 20 and 40. The disease destroys the fatty myelin sheath that insulates nerve fibres and assists the transmission of electrical signals. It can cause symptoms ranging from mild tingling or numbness to full-blown paralysis.
Dr Sorrel Bickley, head of biomedical research at the MS Society said: "The link between vitamin D and multiple sclerosis is a promising area of research.
"This study, although small, provides new evidence about the safety of high doses of vitamin D and the effect this has on the immune system.
"There are more than 100,000 people in the UK living with MS and finding treatments that can slow, stop or reverse the worsening of disability is a priority for the MS Society.
"Therefore, we look forward to seeing larger and longer term studies to help us understand whether this could be an effective and safe treatment.
"Although it's important to get enough vitamin D, there are government guidelines around the dosage, and taking too much can lead to side-effects. We'd encourage anybody with concerns about their levels to speak to their GP."