The experts found tissue that was not diseased responded well to vitamin D, suggesting it could be effective at preventing the onset of rheumatoid arthritis, which affects more than 400,000 people in the UK.
The vitamin proved much less effective once the disorder was established, suggesting much higher doses may be needed in combination with other anti-inflammatories such as steroids.
The study’s co-author, Prof Martin Hewison, said the results had significant implications for people in the UK.
“This is a particular problem within the UK population where almost everyone is going to have vitamin D deficiency, but it’s a particular concern for people with inflammatory diseases,” he said.
“How much vitamin D is needed to overcome that [disorder] is the missing piece of the jigsaw. It’s not going to be the same as is needed to prevent rickets.”
The study is part of a research project which first began in 2011 and builds on previous work showing people with rheumatoid arthritis have vitamin D deficiency and that the vitamin has anti-inflammatory properties.
There have been calls for vitamin D to be added to food in the UK. It is essential for healthy bones, teeth and muscles and a major global study published this year suggested it can reduce the risk of colds, flu and other dangerous infections such as pneumonia.
The Department of Health has said the vast majority of people in the UK get the vitamin D they need through a healthy diet and sun exposure. Foods naturally high in vitamin D include, eggs, salmon and sardines.
But DoH expert advisers want the recommended daily intake to increase from three micrograms to 10 micrograms to protect against bone and muscle conditions. They also say supplements may be necessary for certain groups of people and everyone should consider taking them in the winter months.
Hewison said the levels of vitamin D needed to prevent inflammatory diseases may need to be higher; and the paper, published in the Journal of Autoimmunity on Tuesday, suggests it will certainly need to be so for people who have rheumatoid arthritis.
“Compared to blood from the same patients, the inflamed joint immune cells were much less sensitive to active vitamin D,” he said.
“This appears to be because immune cells from the joints of rheumatoid arthritis patients are more committed to inflammation, and therefore less likely to change, even though they have all the machinery to respond to vitamin D.”
How much vitamin D is needed to overcome rheumatoid arthritis will be the subject of further research by the authors, who also want to determine why the disorder seems leads to vitamin D insensitivity and whether the same effect is seen in other inflammatory diseases, such as multiple sclerosis and inflammatory bowel disease.
Dr Louisa Jeffery, co-author of the report, said: “Our research indicates that maintaining sufficient vitamin D may help to prevent the onset of inflammatory diseases like rheumatoid arthritis.
“However, for patients who already have rheumatoid arthritis, simply providing vitamin D might not be enough. Instead much higher doses of vitamin D may be needed, or possibly a new treatment that bypasses or corrects the vitamin D insensitivity of immune cells within the joint.”
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