Women who breastfeed for more than a year reduce their chance of rheumatoid arthritis by half, research suggests.
Sweden's Malmo University Hospital compared 136 women with the condition to 544 without for the Annals of Rheumatic Diseases study.
They found women who had breastfed for 13 months or more were half as likely to develop rheumatoid arthritis as those who had never breastfed.
Those who breastfed for between one and 12 months had a 25% decreased risk.
The study also found that simply having children and not breastfeeding did not seem to protect the women against developing rheumatoid arthritis.
Around 400,000 people in the UK have the condition.
"The surprising results from this research demonstrate how much more we need to know about rheumatoid arthritis and its causes"
The study notes that women have more than a two-fold higher incidence of rheumatoid arthritis than men.
It is known that breastfeeding is linked to raised levels of a hormone called oxytocin, which can reduce stress hormone levels, lower blood pressure and induce well-being.
However, breastfeeding also raises levels of another hormone - prolactin - which is known to stimulate the immune system, and may actually raise the risk of rheumatoid arthritis.
Different results
Professor Alan Silman, medical director of the Arthritis Research Campaign, said the study shows different results from previous studies.
"In the past we have shown that breastfeeding was a risk factor for developing rheumatoid arthritis in a small group of women soon after giving birth to their first child.
"This is slightly different in that it looks at longer-term risk. One explanation could be that women who breastfeed may lead healthier lifestyles than women who don't, but we don't know the mechanisms that would explain this fully, and not enough work has been done in this area."
Rachel Haynes, from the voluntary organisation Arthritis Care, said: "The surprising results from this research demonstrate how much more we need to know about rheumatoid arthritis and its causes."
But she added: "It is important that women feel able to make informed choices by balancing their lifestyle with the full range of evidence about the pros and cons of breastfeeding."
The Infant Feeding Survey, published in May last year, showed that most women are failing to follow government advice, with fewer than one in 100 women breastfeeding exclusively for the first six months.
While 76% of UK mothers started out breastfeeding - a rise of 7% since 2000 - most resort to formula milk within weeks.
Fewer than half of mothers are still breastfeeding by the time their child is six weeks old, and only a quarter do so at six months.
The study also looked at the role of the contraceptive pill, which has been previously suspected of cutting the risk of rheumatoid arthritis.
It has been thought the pill could offer a protective effect because it contains hormones that are also raised in pregnancy.
But the research found that taking oral contraceptives did not offer the women less chance of developing the disease.
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