Tuesday May 13 2008
“Breast feeding your baby for at least a year can dramatically reduce the chance of contracting rheumatoid arthritis”, the Daily Express reported today. It went on to say that a study has found that women who breastfeed their babies for more than 13 months are half as likely to develop rheumatoid arthritis as women who don't. The scientists behind the research reportedly believe that hormonal changes in women who become pregnant and then breastfeed their baby “could help protect against the condition later in life”.
The study behind these stories is a case-control study that looked at the duration of breastfeeding in women who had rheumatoid arthritis compared to women free of the condition. It found that a longer history of breastfeeding was associated with a reduced risk of rheumatoid arthritis. However, previous studies have had different findings, including some that have suggested that breastfeeding actually elevates the risk of rheumatoid arthritis in the short term.
The study design and the way the data were analysed limit the conclusions that can be drawn from this study. The authors themselves call for larger studies to corroborate their conclusions.
Where did the story come from?
Doctors Pikwer, Bergström and colleagues from the Malmö University Hospital in Sweden carried out the study. The study was funded by Lund University, the Craaford Foundation and the Swedish Rheumatism Association. The study was published in the peer-reviewed medical journal: Annals of the Rheumatic Diseases.
What kind of scientific study was this?
The objective of this nested case-control study was to investigate whether breastfeeding or the use of oral contraceptives affects the future risk of rheumatoid arthritis.
This study used information from a larger study, the Malmo Diet and Cancer Study (MDCS), a community-based health survey in Malmo, Sweden. The survey was performed between 1991 and 1996 and involved more than 30,000 women. The MDCS participants filled in a questionnaire that asked them about issues such as their use of oral contraceptives, how many children they had, whether each child was breastfed and for how long. The medical records of the survey participants were then found and reviewed. All women who had been diagnosed with rheumatoid arthritis since they were enrolled in the MDCS study were included in this study.
In total, the researchers found 136 women with a new diagnosis of rheumatoid arthritis. Each woman was matched for age and year of screening to four control females from the MDCS study, who were alive and free of rheumatoid arthritis at the time of the woman’s diagnosis. The women with rheumatoid arthritis were compared with the control women for their use of oral contraceptives (never, one to five years of use, more than five years of use) and whether they breastfed (never, one to twelve months, 13 months or more). They also compared the women with respect to their smoking status and education.
What were the results of the study?
There was no significant difference in the number of children between women with rheumatoid arthritis and those without. However, the researchers suggest that there was a “trend towards a reduction in risk of rheumatoid arthritis for each child born”.
The researchers also found that ”a longer history of breastfeeding was associated with a reduced risk of rheumatoid arthritis”. For example, compared to women who didn’t breastfeed, those who breastfed for 13 months or more were half as likely to have rheumatoid arthritis (OR 0.46, 95% CI 0.24 to 0.91). There was no statistically significant effect of women breastfeeding for between one and twelve months on the risk of rheumatoid arthritis. The researchers report that when they took into account the effects of smoking and education, longer breastfeeding was still significantly associated with rheumatoid arthritis.
No association was found between a diagnosis of rheumatoid arthritis and the use of oral contraceptives.
What interpretations did the researchers draw from these results?
The researchers conclude that their study has shown that women who breastfeed for 13 months or more have a reduced risk of developing rheumatoid arthritis. Although the results for breastfeeding between one and 12 months were not significant, the researchers say that it appears that the more breastfeeding a woman does, the lower her risk of the disease.
Importantly, the researchers say it is difficult to differentiate the effect of breastfeeding from that of childbirth. They say that a larger study needs to confirm their finding that the reduced risk of rheumatoid arthritis is associated with breastfeeding and not with the number of children born.
What does the NHS Knowledge Service make of this study?
This design of this case-control study has weaknesses that limit the conclusions that can be drawn from its results. The researchers themselves highlight some of these limitations:
- Most importantly, the study included women in their 40s or older who may have breastfed their babies some time ago. This raises two issues; firstly, the women’s recollection of how long they breastfed their children may have been inaccurate, and secondly, in the time between breastfeeding and onset of rheumatoid arthritis, other factors may have been at play that contributed to the condition. These would not necessarily have been measured or accounted for in the analyses.
- The adjustments for smoking and education that the researchers made were based on information collected in the MDCS (i.e. they don’t account for behaviour at the time of diagnosis).
- The group that the researchers compared breastfeeding duration with (i.e. those who didn’t breastfeed) also included women who didn’t have any children. This was because there were too few women in their sample who gave birth, but did not breastfeed. This means it was not possible to truly compare whether breastfeeding reduces rheumatoid arthritis risk compared to not breastfeeding in women who had children.
- It is unclear how the researchers performed some of their calculations. In particular, they have tried to look at subgroups and the small size of these groups of participants reduces confidence in the reliability of their conclusions.
These findings need to be replicated in larger studies, particularly since they are in contrast to the results of previous studies, some that have suggested that breastfeeding actually elevates the risk of rheumatoid arthritis in the short term. Importantly, future studies should involve enough women who have actually had children so that proper comparisons can be made between those who breastfeed and those who don’t (i.e. excluding women who have not given birth).
There are few women who breastfeed for more than a year and it remains to be seen whether there is an ideal time at which the protective effect of breastfeeding, if it was proven, could be expected to begin.
Sir Muir Gray adds…
This is more evidence for the benefits of breastfeeding.