You can read about the study, conducted at the Karolinska Institutet in Stockholm, Sweden, online in the 24 March issue of the American Journal of Clinical Nutrition.
Many women use multivitamins in the belief that they will protect them from chronic diseases like cancer and heart disease, wrote the authors in their background information.
In fact, in February 2009, the Archives of Internal Medicine published details of a large US study of over 160,000 postmenopausal women that found no convincing evidence that long term use of multivitamins changed their risk of developing common cancers, cardiovascular disease or dying prematurely.
But, as the authors of this Swedish study pointed out, the effect of multivitamins on breast cancer only is unclear.
For the prospective study, lead and corresponding author Dr Susanna C Larsson, of the Division of Nutritional Epidemiology, National Institute of Environmental Medicine at the Karolinska Institutet, and colleagues, looked at data from 35,329 women in the Swedish Mammography Cohort, who filled in questionnaires about their use of multivitamins and breast cancer risk factors. The women were cancer free and aged between 49 and 83 when they filled in the questionnaires in 1997.
The results showed that:
- 974 women were diagnosed with incident breast cancer over a mean follow up of 9.5 years.
- 293 of the diagnoses were among 9,017 women who reported using multivitamins.
- Use of multivitamins was linked to a statistically significant 19 per cent increased risk of breast cancer (after adjusting for lifestyle and risk factors like weight, diet, smoking, exercise, and family history of breast cancer, the relative risk of women who reported using multivitamins was 1.19, with confidence interval ranging from 1.04 to 1.37).
- Hormone receptor status did not change the strength of this link significantly.
"These results suggest that multivitamin use is associated with an increased risk of breast cancer. This observed association is of concern and merits further investigation."
Readers concerned about these findings should note that they don't prove that multivitamins caused the women's breast cancer: a prospective study, which this was, can only show whether there is a link or not and try and rule out possible influencers.
As Larsson pointed out in an email to Reuters Health, it is possible that something they did not measure is responsible for the link. Also, even if the link was causal, these findings show that for any one woman, using multivitamins has a small effect on her risk of breast cancer, she noted.
However, given the widespread use of multivitamins, there is an important public health message in this study, said the authors.
Speculating on what the biological reasons might be, the researchers mentioned various previous studies that taken as a whole reveal a conflicting picture. For example, some studies on folic acid suggested it increases breast cancer risk while others suggested it has no effect and may even decrease it.
In the meantime, Larsson recommends women eat a healthy and varied diet instead of using pills to get the nutrients they need.
"Multivitamin use and breast cancer incidence in a prospective cohort of Swedish women."
Susanna C Larsson, Agneta Åkesson, Leif Bergkvist, and Alicja Wolk
Am J Clin Nutr Published online 24 March 2010.
Multivitamins Don't Change Cancer Or Heart Risk In Postmenopausal Women
Sources: AJCN, Reuters, MNT Archives.