“Women who take a daily multivitamin pill to ward off illness may actually be increasing their risk of breast cancer,” reported the Daily Mail.
The news is based on a decade-long study which followed over 35,000 Swedish women aged between 49 and 83 years old. It found that women who regularly took multivitamins were 19% more likely to develop a breast tumour than women who did not take multivitamins. However, it is difficult to draw firm conclusions on whether a link between multivitamins and breast cancer exists, particularly as other studies have had mixed findings on the matter. As the researchers have suggested, further research is needed to clarify whether there is a link and, if so, which component(s) of the multivitamin supplements may be responsible.
It is also important to note that the risk of developing breast cancer in this research was low, with only 2.8% of women affected. Although many people take multivitamins, a balanced and varied diet is usually enough to provide most people with the daily levels of vitamins and minerals they require.
Where did the story come from?
Dr Susanna C Larsson and colleagues from the Karolinska Institutet and a hospital in Sweden carried out this research. The study was funded by the Swedish Cancer Foundation and the Swedish Research Council for Infrastructure. The study was published in the peer-reviewed American Journal of Clinical Nutrition.
The Daily Mail’s report on this research provided balanced coverage of its implications. The newspaper noted some of the limitations of the study and said that “on an individual basis, the risks to women remain small and the vast majority of vitamin users will not develop cancer”.
What kind of research was this?
This was a cohort study that looked at the relationship between multivitamin use and the risk of developing breast cancer.
A randomised controlled trial (RCT) would normally be the best study design for looking at the effects of multivitamins on health. However, this approach may not be appropriate if researchers are solely interested in whether the use of vitamins increases the risk of an adverse outcome such as breast cancer. Some RCTs of multivitamins have been carried out and, even though their focus may not have been the risk of breast cancer, analysis of their results may help provide an answer to this question.
What did the research involve?
The researchers used data from women enrolled in the Swedish Mammography Cohort study. The 35,329 women who enrolled in the study reported on their lifestyles, including whether they used multivitamins and other risk factors for breast cancer. They were followed up over an average of 9.5 years and any women who developed breast cancer were identified. The researchers then compared the breast cancer risk of women who regularly used multivitamins to that of women who did not.
The Swedish Mammography Cohort study obtained its population by mailing all women living in central Sweden who were born between 1914 and 1948. Between 1987 and 1990, the women received a questionnaire asking about various aspects of their diet and risk factors for breast cancer.
In 1997, all surviving participants still living in central Sweden were sent a more detailed questionnaire asking them if they used dietary supplements and, if so, what type and how many tablets they took and for how long they had been taking them. Women were classified as using multivitamins (with or without minerals) if they took at least one tablet a week or had been taking them for at least a year.
The researchers reported that multivitamins in Sweden generally contain doses of vitamins and minerals close to the recommended daily allowances of each constituent: vitamin A (0.9 mg), vitamin C (60 mg), vitamin D (5 micrograms), vitamin E (9 mg), thiamine (1.2 mg), riboflavin (1.4 mg), vitamin B-6 (2.1 mg), vitamin B-12 (3 micrograms) and folic acid (300-400 micrograms). The minerals usually included were reported to be iron (10 mg), zinc (12 mg), copper (2 mg), chromium (50 micrograms), selenium (40 micrograms) and iodine (150 micrograms).
The current analysis included 35,329 women aged 49 to 83 years old who provided information on their supplement use and had no history of cancer in 1997. The women were followed up until December 2007, and any cases of invasive breast cancer were identified using regional and national cancer registries. Deaths were identified from the Swedish Death Registry.
The proportion of women who developed breast cancer was compared between the group that used multivitamins and the group that did not. This analysis took into account a number of risk factors for breast cancer, including age, education, history of benign breast disease, family history of breast cancer, number of children, age when first child was born, age at first period, age at menopause, use of oral contraceptives, use of hormone treatment after menopause, body mass index (BMI), physical activity, smoking, calcium supplement use and alcohol intake.
What were the basic results?
Just over a quarter of women (25.5%) reported using multivitamins, and nearly all of those reported that at least some of the multivitamins they took also contained minerals (23.9%). Women who took multivitamins were more likely to have received post-secondary education, have a history of benign breast disease, have no children and to have used oral contraceptives and postmenopausal hormone replacement than women who did not use multivitamins. Multivitamin users had lower BMIs than non-users and were less likely to smoke.
During the study, 974 women (2.8%) developed breast cancer. Women who took multivitamins were more likely to have developed breast cancer than those who did not. After taking into account other factors that could affect results, multivitamin users had a 19% greater risk of developing breast cancer than non-users (relative risk 1.19, 95% confidence interval 1.04 to 1.37).
When the researchers looked at the number of tablets taken and the duration of use, they found that women who had taken tablets for three years or more and those who took seven or more tablets a week were at increased risk of breast cancer compared to non-users. There was no increase in risk in women who had taken multivitamins for less than three years compared to non-users, and the increase was only just statistically significant in women who took fewer than seven multivitamin tablets a week.
There was no significant increase in risk of breast cancer in women who took supplements containing specific vitamins (vitamins C, E, B-6 or folic acid) compared to those who did not take such supplements.
How did the researchers interpret the results?
The researchers concluded that “multivitamin use is associated with an increased risk of breast cancer”. They say that this “is of concern and merits further investigation”.
This study has some strengths, including its large size. However, it also has some limitations:
- As with all studies of this type, it is possible that factors other than the one of interest (multivitamin use) could have affected the results. The researchers took into account a number of these factors, but they may not have completely accounted for their effects or the effect of an unknown or unmeasured factor.
- The researchers note that other studies of multivitamin use have had mixed findings. Some cohort studies found an increase in breast cancer risk with multivitamin use, but not all of these findings were statistically significant. Other cohort studies found no link between multivitamin use and breast cancer. They also report that a French randomised controlled trial found that a combination of vitamins C and E, b-carotene, selenium and zinc did not affect the risk of breast cancer. A systematic review of existing studies could give a comprehensive picture of what is known to date about this question.
- Vitamin use was based on self-reported questionnaire data taken at one point only. It is possible that some women may not have reported their supplement use accurately, or that their use changed over the 10-year follow-up period. This could potentially affect the results.
- It is not possible to say from this study which of the component(s) of the multivitamin supplements may affect breast cancer risk.
Overall, it is difficult to draw firm conclusions based on this study alone. Further research is likely to be needed to clarify whether there is a link between multivitamin use and breast cancer risk and, if so, which component(s) of the multivitamin supplements are responsible. Most people can get their daily requirement of vitamins and minerals from a healthy, balanced diet.