Breasts, genes and coffee

Friday October 24 2008

“Drinking three cups of coffee a day shrinks women’s breasts,” the Daily Mail reported. It said that researchers had questioned nearly 300 women on how much coffee they drank, and then measured the size of their bust. The study found that “three cups was enough to make breasts shrink”, with the effect increasing with each cup. The newspaper said there is “a clear link between drinking coffee and smaller breasts”, as about half of all women possess a gene that links breast size with coffee intake.

Previous research has found that risk of breast cancer may be affected by a particular oestrogen balance in the body, a balance that seems to be associated with breast volume. This is influenced by a gene that codes (instructs) for an enzyme involved with both oestrogen and caffeine metabolism. The current study looked at whether coffee intake is associated with breast volume, and how this is modified by which variant of the gene CYP1A2*1F a woman carries. Although it did find a link between these factors in women who did not use oral contraception, the media has over-simplified the results, and the study has few implications at the current time. The study only looked at these factors at one point in time, and cannot demonstrate that coffee consumption ‘caused’ the tested breast volume, or causes breasts to shrink.

Where did the story come from?

Dr Helena Jernström and colleagues from Lund University and Malmo University in Sweden, carried out this research. The study was funded by The Swedish Research Council and several other Swedish foundations. The study was published in the peer-reviewed medical journal, the British Journal of Cancer.

What kind of scientific study was this?

This was a cross-sectional study in which the authors aimed to investigate whether coffee intake was associated with breast volume and how this was modified by a particular variant of gene CYP1A2*1F  (the A/A genotype). The gene codes for the CYP1A2 enzyme, which plays a role in both coffee and oestrogen metabolism. Women with the A/A genotype who have a high caffeine intake have previously been shown to have a higher ratio of certain types of oestrogens, which is thought to be protective against breast cancer.

The researchers recruited 269 Swedish volunteers (average age 29) who completed a questionnaire on reproductive issues, use of contraception, smoking, coffee consumption (large 300ml cups or small 150ml cups), and other information (not included in this report). Body measurements were taken at set times during the menstrual cycle. The women’s breast volume was assessed by asking them to take a kneeling position with the breasts hanging down. The approximate volume was then worked out by a simple calculation (base x height divided by three). Laboratory methods were used to analyse the CYP1A21F gene, and levels of oestrogen and other hormonal factors. In their statistical analyses, the researchers looked for any associations between the CYP1A21F genotype (either A/A or not) and drinking three or more cups of coffee per day.

What were the results of the study?

The CYP1A21F A/A genotype occurred in 51% of the women. There was no difference in coffee consumption, or any other personal, social or lifestyle characteristic between the women who had the CYP1A21F A/A genotype and those who did not. Coffee consumption was significantly associated with smoking. Total breast volume was significantly associated with weight, but not with age, use of hormonal contraception, having no children, smoking or the CYP1A2*1F genotype.

In further analyses, the researchers only looked at those women who did not use hormonal contraception (such as the pill). Among the 145 non-users, they found that the association between drinking at least three cups of coffee per day and breast volume was significantly modified by CYP1A2*1F genotype. Women who did not have the A/A genotype and who consumed three cups per day or more had smaller breast volume than those who consumed less coffee. Women with the A/A genotype who consumed three cups per day or more had a slightly larger breast volume than those with the same genotype who drank less coffee.

What interpretations did the researchers draw from these results?

The authors conclude their main finding of “a significant interaction between coffee consumption, CYP1A21F genotype and breast volume among young healthy women who did not use hormonal contraceptives”. The researchers say that this interaction was mainly driven by the fact that coffee consumption of three cups a day or more was associated with lower breast volume in women who did not have the CYP1A21F A/A genotype.

What does the NHS Knowledge Service make of this study?

This is complex research, following on from studies investigating the relationship between the CYP1A2*1F genotypes, breast volume, and breast cancer. The research has been over-simplified by the media, and the study has few implications at the current time. The cross-sectional study design only demonstrates associations and does not prove that level of coffee consumption ‘caused’ the breast volume taken at the time of the study, or caused the breasts to ‘shrink’. Additionally, there may be issues regarding both the accuracy of the breast measurement and that of coffee consumption (which is not detailed in this report). The observed relationship is based on an assessment of only 145 women, and further research will need to follow up the findings.

Many women will have consumed coffee for much of their lifetime. These women should not be concerned that their breast size will be affected if they continue to consume in moderation.

Analysis by Bazian
Edited by NHS Choices