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Does coffee make your breasts smaller? Every few years, a meme circulates on Facebook and other social media sites warning women that their coffee habits could be shrinking their boobs. The story behind the myth is one that highlights what happens when people who don’t understand science try to report on scientific studies.
Back in 2008, news and media outlets from Fox News to Glamour blared the headline: New Study: Drinking Too Much Coffee Shrinks Your Boobs? The story even made its way overseas to the UK’s famous tabloid, the Daily Mail. According to those stories, a small study about breast cancer had found that women who drink three or more cups of coffee daily had smaller breasts than participants who drank less than three cups of coffee daily – about 17% smaller, to be exact. The question: did drinking too much coffee actually make women’s breasts smaller?
The study was published in the British Journal of Cancer, hardly a sensationalist publication – and the actual study was not really all that sensational. Titled “Coffee intake and CYP1A2*1F genotype predict breast volume in young women: implications for breast cancer,” the Swedish researchers had a group of 269 women from families with a high risk of breast cancer fill out a questionnaire about their use of coffee. They also measured the breast volumes of each of the women and tested samples of their blood for specific hormones and genes.
They were looking for a relationship between coffee consumption, breast size, and breast cancer risk – and they had a specific reason for doing so. Previous research had shown a correlation between coffee drinking and a reduced risk of breast cancer. Other research had found a correlation between breast size and reduced risk of breast cancer. The Swedish group was looking to see if there was a direct correlation between breast size and coffee consumption. They did find one – but the findings are far more complicated than the simple headlines and news stories made them appear.
The original studies that showed a correlation between coffee consumption and breast cancer found that women who carried a certain gene allele – CYP1A2*1F C-allele – were less likely to develop breast cancer if they were moderate to heavy coffee drinkers. That protective benefit didn’t extend to women with a CYP1A2*1F with an A/A genotype (no C-allele). The researchers also ruled out women who used hormonal birth control, leaving only 145 women for their calculations.
What they found was that among women who carried the C-allele, women who drank three or more cups of coffee a day had smaller breasts than those who drank at least three cups of coffee daily. In women with the A/A genotype, there was no significant difference in breast size based on coffee consumption – though the paper reports that the heavy coffee drinkers among the A/A group actually had somewhat larger breasts than those who drank less coffee. In other words, though the difference wasn’t statistically significant, it was common enough that the research team mentioned it in their published study.
And About that Shrinkage…
The researchers only measured the women’s breasts once, so there’s no way to say whether or not their boobs actually “shrank.” What the study actually determined is that women with a CYP1A2*1F C-allele who drink 3 or more cups of coffee daily tend to have smaller breasts than women with the same genotype who drink less coffee.
Correlation vs. Causation
A well-known proverb among researchers is “correlation does not equal causation.” Just because you note a statistical link between two things, it doesn’t mean that one thing causes the other thing. While scientists try very hard to eliminate “confounding factors” – things that may have an effect on the results they’re studying – a statistical link is not proof of causation. And in this case, the lead researcher on the study recently told a writer at Fusion that the study did not find “any causal link” between coffee consumption and breast size. In other words, no, the study did not find that drinking too much coffee will make your boobs smaller. Enjoy your coffee. You’re welcome!