While conducting a study about a link between breast size and heart cancer, Helena Jernström, an oncologist at Lund University in southern Sweden, discovered a gene that that about half of women possess is involved in breast cancer - and so is coffee.

The researchers found that coffee intake and the CYP1A2*1F genotype predict breast size in young women. They studied healthy premenopausal, non-hormone women and among those who drink 3+ cups per day of coffee , smaller breast size was associated only with C-allele carriers (Pinteraction=0.02), which they stated is consistent with reports that coffee protects only C-allele carriers against breast cancer.

So, smaller breasts mean less breast cancer and coffee means smaller breasts.   

"Drinking coffee can have a major effect on breast size," said Jernström.  She previously studied the link between breast cancer and coffee and the results showed that the effect of coffee varies depending on which variant the women have of CYP1A2, which codes for an enzyme that metabolizes both estrogen and coffee. Half of the women had a variant called A/A, while the others had either A/C or C/C.

“Those women who had one of the C variants, and who had drunk at least three cups of coffee a day, developed breast cancer considerably more seldom than women with the A/A variant with the same coffee consumption. Their cancer risk was only two thirds of that of the other women."

She stresses, however, that it is too early to make any dietary recommendations regarding coffee consumption.

Article: H Jernström, M Henningson, U Johansson and H Olsson, 'Coffee intake and CYP1A2*1F genotype predict breast volume in young women: implications for breast cancer', British Journal of Cancer; doi: 10.1038/sj.bjc.6604687