Multiple studies have linked alcohol consumption to breast cancer risk, but published in JAMA is a study reporting on this relationship in greater detail. Until now, the risk of lower levels of consumption has not been well quantified. As well, the role of drinking patterns (i.e., frequency of drinking and “binge” drinking) and consumption at different times of adult life are also not well understood.

In this research, the authors report on 105,986 women enrolled in the Nurses’ Health Study followed up from 1980 until 2008 with an early adult alcohol assessment and 8 updated alcohol assessments. During 2.4 million person-years of follow-up, 7,690 cases of invasive breast cancer were diagnosed.
 
What was found was that increasing alcohol consumption was associated with increased breast cancer risk that was statistically significant at levels as low as 5.0 to 9.9 g per day, which is equivalent to only 3 to 6 drinks per week.
 
At 1 drink daily, the risk rose by a factor of 1.2.  What that actually means is that the 10 year risk increased by 0.7%. While that number might seem low, taken over large numbers of women, it adds up. If the baseline risk is 2.8%, now the number rises to 3.5% 10-year risk.
 
At 2 or drinks a day, the risk rises to 4.1% over 10 years, or an increase of 1.3% over 10 years, which is actually a 50% relative increase in risk -- now we begin to see some 413 cases per 100,00 women years. 
 
Binge drinking, but not frequency of drinking, was associated with breast cancer risk after controlling for cumulative alcohol intake but overall, the cumulative amount is what puts you at risk.
 
Alcohol intake both earlier and later in adult life was independently associated with risk but the risk manifested itself later on in life- in the post menopausal period of time.

The mechanism of action is thought to be similar to hormones in that alcohol can affect circulating estrogens. the impact on cancers were more likely to be hormone positive tumours . Alcohol can increase sex hormone levels by increasing a process called aromatisation that converts precursor hormones to estrogen. It might also decrease the break down of male hormones in the liver or it could possibly affect hormone production in the adrenal glands. It could also impact on the breast tissue itself enhancing growth in the breast or estrogen positive cell lines.
 
On the other hand, low doses of alcohol has been shown to be cardio-protective. So what messaging can women take home? The discussion becomes one about risk and how prepared you are to control lifestyle risks remembering that these risks are only one component of breast cancer likelihood. While it might be easier for women who drink 3 drinks a week to quite, those who rink more liberally might find it harder to quite. The next question we have to ask is if cutting out alcohol will decrease the risk and that is data that we do not have.
 
In the interim, there is more data for us to consider when making lifestyle choices. 

WATCH: My drinking and breast cancer chat on Canada AM