A new analysis concludes that all levels of alcohol use - even light drinking - are associated with raised risk for breast cancer, with higher consumption linked to higher risk. The researchers also summarize the biological mechanism behind the link and the impact on global breast cancer incidence and deaths due to drinking.

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The researchers estimate that 18.8 percent of global breast cancer cases and 17.5 percent of the deaths in 2012 were among women classed as light drinkers.

The study paper, by Dr. Kevin D. Shield from the International Agency for Research on Cancer (IARC) in Lyon, France, and colleagues, is published in the journal Alcoholism: Clinical and Experimental Research.

The findings address a long-standing debate about whether light alcohol consumption is linked to raised risk of breast cancer.

The link between alcohol consumption and cancer was officially declared in 1987, when a working group of the IARC - an agency of the World Health Organization (WHO) - listed cancers of the oral cavity, pharynx, larynx, esophagus, and liver as "causally related to the consumption of alcoholic beverages."

Since then, many studies have found links between alcohol consumption and breast cancer, and another review in 2007 by the IARC added breast cancer and colorectal cancer to the former list of cancers.

The authors also note that as recently as 2014, studies have revealed convincing evidence that alcohol raises the risk of breast cancer. However, the question of whether light drinking is linked to breast cancer remains somewhat controversial.

The new study is in three parts. First, the authors review and summarize the literature on the biology of the link between alcohol use and breast cancer. Then, they examine evidence about the size of risk and how it relates to levels of alcohol consumption. Finally, they assess the global impact of such a link, with an emphasis on light alcohol consumption.

'Dose-response relationship'

The first part of the study summarizes three biological ways alcohol affects breast cancer risk: how it changes hormone levels, how the body produces cancer-causing compounds when it metabolizes alcohol, and how it blocks a key metabolic pathway known as the "one-carbon metabolism pathway."

In the second part of the study, the researchers searched for published meta-analyses of the risk relationship between alcohol consumption - including light drinking - and the risk of breast cancer. Meta-analyses are large studies that pool and analyze data from several studies.

All but two of the 15 meta-analyses that met the researchers' criteria showed a dose-response relationship between alcohol consumption and the risk of breast cancer - even at low levels of consumption. Thus, even light alcohol drinkers are at higher risk for breast cancer than non-drinkers, they found.

Finally, when they assessed the impact of these findings on the global burden of breast cancer, the team estimated that 144,000 breast cancer cases and 38,000 breast cancer deaths in 2012 were caused by alcohol use.

Moreover, in 18.8 percent of the cases and 17.5 percent of the deaths, the women were light drinkers.

The researchers got the data on global alcohol consumption from the Global Information System on Alcohol and Health (GISAH), and the data on cancer mortality came from the GLOBOCAN database.

They used a method called Population-Attributable (PAF) methodology to produce the estimates from these data sets. PAF calculates the number of cases or deaths that would not occur if the risk factor were reduced to an alternative scenario (for example, no alcohol use).

According to the WHO, the harmful use of alcohol kills 3.3 million people worldwide every year and there are 60 different diseases where alcohol features as a significant cause.

The global consumption of alcoholic drinks in 2010 was equal to 6.2 liters of pure alcohol per person aged 15 and over.

In the United States, dietary guidelines suggest if people choose to drink alcohol it should be in moderation - up to one drink per day for women and up to two drinks per day for men. Plus, they "do not recommend that individuals who do not drink alcohol start drinking for any reason."

Dr. Shield and colleagues note:

"All levels of evidence showed a risk relationship between alcohol consumption and the risk of breast cancer, even at low levels of consumption. Due to this strong relationship, and to the amount of alcohol consumed globally, the incidence of and mortality from alcohol-attributable breast cancer is large."

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