Alcohol and breast cancer
The relationship between alcohol and breast cancer has been a subject of much research, and debate. It is commonly considered a risk factor for breast cancer in women. The International Agency for Research on Cancer has declared that there is sufficient scientific evidence to classify alcoholic beverages a Group 1 carcinogen that causes breast cancer in women. Group 1 carcinogens are the substances with the clearest scientific evidence that they cause cancer, such as smoking tobacco.
A woman drinking an average of two units of alcohol (also known formally as ethanol) per day has 8% higher risk of developing breast cancer than a woman who drinks an average of one unit of alcohol per day. A study of more than 1,280,000 middle-aged British women concluded that for every additional drink regularly consumed per day, the incidence of breast cancer increases by 1.1%. A study of 17,647 nurses found that high drinking levels more than doubled risk of breast cancer with 2% increase risk for each additional drink per week consumed. Binge drinking of 4–5 drinks increases the risk by 55%.
However, studies of mortality show that drinkers have no greater risk of dying from breast cancer. An analysis of various causes of death of middle aged and elderly Americans  found that, of the 251,420 women in the study, 0.3% of the zero and super-light drinkers died from breast cancer, over the 10 years of study observation. And exactly the same proportion, 0.3%, of the moderate to heavy drinkers (1 to 4+ drinks per day). In another mortality study of 85,000 women, the chance of death from breast cancer during the 12 year follow-up period was 0.4%, and again this was identical for zero-to-super-light drinkers as with moderate-to-heavy drinkers.
This paradoxical difference between the results for diagnoses and those for mortality appears to be due to drinkers screening more for breast cancer  potential reasons for the higher rates of screening among drinkers are because they are wealthier, more urban, more health conscious, closer to screening clinics. Studies that control for screening rates show no association between drinking and being diagnosed with breast cancer.
Meta-analysis of the epidemiological studies looking at drinking and breast cancer mortality/survival after diagnosis shows no association between levels of drinking (before or after diagnosis) and risk of breast cancer death, nor reoccurrence of the cancer. Two recent studies looking at patients already diagnosed with breast cancer both found that women who drank before their breast cancer diagnosis had no higher risk of dying from the cancer than the non-drinkers. Similarly a large study with long follow-up of women with breast cancer  showed breast cancer patients had better chances of survival if they were regular drinkers before diagnosis. If they altered their drinking after diagnosis this did not alter their chance of dying from breast cancer. But an increase in drinking was associated with an overall improvement in life expectancy (largely due to substantially fewer heart disease deaths among those who increased their alcohol consumption).
The mechanisms of increased breast cancer risk by alcohol are not clear, and may be:
- Increased estrogen and androgen levels
- Enhanced mammary gland susceptibility to carcinogenics
- Increased mammary DNA damage
- Greater metastatic potential of breast cancer cells
Their magnitude likely depends on the amount of alcohol consumed.
Susceptibility to the breast cancer risk of alcohol may also be increased by other dietary factors, (e.g. folate deficiency), lifestyle habits (including use of hormone replacement therapy), or biological characteristics (e.g. as hormone receptor expression in tumor cells).
In daughters of drinking mothers
Studies suggest that drinking alcohol during pregnancy may affect the likelihood of breast cancer in daughters. "For women who are pregnant, ingestion of alcohol, even in moderation, may lead to elevated circulating oestradiol levels, either through a reduction of melatonin or some other mechanism. This may then affect the developing mammary tissue such that the lifetime risk of breast cancer is raised in their daughters."
In some population studies moderate alcohol consumption is associated with increase the breast cancer risk.
Studies that control for screening incidence show no association with moderate drinking and breast cancer, e.g. Moderate drinkers tend to screen more  which results in more diagnoses of breast cancer, including mis-diagnoses. A recent study of 23 years of breast cancer screening in the Netherlands concluded that 50% of diagnoses were over-diagnoses.
A meta analysis of cohort studies of alcohol consumption and breast cancer mortality showed no association between alcohol consumption before or after breast cancer diagnosis and recurrence after treatment.
In men, breast cancer is rare, with an incidence of fewer than one case per 100,000 men. Population studies have returned mixed results about excessive consumption of alcohol as a risk factor. One study suggests that alcohol consumption may increase risk at a rate of 16% per 10g daily alcohol consumption. Others have shown no effect at all, though these studies had small populations of alcoholics.
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