Even indulging in just a few drinks a week raises a woman’s risk of breast cancer, according to a large Harvard study released Tuesday.

The analysis of data collected from nearly 106,000 nurses over 28 years found that those who imbibed as sparingly as three to six glasses of wine or any other alcoholic drink per week were slightly more likely to develop breast cancer than teetotalers.

Although doctors have long known that women who drank more than about one drink a day were more likely to be diagnosed with breast cancer, the new analysis marks the first clear evidence that even those who consume that amount or less are at increased risk.

“What is novel about our study is we had enough power to ask the question at lower levels of alcohol consumption,” said Wendy Y. Chen, an assistant professor of medicine at Harvard Medical School who led the study published in Wednesday’s Journal of the American Medical Association. “We found a significantly increased risk starting at three to six drinks a week.”

The findings are the latest seemingly head-spinning medical advice about alcohol. For years, doctors advised that women could safely consume about a drink a day, which could be healthful by reducing their risk for heart attacks. Men could get away with two. More than that has long been known to have more risks than benefits, especially for breast cancer among women. Scientists believe alcohol can cause breast cancer by hiking estrogen levels.

Many experts urged caution, however, about overreacting to the new findings. The slight increased risk for breast cancer from such low alcohol consumption was probably still outweighed for many women by the reduction in the risk for heart attacks, which by far kills more women than breast cancer.

“I’m sure a lot of women will be thinking, ‘They told me last week a glass a wine was good for me. Now, they’re saying it will raise my risk for breast cancer,’” said Steven A. Narod of the Women’s College Research Institute in Toronto, who wrote an editorial accompanying the study. “I would not want the average woman who is enjoying one, two, three drinks a week feeling guilty of negligence. At the level of one drink a day, I don’t think it’s a problem.”

But Chen and others said the new findings should prompt women to individually calculate their risks and benefits of alcohol consumption.

“You need to weigh the benefits that would come for cardiovascular disease against possible increased in breast cancer risk. We know that low levels of consumption do appear to be good in terms of heart health,” said Louise Brinton of the National Cancer Institute. “It’s a personal decision.”

For the study, Chen and her colleagues analyzed data collected from 105,986 women ages 30 to 55 who participated in the Nurses’ Health Study, an ongoing project scrutinizing a host of women’s health issues, between 1980 and 2008.

A total of 7,690 of the women were diagnosed with invasive breast cancer. Those who consumed a low level of alcohol - between about 5 and 10 grams a day, which works out to about three to six glasses of wine a week - were 15 percent more likely to be diagnosed with breast cancer.

The risk appeared to increase by 10 percent with each 10 grams of alcohol intake per day, which is about the amount of alcohol in a single drink. Those who consumed at least 30 grams of alcohol a day on average - at least two drinks a day - had a 51 percent increased risk of breast cancer compared to those who abstained. It didn’t matter whether it was beer, wine, scotch, vodka, gin or any other alcohol. Those who drank less than about three drinks a week had no increased risk. Binge drinking was also associated with an increased risk.

Chen noted that it was the average lifetime consumption that was key.

“Let’s say you usually hardly have a drink but you are on vacation and have one glass a day on vacation, that’s not a problem,” Chen said. “That’s an important thing to emphasize - it’s not just what people do in the short term but their cumulative intake over time.”

While alcohol consumption after menopause appeared to increase the risk more than drinking earlier in life, the overall average lifetime consumption appeared to be the most important, Chen said.

“Let’s say you have someone who didn’t drink at all when they were younger. Now they can drink more. Those who drank more when they were young, they should think about cutting back,” she said.

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By Rob Stein