Here’s some news that might make a lot of women feel as if they need a drink: The compound in red wine suspected of having a host of health benefits has for the first time shown promising test results in people. But a new Harvard study indicates that indulging in as few as three drinks a week may boost a woman’s risk for breast cancer.

So, should that be a glass of merlot? Or just plain water?

Scientists, of course, say: It depends.

“If you are someone with a family history of breast cancer but are healthy, at a good weight, exercise regularly, have a healthy diet and don’t have a risk for heart disease, then you may make one decision,” said Wendy Y. Chen of the Harvard Medical School. “Another woman who has some cardiovascular risk factors and no history of breast cancer may make a different decision.”

The findings, both released Tuesday, are the latest seemingly head-spinning medical advice about alcohol. For years, doctors advised that women could safely consume about a drink a day. Men could get away with two. More servings have long been known to have more risks than benefits, especially for breast cancer among women. Scientists think alcohol can cause breast cancer by raising estrogen levels.

Many experts urged caution 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 a possible reduction in the risk of heart disease, which kills far more women than breast cancer.

“Women who abstain from all alcohol may find that a potential benefit of lower breast cancer is more than offset by the relinquished benefit of reduced cardiovascular mortality associated with an occasional glass of red wine,” Steven A. Narod, of the Women’s College Research Institute in Toronto, wrote in an editorial accompanying the study in Wednesday’s New England Journal of Medicine.

The new analysis marks the first clear evidence that even those who consume a drink a day or fewer are at increased risk. Chen and her colleagues analyzed data collected between 1980 and 2008 from 105,986 women in the Nurses’ Health Study, an ongoing project scrutinizing women’s health issues. A total of 7,690 of those women received a diagnosis of invasive breast cancer. Those who consumed 5 to 10 grams a day, or three to six glasses of wine a week, were 15 percent more likely to receive a diagnosis of breast cancer.

It didn’t matter whether women, whose ages ranged from 30 to 55, drank beer, wine, scotch, vodka, gin or any other alcohol. Those who drank fewer than about three drinks a week had no increased risk. Binge drinking was also associated with an increased risk. Chen said that average lifetime consumption is 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.”

The second study involved resveratrol, which is found in trace amounts in red wine. Many scientists have been excited about resveratrol since studies in yeast, fruit flies and laboratory mice indicated that the substance could mimic the benefits of consuming a low-calorie diet, which has been shown to extend longevity in many species.

In tests on mice, resveratrol appeared to protect the animals from obesity and diabetes, boost their endurance, reduce their chances of suffering the ill effects of obesity and extend their lives.

Such findings prompted some scientists to speculate that the resveratrol in red wine might help explain the “French paradox,” which is that French people consume a relatively rich diet but live as long as anyone else. Some people have even started taking resveratrol, which is sold over the counter in health and grocery stores.

But resveratrol had never been tested in people, and the potential risk of high doses for long periods remains unknown.

In the new study, researchers gave 150 milligrams or a placebo to 11 healthy obese men for 30 days, then switched those on placebo to resveratrol, and vice versa, for another month.

Resveratrol appeared to produce all of the same effects in the human subjects as it had in animals, such as lowering the metabolic rate; cutting the accumulation of fat in the liver; reducing blood sugar, blood pressure, triglycerides and inflammation; and boosting the efficiency of muscles. There were no apparent side effects.

“We are very excited,” said Patrick Schrauwen of Maastricht University in the Netherlands, whose research was posted online by the journal Cell Metabolism. “We found a lot of effects that all point in the same direction of better metabolic health.”

The dose was much lower than that used in animal studies and the amount many people take on their own, Schrauwen said. But a person would have to drink about two gallons of red wine a day to get the equivalent amount of resveratrol.

The findings were praised by other researchers who have been studying resveratrol and other compounds that increase proteins known as sirtuins thought to have the beneficial effects.

“This study comes at a time when obesity and diabetes are reaching epidemic proportions,” said David A. Sinclair of Harvard Medical School. “Sirtuins offer the promise that we can find ways to prevent the effects of obesity and sedentary lifestyles.”

Schrauwen and others said that more research is needed to determine whether resveratrol’s effects translate into health benefits, especially for those who are not obese, and whether it is safe over the long term.