A glass of wine with dinner might not be a bad idea for your waistline, according to a new study by Harvard researchers, which found that middle-aged and older women who drink moderately experience significantly less weight gain than those who abstain.

This was the first long-term study to follow a large group of women that provides solid evidence for the inverse association between alcohol consumption and weight gain.

Because alcohol is considered to be a substantial source of calories that can contribute to weight gain, these findings can be “counterintuitive,” said Lu Wang, the lead author and an instructor in medicine at Harvard Medical School.

The researchers tracked a group of over 19,000 women aged 38 to 89 and periodically assessed their body mass index, eating habits, and other health indicators. The women were all healthcare professionals who started the study in good health and with a BMI within the normal range.

In the study, light to moderate drinking was defined as consuming an average of two to three alcoholic drinks daily.

At the end of a thirteen-year period, researchers concluded that, overall, the women who drank moderately showed significantly less weight gain than women who did not drink.

“It did surprise us that the association between moderate alcohol consumption and risk of developing obesity was so strong,” said JoAnn E. Manson ’75, a Medical School and School of Public Health professor and one of the study’s authors.

Although the study did not point to any mechanisms to explain this association, the researchers hypothesized that the cause could lie in both the women’s behavior and biology.

Previous studies have shown that women who drink in moderate amounts compensate by consuming fewer non-alcoholic calories and, unlike men, women who drink with meals also tend to reduce their food consumption, Wang explained.

Biological pathways, such as alcohol digestion or fat accumulation in women, may also play a part in this link, Wang said. She added that these areas are potential avenues for future research.

While the study provides a favorable reason for drinking, the researchers emphasized that people should extrapolate from these findings with caution. Manson said that the alcohol amount should be kept moderate, accompanied by healthy eating and regular exercise.

“The key point is that people should definitely not begin drinking for the express purpose of weight control,” Manson said, referencing the host of health problems that have been shown to be associated with alcohol consumption in women.

The study was published Monday in the journal Archives of Internal Medicine.

—Staff writer Helen X. Yang can be reached at hxyang@fas.harvard.edu.