"Food choices changed on the days that people drank... and changed in an unhealthier direction for both men and women," said Rosalind Breslow, a nutritional epidemiologist at the National Institute on Alcohol Abuse and Alcoholism and the lead author of the study.
She said the new information gives people an opportunity to be more aware of what they're eating on the days they imbibe.
In a previous study, Breslow found people who drink more tend to have poorer diets in general, compared to those who drink less. For the current research, she and her colleagues looked at volunteers' diets on both the days they drank and the days they abstained.
The data came from a large U.S. health and lifestyle survey conducted in 2003 through 2008.
More than 1,800 people answered a diet questionnaire on two days within a 10-day span -- one day when they drank and another when they did not. When people did imbibe, they had an average of two to three alcoholic beverages at a time, most commonly beer and wine.
Breslow's team found that on non-alcohol days, men in the study ate about 2,400 calories, based on their diet reports, and women consumed 1,700 calories, on average.
When they also drank, men took in about 400 more daily calories and women about 300 more.
For women, the extra calories could be explained by the alcohol alone, but for men, between 100 and 200 were from food.
The types of food people ate -- not just how much -- changed on the days they drank as well. For example, men and women both ate about nine percent more fat when they drank alcohol, the researchers reported in The American Journal of Clinical Nutrition.
Men reported eating more white potatoes and meat on their drinking days and both men and women drank less milk.
"Why that's happening, it's very hard to speculate based on this," said Dr. Suthat Liangpunsakul, who studies alcohol consumption at the Indiana University School of Medicine in Indianapolis and was not involved in the study.
Breslow said there are a number of possible explanations.
It could be that social events that involve drinking often also involve less-healthy foods, or that people are more impulsive when they drink and do not stop themselves from indulging.
Although food choices tended to go in a less-healthy direction on drinking days, "we can't say that because these people were taking in more calories that they would be gaining weight, because we didn't study that," Breslow told Reuters Health.
The researchers do not know how people ate on the other days of the week, she pointed out, and they might have been compensating for their poor eating habits on drinking days.
Liangpunsakul agreed that it's difficult to determine just how important less-healthy diets on drinking days might be to people's health.
"We don't know if it will (influence) obesity or weight changes," he told Reuters Health.
Breslow said people should be aware that alcoholic beverages add calories and be mindful of how they eat when they drink alcohol, focusing on whole grains, healthy oils and vegetables. And, she added, people should drink in moderation.
Current federal guidelines recommend women drink no more than one alcoholic beverage each day and men no more than two.