The study, published in the journal BMJ, was based on a random sample of 43,396 women ages 30 to 49, each of whom completed a dietary questionnaire. The researchers used the data to create a 20-point scale, with higher scores indicating a lower ratio of carbohydrates to protein.

During an average of 15 years of follow-up, there were 1,270 cardiac events, mostly ischemic heart disease and strokes. After controlling for numerous risk factors, the researchers found that women had a 5 percent increase in cardiovascular events for each increase of 2 points on the scale. (That translates into a daily 20-gram decrease in carbohydrates and a 5-gram increase in protein.)

Previous studies have had mixed results. The 2006 Nurses’ Health Study showed an increase in cardiovascular mortality for the low-carbohydrate diet, but it was not statistically significant. Several European studies, though, have found statistically significant increases.

The study’s lead author, Dr. Pagona Lagiou, a professor of epidemiology at the University of Athens, said, “To those who want to control body weight, I would suggest that they try to increase their physical activity and reduce their portion sizes.”