We’ve all done it. Perhaps it isn’t long after New Year’s and we’re still adhering to our resolution to eat a healthier diet. But, hey, it’s the weekend and can’t we let our hair down a little? How about a guilty pleasure, like a cheeseburger and fries?

Cheating once in a while is no big deal, right?

Ahh, guess again. Research out of Rush University Medical Center in Chicago found that intermingling some junky food in an otherwise wholesome Mediterranean diet of lots of fruits, vegetables, nuts, fish and healthy fats deflates some of the healthy benefits you might receive from eating well.

The study looked at the diets of 5,000 older adults participating in the Chicago Health and Aging Project that was conducted from 1993 to 2012. Aside from tracking the foods eaten, the investigation included cognitive testing involving memory and brain processing skills.

The people with the slowest cognitive decline tended to closely follow the Mediterranean diet. Including a less healthy Western diet of sweets, refined grains and red and processed meats diminished benefit from their otherwise healthy food consumption.

It’s no surprise that a Mediterranean diet is good for us. In addition to its cognitive benefits, it’s been shown to lower the risk of heart disease, cancer and diabetes..

Indeed, those adhering to a strong Mediterranean plate were cognitively nearly six years younger than those who ate the poorest, least-nutritional foods.

Scientists say their results can’t be generalized. Even so, it can offer a lesson in the benefits of sticking with a good diet. Don’t cheat. Your brain will thank you for it.