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Binging on food—whether during a Sunday football showdown or during the holidays with the family—is never a great idea. Even if you try to offset the occasional burger splurge with some follow-up exercise or extra attention to the crudité plate, there’s no denying that a fatty food bomb can leave you feeling pretty awful.

And, as it turns out, even a single cheat day’s worth of pigging out on pizza or sausage dip may be harming our body more than we thought, a new study has shown.

In a small study at Loughborough University in the U.K., researchers fed 15 healthy, young adults—nonsmokers, physically active, and free of cardiovascular disease or diabetes—a diet packed with greasy stuff like sausages, bacon, cheese, burgers, and cheesecake for one day. They then gave the participants a sugary drink followed by an oral glucose tolerance test.

The finding? After eating all that fat, the participants’ bodies were substantially worse at handling sugar, which is linked to a reduction in insulin sensitivity.

“Our pilot data suggests that a single day of high-fat overfeeding is sufficient to impair whole-body insulin sensitivity in young, healthy individuals,” said Siôn Parry, a doctoral candidate at the Loughborough University School of Sport, Exercise and Health Sciences. “This may have implications for those with binge-eating disorders, or those who overeat during holiday periods or at times of celebration…We also do not know how long the negative effects of this diet last, or whether repeated periods of short-term, high-fat overfeeding leads to a progressive worsening of glycemic control.”

Bottom line: Even if you’re ready to go on a high-fat keto diet, be sure to prioritize healthy fats that won’t hammer your system quite as hard.