Many popular diets promise a miracle plan that will allow you to lose weight without health consequences. A study by Dr. Kristen D’Anci, published recently in Appetite, complicates these “easy” solutions by showing that even when there are no obvious physical side-effects, different diets can cause different cognitive impairments.

Dr. D’Anci compared two groups of women, those on a low-carbohydrate diet and those following the balanced reduced-calorie diet recommended by the American Dietetic Association. The subjects were assessed with a wide range of cognitive tests two days before beginning the diet, and then 1, 2, and 3 weeks after beginning. There were significant differences in memory, mood, and attention.

Low-carbohydrate dieters performed worse than the low-calorie dieters on memory tasks, but were able to respond more swiftly to an attention vigilance task. They also reported less confusion on the Profile of Moods Scale. The researchers asserted with confidence that impaired memory was directly related to low-carbohydrate diet because the impairment peaked when glycogen levels were at their lowest, and recovered entirely when carbohydrates were reintroduced into the diet.

Dieters should pay close attention to these findings, because they suggest that each diet comes with unique mental consequences. It is not enough to evaluate a diet by its effect on your waistline; you should ask questions about how it will affect your mind.