Researchers compared the results of participants on low-fat and low-carb diets over the course of a year. Both lost the same amount of weight and both experienced a boost in positive mood the first few weeks of dieting.
But the good mood didn't last long for those on the low-carb diet. The study in the Archives of Internal Medicine suggests one reason is low-carb diets are less flexible and tend a be socially difficult to keep up. Also, carbohydrates have been linked with the production of positive mood chemicals in the brain. It's possible the lack of carbohydrates themselves affect mood.