SYDNEY – In the ongoing debate about which diet is the best, here’s a question you may not have heard before: Which diet is most likely to make you happy?

A new study, surprisingly, indicates that when it comes to elevating your mood, not all diets are created equal.

Grant Brinkworth of the Commonwealth Scientific and Industrial Research Organization-Food and Nutritional Sciences in Adelaide, Australia, and colleagues studied 106 overweight and obese subjects. Fifty-five of them were put on low-carb, high-fat diets, and 51 were put on a high-carb, low-fat diets.

After a year, both groups had lost about the same amount of weight–an average of about 30 pounds per person. Both groups also scored about the same on tests that measure general thinking abilities. And both groups reported feeling an improvement in mood after the first eight weeks.

But only those on the low-fat diet continued to feel better after that. The moods of those on the high-fat diet fell back to where they were before their diets, the researchers reported in the Nov. 9 issue of the Archives of Internal Medicine.

The findings suggest that something about the low-carb diet negates the positive effects of losing weight on mood. They’re not sure what that might be. They speculate that it’s just too difficult to eat a low-carb diet in a culture full of pasta and bread. But maybe there is something about the effects of protein and fat on brain chemicals such as serotonin.