Both low-carbohydrate diets, such as the Atkins program, and low-fat diets appear to help people lose pounds over the course of a year. But only low-fat diets offer a long-term boost to the mood, finds a new study in the Archives of Internal Medicine.

For the new study, 106 overweight and obese adults were randomly assigned to one of two diets for one year:

• a low-carb, high-fat plan
• or a high-carb, low-fat diet.

Both diets restricted calories to about 1,433 to 1,672 a day. Changes in mood, well-being, and cognitive functioning (thinking, learning and memory skills) were assessed periodically during and following the one-year diet.

A year later, average weight loss was about the same in each group: 13.7 kilograms or about 30 pounds.

Participants in both groups showed mood improvements after the first eight weeks, presumably as the participants enjoyed their early weight loss and felt good about their lifestyle changes. But that mood boost lasted only in the low-fat group.

After a year, the mood of those in the high-fat group returned to what it had been before they started dieting, the study found.

"This outcome suggests that some aspects of the low-carbohydrate diet may have had detrimental effects on mood that, over the term of one year, negated any positive effects of weight loss," the authors write.

It's not clear why those on the low-carb plan saw their moods slip. It might be that there was social difficulty in adhering to a low-carb plan, which is counter to the typical Western diet full of pasta and bread. It might be that the strict, highly structured nature of the diet was difficult for dieters.

Another explanation might be that the protein and fat intake affected brain levels of serotonin, a neurotransmitter related to mood and happiness.

Carbs such as crusty breads, pastries and pastas act like comfort food to the brain and can increase serotonin concentrations in the brain, whereas added fat and protein can reduce serotonin concentrations.

The study authors don't think there were differences in the nutrient content of either diet, since both groups experienced similar changes in thinking and memory performance over time.

"Further studies are required to evaluate the effects of these diets on a wider range of cognitive domains," the authors conclude.