Which is better for weight loss -- a high-protein diet or a high-carb diet? That endless debate got a new twist Monday.
In a yearlong study, Australian researchers found that both diets worked equally well when it came to shedding pounds but those on the low-carb diet were in considerably worse moods.
The report, published in the Archives of Internal Medicine, assigned 106 overweight and obese men and women to either a low-carb diet high in fat and protein or a high-carb diet low in fat and protein.
The participants' weight was noted at weeks eight, 24, 40 and 52, and their emotional state was evaluated via three standard questionnaires measuring aspects of mood, including anxiety, depression and anger.
Both groups lost an average of 30 pounds, slightly more than in most studies of this type. And in early weeks of the study, both groups showed an improvement in mood.
However, although mood improvements remained in the high-carb group, they went back to original levels in the low-carb group.
On the low-carb plan, diets consisted of 4% carbohydrates, 35% protein and 61% fat, while the high-carb diet comprised 46% carbohydrates, 24% protein and 30% fat. Each group was allowed the same number of calories daily, about 1,400 for women and about 1,700 for men.
Most studies on diets typically focus on pounds lost, pounds kept off and cardiovascular function. But there is good reason to also consider mood, said study lead author Grant Brinkworth of the Commonwealth Scientific and Industrial Research Organisation/Food and Nutritional Sciences in Adelaide.
"Anything that adversely affects mood could affect someone's ability to maintain weight loss," Brinkworth said.
The results could be evidence that it's tough to stick to a low-carb diet in cultures where high-carb foods are abundant, the authors speculated.
"Over the long term, trying to maintain that dietary pattern may mean coming across a lot of challenges," Brinkworth said. "That may cause a negative mood impact, even though you're getting a good weight loss."
But the reason for the mood dip could also be biological -- a low-carb diet can cause a drop in levels of the brain chemical serotonin. Low serotonin levels are linked to depression.
Brinkworth said that longer studies might be necessary to see which dietary plans get the best results. And even then, since most dieters aren't under the constraints of a research study, real-life results may differ from what his team found.
Still, the study does raise the idea that people should pay attention to their moods while dieting.
"It is something to consider," Brinkworth said. "If you go into a more negative state, it may potentially expose you to weight gain in the long term."
Dr. David Heber, director of the UCLA Center for Human Nutrition, said that most people find diets to be stressful. "If you're going to feel crummy, there's no point in doing it," he said.