A low-fat diet seems to boost dieters' mood more than a low-carbohydrate diet, Australian researchers have found.

Very low-carbohydrate diets are often used to help overweight and obese people lose weight, but the long-term effects on psychological well-being are unclear.

To investigate, researchers randomly assigned 106 overweight and obese adults to follow either a very-low-carbohydrate, high-fat diet or a high-carbohydrate, low-fat diet for one year. They assessed changes in body weight and mood and well-being periodically during and one year after the study ended.

After one year, the dieters lost an average of 30.2 pounds, with no difference between the two groups.

After the first eight weeks, tests showed that dieters in both groups experienced an improvement in their mood.

However, most measurements of mood revealed a lasting improvement in only those dieters following the low-fat diet. In general, mood in the dieters on the high-fat plan returned toward more negative "baseline" levels, the researchers report.

"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," they note in the latest issue of Archives of Internal Medicine, published November 9.

These results were a surprise, study chief Dr. Grant D. Brinkworth, of the Commonwealth Scientific and Industrial Research Organization in Adelaide, Australia told Reuters Health.

"With mood, we would have thought that the weight loss effects would have been the most potent effects," Brinkworth said. "We already saw improvement in mood over eight weeks. We would have thought that would have been sustained in both groups due to weight loss effects. But the interesting thing is that they tended to regress toward baseline levels in the low-carb group."

The social difficulty of sticking to a low-carbohydrate plan, which is counter to the typical Western diet full of pasta and bread, is one possible explanation for the findings, the researchers say.

Brinkworth added: "When you look at our food supply, Australia and the States are very similar, it's very biased toward high carbohydrate diet and intake of high carbohydrate fruit. So if you try to follow these low carb diets over the long term, if it's not your preferential or habitual type of diet, you may come across some significant challenges," which dampen a person's mood.

SOURCE: Archives of Internal Medicine, November 9, 2009.