Low carbohydrate diet can leave you grumpier, it has been found.
Scientists at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Flinders and South Australia Universities compared the moods of people on different diets.
After randomising 118 obese people aged 24 to 64 to either a low fat or low carbohydrate restricted energy diet, researchers followed participants for 12 months and measured both body weight and mood scores.
While mood scores for both groups improved dramatically in the first eight weeks, this may be due to associated early weight loss, the study authors said.
Of the 55 people who completed the trial, researchers found participants in the low fat group maintained the improvement over the course of the trial, while mood scores in the low carbohydrate group returned towards more negative baseline levels.
"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 wrote in the Archives of Internal Medicine.
This may be due the difficult nature of adhering to a restrictive diet or neurological reasons such as the effects of the diet on serotonin levels, researchers said.
As both groups recorded similar scores in cognitive tests over the study there was no evidence the dietary composition of either diet effected cognitive functioning.
"Everyone achieved and got benefits initially - they all improved their mood - but over time there was a gradual loss of those benefits for the people who were consuming the low carbohydrate diet," said Jon Buckley, a co-author and deputy director of the Nutritional Physiology Research Centre at the University of South Australia.
"This was despite achieving the same levels of weight loss.
"No one got grumpier than when they started out, but those on the low fat diet, if you like, got happier and those on the low carb diet didn't get that same benefit, they in some cases reverted back to where they were at base line."
He says the reason low-fat diet produces greater happiness may be because people feel they are actively doing something for themselves.
Another theory involves people eating extremely low carbohydrate diets having restricted blood flow.
"That can potentially impact on mood because we know conditions like depression are associated with changes in cerebral blood flow," he said.
It remains somewhat of a mystery why people eating more carbohydrates are happier than other dieters, as the body is not solely dependent on carbohydrates to maintain mood.
"Your body has other mechanisms for producing glucose if you're not taking it in," he said.
Another mystery - according to Associate Professor Buckley - is whether a regular low fat diet is healthier than the Atkins style menu.
"We found that you get better improvements on some things and lesser improvements on others. So it's still difficult to work out which is the best dietary plan," he said.
The Dietitians Association of Australia recommends a balanced, low fat diet.