A study has found that although people lose weight on trendy low-carbohydrate diets, they also tend to become worse-tempered than those on low-fat diets.

Low-fat diets work equally well but improve the dieters' frame of mind.

CSIRO researchers placed 106 overweight people on diets for 12 months, randomly splitting the group between low-fat and low-carb, The Courier-Mail reports.

Researcher Grant Brinkworth and colleagues found a low-calorie, low-fat diet more beneficial to dieters' moods than low-carb with the same number of calories.

Dr Brinkworth said changes in body weight, mood, well-being and cognitive functioning - thinking, learning and memory skills - were assessed periodically during and after the diets.

"Potential explanations include the social difficulty of adhering to a low-carbohydrate plan, which is counter to the typical western diet full of pasta and bread; the prescribed, structured nature of the diet; or effects of protein and fat intake on brain levels of serotonin, a neurotransmitter related to psychological functioning," Dr Brinkworth said.

Writing in the Archives of Internal Medicine, Dr Brinkworth said there was no evidence the nutrient content of either diet was associated with changes in cognitive function, since both groups experienced similar changes in thinking and memory performance.

After one year, the overall average weight loss of the two groups was 13.7kg.

After the first eight weeks of dieting both groups experienced a more positive state of mind which reflects that obese people who lose weight generally are in a better mood.

But a lasting improvement continued only in those following the low-fat diet.

Researchers said despite consistent advice that a high-carbohydrate, low-fat, energy-restricted diet for obesity treatment was best, the obesity epidemic had led to widespread interest in alternative dietary patterns.

These included low-carbohydrate diets, typically high in protein and fat - particularly saturated fat.

The study was done with National Heart Foundation and National Health and Medical Research Council grants.

Meanwhile, a French team has found that people who walk slowly are three times more likely to die from heart disease than those who walk faster.

The findings highlight the role of fitness in preserving life and function in older age.

The study at the University Pierre et Marie Curie in Paris monitored 3208 men and women aged 65 to 85 years in three cities.