Not all calories are equal when it comes to maintaining a healthy weight, according to research from the US.
A new study, published in the prestigious Journal of the American Medical Association, compared three well-known diets to work out which is the most successful long-term.
"We know that many people can lose weight for a few weeks or months, but most people have difficulty maintaining that for a long time," said author Dr David S Ludwig, who researches obesity prevention at Boston Children's Hospital.
In the study, 21 obese young adults were put on a portion-controlled diet so they lost about 14 percent of their body weight.
Then, the participants were put on month-long trials of three popular diets to see how they compared for weight loss maintenance.
One diet was based on the Atkins low-carbohydrate approach, another consisted of low-fat foods and the third was based around eating low-glycaemic-index foods.
Each diet provided the same number of calories but differed in the amount of carbohydrates, protein and fat. The researchers then measured how many calories people burned each day.
While the low-carbohydrate diet, which consisted of more fat and more protein, allowed people to burn 350 calories each day, the researchers found the broader health implications showed it's not the best choice.
The participants had increased levels of stress hormones and inflammation, which are key risk factors for heart disease, cancer and osteo-arthritis.
They found the low-glycaemic index diet, which consisted of complex carbohydrates such as brown rice and wholegrain bread, proved the most beneficial. People on that diet burned an extra 150 calories a day more than those on a low-fat diet.
"The low-glycaemic index diet gives you many of the same [calorie burning] advantages of the low-carb diet without any of the extremes," Ludwig said.
For decades, low-fat diets were recommended to overweight people, but this study shows they not only burned the least amount of calories each day, but experienced the worst outcomes in terms of insulin resistance, cholesterol and lipid levels.
"From a metabolic perspective, all calories are not alike," Ludwig said. "The quality of the calories going in affects the number of the calories going out."
Jennie Brand-Miller, a professor of human nutrition at the University of Sydney Boden Institute, said this study proves that a low-glycaemic index is the healthiest choice for people wanting to maintain a healthy weight.
"What they've shown is that there are differences between the diets that are in practice in the moment in how they affect your metabolic rate, which is like your 'engine revs', after you've lost weight," she said.
"The message that came out of that paper is that the low-glycaemic diet kept your 'engine revs' running reasonably high and it had the best effect on your health risk factors, like blood fats, insulin levels and inflammatory factors."
Professor Brand-Miller said the fact you can burn more calories on the Atkins-style diet explains why so many people find it effective, but added that the broader negative health impacts mean it's not sustainable.
"The other thing that was bad about the low-carb diet was the fact that cortisol levels were higher, which is your stress hormone," she said.
"It's a measure that your body is not in an ideal state and is finding life a bit stressful. That can interfere with lots of things and in the longer term it encourages people to overeat because it makes you hungry. People can go for a certain length of time on an Atkins-style diet but they are not feeling terrific and in the end don't adhere to that diet."
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