Scientists fed 19 obese adults controlled diets over two separate fortnights, where the slimmers cut carbs by 30 per cent during the first period and then reducing fat intake by the same amount.
Contrary to regimes such as the Atkins diet, where participants avoid carbs entirely, the study found that the low-fat diet delivered better results.
The team from the US National Institute of Diabetes and Digestive and Kidney Diseases predicts that over long periods the body will automatically minimise the difference to fat loss caused by alternative diets of the same calorie count.
Lead scientist Dr Kevin Hall said: “There is one set of beliefs that says all calories are exactly equal when it comes to body fat loss and there’s another that says carbohydrate calories are particularly fattening, so cutting those should lead to more fat loss.
“Our results showed that, actually, not all calories are created equal when it comes to body fat loss, but over the long term, it’s pretty close.”
The results published yesterday in the journal Cell Metabolism confirmed the results of computer simulations carried out by the same team which predicted that only the carb-restricted diet would lead to changes in the amount of fat burned.
It found that the reduced-fat diet would lead to greater overall fat loss. Dr Hall said: “A lot of people have very strong opinions about what matters for weight loss, and the physiological data upon which those beliefs are based are sometimes lacking.
“I wanted to rigorously test the theory that carbohydrate restriction is particularly effective for losing body fat, since this idea has been influencing many people’s decisions about their diets.”
Dr Hall said his latest study offers some of the “most precise human data yet” on whether cutting carbs or fat results in the most fat loss but he cautioned against drawing sweeping conclusions from it.
The sample size had to be limited to 19 because the research was so expensive and the menu fed to participants might not be reflective of a normal diet, he warned.
He added: “We are trying to do very careful studies in humans to better understand the underlying physiology that will one day be able to help generate better recommendations about day-to-day dieting.
“But there is currently a gap between our understanding of the physiology and our ability to make effective diet recommendations for lasting weight loss.”
However Scottish nutritionist Emma Conroy warned against drawing conclusions from the study as she said that the majority of evidence supports a low-carb diet, and low-fat diets are usually doomed to fail.
Ms Conroy, who runs Edinburgh Nutrition, said: “The evidence points overwhelmingly towards a low-carb diet for a variety of reasons.
“Our bodies have a sense of how much fat we are carrying, so if we start losing it quickly then it may start sending signals that something is wrong and start trying to preserve fat stores.
“A low-fat diet may cause more weight loss but we have to ask ourselves whether we are losing muscle as well, rather than just fat.”
She warned that a low-fat diet might push people to consume foods that are higher in sugar content.
Ms Conroy added: “I would caution against adopting any major dietary changes from this as the overwhelming weight of evidence favours a low carbohydrate diet.”