Low-fat dieting 'beats carb-cutting'
A low-fat diet beats carb-cutting as the most effective way to slim, research has shown.
The dilemma over which strategy to adopt has taxed many a weight-watcher.
While reducing carbohydrate lowers insulin levels and burns fat, avoiding fatty foods delivered better results overall, scientists found.
The findings, from a study of 19 obese volunteers, confirmed the predictions of an earlier computer simulation conducted by the same team.
Lead scientist Dr Kevin Hall, from the US National Institute of Diabetes and Digestive and Kidney Diseases, 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."
The researchers watched every morsel of food consumed by the volunteers over a pair of two-week periods.
During the first two weeks, 30% of participants' original daily calorie intake was cut through carbohydrate restriction alone while fat consumption remained the same.
Over the following two weeks, conditions were reversed. Again, calories were cut by 30%, but this time by reducing the amount of fat eaten. Carbohydrate consumption was unaltered.
Body fat loss as a result of dietary fat restriction was significantly greater than that achieved by cutting carbs, the results reported in the journal Cell Metabolism showed.
This was despite the fact that more fat was burned with the low carbohydrate diet.
However, the simulations predicted that over long periods of time the body was likely to try to minimise body fat differences between the two diets.
"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," said Dr Hall.
"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."
Cumulative fat loss was at least 110 grams greater with the reduced fat diet than with the reduced carbohydrate diet, although "substantially larger" differences were observed.
But Dr Hall cautioned against making "sweeping conclusions" about how to diet.
He pointed out that the research was limited by its small sample size, with only 19 participants recruited, and the menus followed were not a true reflection of "normal dieting".
Nor did the study take into account which diet people would find easier to stick to over extended periods.
Dr Hall 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."
Jamie Blackshaw, healthy weight and obesity lead at Public Health England, said: "The best way to lose weight is by reducing calories in the diet and increasing physical activity. Aim to lose around one to two pounds a week to achieve a healthier weight. The NHS Choices weight loss plan provides lots of useful advice and recipes."