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For fat loss, low-fat diets beat low-carb diets handily, new research finds

It is a central dogma of the low-carb lifestyle: that while avoiding carbohydrates will force the human body into fat-burning mode, any diet that fails to suppress insulin will trap body fat in place and thwart a dieter's hope of shifting to a leaner, healthier body type.

But researchers from the National Institutes of Health have found that the hallowed creed of Atkins acolytes doesn't hold up in the metabolic lab, where dieters can't cheat and respiratory quotients don't lie.

The authors of a study published Thursday in the journal Cell Metabolism conducted a high-tech throw-down pitting a carbohydrate-restricted diet against a weight-loss regimen that reduces dietary fat. Confined for a total of four weeks in an NIH metabolism lab, research subjects got equal calories in each condition (low carb, low fat, each for two weeks).

The subjects in each condition also had equally scant opportunities to cheat, shave or misremember what they ate. Night and day, machines measured not only how much fuel their bodies were burning, but what kind of fuel.

In the end, the obese subjects lost weight regardless of which diet they were on (and low-carb dieters lost a little over a pound more than did low-fat dieters over two weeks). But obese subjects on a low-fat diet lost more body fat than did those on a diet low in carbohydrates. 

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The differences were barely perceptible over a subject's two-week stay in each of the two diet conditions. But the study's authors devised a computer model and projected that over six months, subjects who stuck with a low-fat weight-loss diet would lose 6.5 pounds of body fat more than those who adhered to a diet that restricted carbs.

"In contrast to previous claims about a metabolic advantage of carbohydrate restriction for improving body fat loss, our data and model simulations support the opposite conclusion when comparing the reduced-fat and reduced-carbohydrate diets," the authors wrote. "Furthermore, we can definitively reject the claim that carbohydrate restriction is required for body-fat loss."

Curiously, low-carb dieters did show sustained increases in fat oxidation, while low-fat dieters showed no such increase. One difference between the two groups may have played a key role in helping the low-fat dieters lose more body fat: compared with carb-restricted dieters, those on reduced-fat diets tended to burn more calories while they slept.

In the war of words between low-fat zealots and the carbohydrate-averse, these findings are a small but significant victory for proponents of reduced-fat diets. But while this battle has gone to the low-fat camp,  lead study author Kevin D. Hall said, the war is far from over.

"Our study suggests it's probably the calories in a diet that matter much more than the carbohydrates or the fat," Hall said. Getting those calories down--and keeping them down for the long haul--is the key challenge for dieters, he added.

There may be some satisfaction in puncturing low-carb champions' claims of metabolic superiority, Hall said. And the findings should reassure those who prefer to whittle their dietary fat and enjoy the occasional carb that their efforts to shape up are not in vain, he added.

But for dieters outside the metabolism lab, "it's whether you can stick to a diet that's the most important thing," said Hall. For some dieters, paring back on carbs is a small price to pay for the satisfaction of consuming a bit more fat. For others, putting toast and pasta off-limits is a formula for diet sabotage.

Those real-world preferences, and the neural bases for them, will be the subject of ongoing research in Hall's biological modeling lab at the NIH's National Institute of Diabetes and Digestive and Kidney Diseases.

As the 19 subjects recruited for the current study dieted their way through four weeks of low-carb and low-fat regimens, Hall and his colleagues conducted brain scans and other tests to glean how diets with differing nutrient compositions affected their mood, motivation and sense of satisfaction. That, said Hall, should begin to shed light on the factors that shape individuals' ability to stick to diets.

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