Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity, Cell Metabolism, 13 August 2015
- 19 adults with obesity were confined to a metabolic ward for two 2-week periods
- Cutting carbohydrates increased net fat oxidation, but cutting fat by equal calories had no effect
- Cutting fat resulted in more body fat loss as measured by metabolic balance
- Mathematical model simulations predicted small long-term differences in body fat
Dietary carbohydrate restriction has been purported to cause endocrine adaptations that promote body fat loss more than dietary fat restriction. We selectively restricted dietary carbohydrate versus fat for 6 days following a 5-day baseline diet in 19 adults with obesity confined to a metabolic ward where they exercised daily. Subjects received both isocaloric diets in random order during each of two inpatient stays. Body fat loss was calculated as the difference between daily fat intake and net fat oxidation measured while residing in a metabolic chamber. Whereas carbohydrate restriction led to sustained increases in fat oxidation and loss of 53 ± 6 g/day of body fat, fat oxidation was unchanged by fat restriction, leading to 89 ± 6 g/day of fat loss, and was significantly greater than carbohydrate restriction (p = 0.002).
People ended up losing weight on both diets, but they lost slightly more on the reduced-carb diet. That didn’t surprise Hall at all. “We’ve known for quite some time that reduction of dietary carbohydrates causes an excess of water loss,” he says, so the weight loss may be due to water loss. As expected — and in keeping with the theory about carb-cutting — insulin levels went down and fat burning went up.
But on the low-fat diet, people lost more fat, “despite not changing insulin one bit,” Hall says.
How is this possible? The exact mechanism is yet to be determined, but Hall has some ideas. “When we cut fat in people’s diets, the body just doesn’t recognize that we’ve done that … in terms of metabolism, so it keeps burning the same number of calories [and fat] as it did before,” he says. This surprised him; Hall thought that the body would somehow respond to the reduction in fat, but it didn’t.
“Insulin is a hormone that is particularly reactive to changes in carbohydrate,” says Hall. “What I was sort of hoping to find was an analogous hormone that was responsive to changes in fat in the diet and altered metabolism.” But they didn’t find it. “It might not exist,” he says.
What they did find was that cutting 800 calories of fat resulted in the body burning just as much fat as before. In contrast, on a low-carb diet, metabolism changes: insulin levels went, carb-burning went down and fat-burning went up, but only by about 400 calories a day, Hall says. That means that low-carb dieters had a net deficit of about 400 calories per day—but those on the low-fat diet had a net deficit of about 800 calories per day, resulting in slightly less body fat.
“The takeaway for me is that the theory about metabolism that has previously been used to recommend low-carbohydrate diets probably doesn’t hold water.” “In fact, if anything, the reduced fat diet seemed to offer a slight metabolic advantage.”
The results of this study sure do place a check mark in the column of “The fat you eat is the fat you wear.”