New study: Low-fat vs low-carb diets

Comparing low-fat and low-carb diets

Researchers have found that, contrary to popular claims, low-fat diets can lead to greater body fat loss than low-carb diets. This is despite the fact that low-carb diets reduce insulin and increase fat burning.

Model weight loss

Since 2003, Kevin Hall, PhD - a physicist turned metabolism researcher at the National Institute of Diabetes and Digestive and Kidney Diseases - has been using data from dozens of controlled feeding studies to build mathematical models of how different nutrients affect human metabolism and weight.

His model simulations showed that only the carb-restricted diet would lead to changes in the amount of fat burned by the body, whereas the reduced-fat diet would lead to greater overall body fat loss, but he needed the human data to back it up.

Monitoring every morsel of food

Studying the effects of diet is difficult because participants may cheat on the diet. To counter this, researchers confined 19 consenting obese adults to a metabolic ward for a pair of two-week periods and controlled every morsel of food the participants ate.

During the first two-week period, 30 percent of calories were cut through carb restriction, while fat intake remained the same. During the second two-week period the conditions were reversed.

More fat lost on low-fat diet

At the end of the two dieting periods, the mathematical model proved to be correct.

Body fat lost with a low-fat diet was greater compared with a low-carb diet, even though more fat was burned with the low-carb diet.

However, over prolonged periods the model predicted that the body acts to minimise body fat differences between diets that are equal in calories but varying widely in their ratio of carbohydrate to fat.

The best diet is…

Hall cautions against making sweeping conclusions from this study about how to diet.

"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," Hall says. "But there is currently a gap between our understanding of the physiology and our ability to make effective diet recommendations for lasting weight loss."

Hall recommends that for now, the best diet is the one that you can stick to. His lab will be continuing their investigations and hope the results may reveal why people respond differently to different diets.

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