Low-carb diets do not burn fat

roponents of low-carb diets will tell you that a high-fat, high-protein, low-carbohydrate diet will induce your body to burn fat. The theory is that a higher intake of carbohydrates in the diet causes greater insulin secretion, and that results in the body directing its circulating fat into storage. With less fat circulating for the body to use for fuel, the body decreases its basal metabolic rate and increases food intake. This results in weight gain. Decreasing the proportion of carbohydrate as compared to fat in the diet without cutting protein or calories, they argue, will then theoretically reduce insulin secretion, raise your basal metabolic rate, and induce the body to use stored fat for fuel.

The problem, of course, is that humans are notoriously unreliable. Even when the researchers provide all meals to study volunteers, it's difficult to draw really solid conclusions because people might not follow the diet as closely as they should, not consuming the exact number of grams of fat, carbohydrate, or protein (that is to say, cheating on their diet). Further, the most accurate way to measure the number of calories a person burns is by using a metabolic chamber that the subject lives in for 24 hours - and as you might guess, those are extremely expensive to build and use. Finally, the most accurate measure of a person's body fat is done by using sophisticated X-ray equipment, not calipers or indirect estimates using Body Mass Index.

Fortunately, a team of researchers with the National Institute of Health (NIH) were able to put together the funding to do a truly solid piece of research (Am J Clin Nutr 2016;104:324-33). Seventeen overweight or obese men who were otherwise healthy agreed to move into the study lab for the duration of the 8-week study, never leaving the lab. Each man was prescribed 90 minutes of light stationary cycling each day and wore accelerometers at all times so that the researchers could better estimate their daily caloric burn.

The study compared the results of two different 4-week diet plans. For the first four weeks of the study the participants consumed a high-carbohydrate diet similar in macronutrient proportions to their usual diets. The diets were specially designed by dietitians to use few processed foods and little added or liquid sugars and provide all required nutrition to maintain their current weight. The second four weeks of the study the participants' diets were switched to a low-carbohydrate and higher-fat diet that contained the exact same amount of protein as the diet from the previous four weeks. This second diet more than doubled their intake of total fat, from 93 grams to 212 grams, and cut their carbohydrate intake by 90% (from 300 grams to 31).

During each of the two dietary periods the participants spent two days each in a metabolic chamber so that the authors could precisely measure the number of calories they burned, whether awake, exercising, or asleep. Further, the authors used a method known as the "doubly-labeled water method" (another gold standard) to measure caloric expenditure when the participants were not living in the metabolic chamber. At the beginning, midpoint, and end of each dietary period the authors measured the participants' body composition (fat vs. lean mass) using the standard X-ray spectrometry.

The results are quite interesting. All of the subjects lost weight over the course of the two dietary periods. On average, during the baseline diet (the first dietary period) they lost about 1 kilogram, with about half of that being lost body fat. When they switched to the low-carb diet, in the first two weeks the subjects lost about another 1.6 kilograms - which represented mostly water, as they only lost about .2 kilograms of body fat. Over the entire 4-week low-carb diet period they lost a total of 2.2 kilograms, on average, with total fat loss of only about .5 kilograms.

The subjects did, however, decrease the amount of insulin they secreted while on the low-carb diet - but urinalysis showed that they were not burning more fat, but rather, more protein. Interestingly, the metabolic chamber days revealed that the subjects increased the amount of calories they burned by only about 100 calories per day (after taking into account each participant's body weight and its proportion of fat mass to lean mass). This decreased over the course of the study, and the authors note that this tiny amount is near the limits of the chamber's ability to measure.

What this means for you

For years we've called the Atkins diet and other, similar diets, just plain silly. This small study used the very best, most sensitive equipment available and showed that a low-carb diet does not burn fat - rather, it burns muscle. Better to follow a reasonable diet that allows you to eat real food.