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The study that didn’t end the low-fat/low-carb diet “wars”

A recent metabolic ward study set the low-carb world on fire, and produced many inaccurate media headlines disparaging low-carb diets. We cover the study and its implications, detail by detail.

Study under review: Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity


Some of you may have already come across our blog post about this recent blockbuster of a paper published by Dr. Kevin Hall et al. If you have, stick around for our extended F.A.Q. section where we tackle the numerous questions brought up about the study. If you haven’t read the blog, let’s dive right into the trial analysis.

For some, the central dogma behind the hypothetical superiority of low-carb diets for fat loss is the insulin hypothesis of obesity. Part of this hypothesis states[1] that by restricting carbohydrates you will see a stepwise decrease in insulin secretions. Because insulin plays a part in the regulation of fat storage, it has been theorized that the less insulin secreted the more free fatty acids will be released from adipose stores leading to increased fat oxidation and rapid fat loss. These assumptions have led to the idea that low-carb diets will induce greater fat loss over a low-fat diet even when calories are held constant. Gary Taubes, an advocate of the low-carb approach, posited the following in his latest book, Why We Get Fat (p. 144-47):

“…any time we try to diet by any of the conventional low-fat methods, and any time we decide to “eat healthy” as it’s currently defined, we will remove the most fattening carbohydrates from the diet and some portion of total carbohydrates as well. And if we lose fat, this will almost assuredly be the reason why…This is something that even researchers who run clinical trials testing the effectiveness of different diets rarely recognize.”

Simply put, Taubes suggests that by reducing both carbs and fat in low-fat diets it is possible that reductions in carbohydrate intake could be responsible for any fat loss seen. Taubes is correct in that researchers who run diet trials often alter the amount of fat and carbohydrate participates eat, making it impossible to determine if restricting one will lead to greater fat loss over the other. Previous studies on low-fat and low-carb diets have changed multiple variables simultaneously. So even though they end up comparing low-fat and low-carb, they do not specifically reduce one macronutrient or the other from a baseline diet without changing other variables. In the present study, Dr. Hall and his team set out to eliminate that confounding variable by subtracting either fat or carbs from the diet without changing anything else. This was done under tightly controlled conditions, to determine if indeed there is a metabolic fat loss advantage to going low-carb.

One important concept to understand before reading through this breakdown is that the study was not looking at the real-world efficacy of diet interventions. That is to say, this was not a free living low-fat vs. low-carb study where researchers educate groups of volunteers and let them eat self-directed low-fat or low-carb diets in their own homes to see how they fare. The investigators designed this intervention to examine some specific mechanisms of weight loss discussed in the sections below.

One version of the insulin hypothesis states that in order to lose body fat you must restrict carbs to bring down insulin, high levels of which will prevent fat loss. Dr. Hall’s study has been designed to test this hypothesis to see if reduced-carb diets confer a fat loss advantage over reduced-fat diets when calorie intake is strictly controlled.

Who and what was studied?

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