Study “Wounds” the Carbohydrate-Insulin Theory of Obesity

Last week a very interesting and, by all accounts, very well-done study made waves among the nutritional science community. For many years, the idea that reducing carbohydrates is the most effective way to reduce fat due to its effect on insulin has been rapidly gaining in popularity.

Prominent researchers like Dr. Robert Lustig (who famously called sugar a “poison”), and Gary Taubes (author of Good Calories, Bad Calories) have promoted the idea that it’s not just about how much you eat, but what you eat, that leads to obesity. Specifically, that carbohydrates and sugar cause a cascade of problems including insulin resistance, obesity, and type 2 diabetes. This new study, however, casts serious doubt on the hypothesized mechanism by which consumption of carbohydrates, in particular, would lead to these problems.

The study, led by Dr. Kevin D. Hall, was published in the highly respected Cell Metabolism journal on August 13 and found that restricting dietary fat led to body fat loss that was 68 percent higher than a diet that reduced the same number of calories through carbohydrates for obese adults. The study was small, with only 19 participants, and short, lasting only four weeks.

However, it was a well-designed and tightly controlled study. As neurobiologist Stephan Guyenet put it, “this study's methods were downright obsessive. The overall study design and diets were extremely tightly controlled, and the researchers took a large number of measurements using gold-standard methods.” The participants were randomly assigned to either the low-carb group or the low-fat group. After five days of baseline eating, the participants had their calories restricted by reducing either fat or carbohydrates by 30 percent (sugar was the same in both groups).

Unsurprisingly, the low-carb diet led to an impressive 22 percent reduction in insulin secretion and a higher rate of fat burning. However, while the low-fat diet had no effect on insulin secretion, studies in this group lost more fat than those on the low-carb diet.

The results are significant because, even though it was not the intent of the researchers, they ended up conducting the first real test of the carbohydrate-insulin theory of obesity. While the theory was correct in that carbohydrate reduction would reduce insulin secretion, resulting in increasing fat oxidation and fat loss, it was incorrect in a very significant way.

According to the carbohydrate-insulin theory, a diet that doesn’t reduce insulin should not produce fat loss. This study seems to cast doubt the most significant aspect of the carb-insulin theory: how it’s supposed to work. As Guyenet said, “[s]ince dropping carbohydrates from the diet lowered insulin but slowed fat loss when compared to dropping dietary fat, this study falsifies one of the most popular incarnations of the carbohydrate-insulin hypothesis of obesity.”

However, the study isn’t perfect—at least not for the purpose of determining if low-fat or low-carb are more effective for losing fat. As Guyenet noted, because of the baseline diets, the low-fat diet was far more extreme than the low carb diet in this study. The low-fat diet had participants eating just 8 percent fat in their diet, while the low-carbers still got 29 percent of their calories from carbohydrates. Some practitioners of paleo or low-carb lifestyles advocate achieving a “ketogenic” diet, which, put simply is a diet so low in carbohydrates (around 5 percent of total calories) that the body is forced to rely on fat for energy instead of the glucose from carbohydrates.

What this study really demonstrates is that, in the short term, a diet extremely low in fat content is more effective in reducing body fat than a diet of moderately reduced carbohydrates and that we need a longer-term study matching the diets in extremeness, to see if there really is an advantage to cutting one macronutrient over the other. As Guyenet notes “[a]ccording to their metabolic model, if the low-carbohydrate diet had been even lower in carbohydrate (with a corresponding increase in fat intake to maintain calories), it would have matched the very-low-fat diet in the fat loss department. Their data suggest that both macronutrient extremes are a bit more effective for fat loss than being in the middle, even when calories are held constant…If both diets were equally extreme, Hall's model predicts that fat loss would have been similar.”