In the second part of this Seminal Review, we break down the great carb war debate and shed some light on what the data tells us.
Overview
In 2017 top metabolism and obesity researchers thoroughly examined the studies investigating weight loss and energy expenditure between a high-carb or a low-carb diet. Results showed that neither was superior in fat loss or energy expenditure. Since then, other researchers have re-analyzed this data, including additional studies that suggest there is a stronger effect for low-carb diets compared to higher-carb diets. However, as Hall and colleagues bring up, some misleading methodologies may portray the data more favorably for the low-carb proponents.
What’s the Problem?
When it comes to fat loss, much of the debate on an ‘optimal’ fat loss diet has centered around the purported ‘metabolic boosting’ benefits. You hear buzzwords like this on social media constantly and what it means is how different diets impact energy expenditure. If a particular dietary strategy could increase energy expenditure, it could be a valuable tool for those challenged with weight loss. Some early research suggests that low-carb diets may increase energy expenditure and, therefore, fat loss greater than other diets. However, a big problem with these studies is studies don’t equate protein intake between the control diet and the low-carb diet, and as we know, protein can increase energy expenditure 3. The question then is whether it was the increased protein intake in the low-carb group or the reduced carbs. Subsequent studies equating calories and protein did not show the same results 4. Still, some proponents of low-carbohydrate diets insist that low-carb diets offer a metabolic advantage compared to other diets. Much of this support for low-carb diets comes from the Carbohydrate-Insulin Model (CIM) for obesity 5. The CIM suggests that consuming refined carbohydrates increases insulin, trapping fat in fat cells, and making fat inaccessible to the rest of the body for energy. Furthermore, this model suggests that this response from refined carbs increases hunger and energy expenditure decreases. In this model, people do not become overweight because they overeat. They overeat because they become overweight. In 2017 Hall and colleagues published a meta-analysis concluding minimal differences in weight loss and energy expenditure between low-carb and low-fat diets 1. A couple of years ago, another group of researchers updated this meta-analysis, including some new studies resulting in contrasting conclusions. The researchers' updated meta-analysis suggests a benefit to weight loss and energy expenditure from a low-carb diet 2. Shortly after the updated meta-analysis, both research teams published correspondence regarding the findings from Ludwig et al. (2020). Before we break down this war on carbs, let’s build off our energy balance discussion quickly so we can fully understand some of the problems in these studies comparing diets and essentially calories vs. CIM.
