November 16, 2018

Why you shouldn’t trust the hype on that new low-carb diet study

Daily Briefing

    A new study published in BMJ suggests that diets low in carbs could enable people to burn more calories than other diets—but several nutrition experts say the results may not mean much for real-world dieters.

    Cheat sheet series: Evidence-based medicine 101

    Study details

    For the study, researchers recruited 164 overweight adults at Framingham State University.

    First, the participants were put on strict diets that lowered their body weight by around 12%. According to David Ludwig, a study author and an endocrinologist at Harvard Medical School and Boston Children's Hospital, this stage of the study was designed to stress the participants' metabolism. "At that point, their bodies are trying to regain the weight," he said. "It pushes the body and predisposes to weight regain."

    Next, the participants were assigned one of three diets:

    • A diet with 20% carbohydrates, 60% fat, and 20% protein;
    • A diet with 40% carbohydrates, 40% fat, and 20% protein; or
    • A diet with 60% carbohydrates, 20% fat, and 20% protein.

    The researchers then tracked the participants for five months and monitored biomarkers to ensure the participants remained on their diets.

    This diet burned more calories than the others

    The researchers found that after five months of the diet, the participants on low-carb diets burned around 250 more calories per day than those on the other diets. Ludwig said that 250 per day calorie burn could potentially lead to a 20-pound weight loss over three years.

    Participants on the low-carb diet also were less likely to feel hungry, as they saw the steepest declines in ghrelin, a hormone that promotes hunger and body fat and reduces energy expenditure. The researchers also found individuals on the low-carb diet who produced high levels of insulin burned the most calories—around 400 more each day.

    Some experts praise results, but some preach caution

    Ludwig said that the study results show "that all calories are not alike to the body from a metabolic perspective and that restricting carbohydrates may be a better strategy than restricting calories for long-term success."

    He added that the findings do not challenge the benefits of whole fruits, beans, and other non-refined carbohydrates but instead suggest reducing foods with refined carbohydrates could assist in weight loss.

    Dariush Mozaffarian, the dean of the Friedman School of Nutrition Science and Policy at Tufts University called the findings "profound" and said they contradict the conventional wisdom of counting calories for weight loss. "It's time to shift guidelines, government policy, and industry priorities away from calories and low-fat and toward better diet quality," he said.

    However, other experts said the findings may not be applicable to most people.

    Christopher Gardner, a researcher at Stanford University who was not involved with the study, said past research has shown when people are asked to follow low-carb diets independently, they are about as effective as other diets for weight loss. This is mainly because researchers have not found an effective way to get people to adhere to diets over long periods of time. "If you prove a mechanism works but you can't get people to do it, it won't help," Gardner said.

    Some experts also questioned the study's methodology. For example, Kevin Hall, a scientist and obesity expert at the National Institute of Diabetes and Digestive and Kidney Diseases, said the researchers used relied on doubly labeled water—a method used to track metabolism that Hall said has not been shown to be reliable.

    While Ludwig agreed the findings needed to be replicated in other studies, he disagreed with critiques of their chosen methodology. He said the researchers "used a gold standard method that has been validated across a wide range of experimental conditions and universally adopted in the field" (O'Connor, "Well," New York Times, 11/14; Thomas, CNN, 11/14; Rettner, LiveScience, 11/14; Belluz, Vox, 11/16).

    Learn more about why study design matters

    Been awhile since your last statistics class? It can be difficult to judge the quality of studies, the significance of data, or the importance of new findings when you don't know the basics.

    Download our cheat sheets to get a quick, one-page refresher on some of the foundational components of evidence-based medicine.

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