On 16th November 2018, NY Times reporter Anahad O’Connor tweeted that the number one most emailed story on the NY Times website at that moment was “How a low-carb diet might help you to maintain a healthy weight” (Ref 1). The UK Telegraph covered the story as “Low-carb dieters can lose more than 1.5 stone over three years, study claims” (Ref 2). I’ll come back to the Telegraph headline, as I think it was wrong. The headlines came from a study that had just been published in the BMJ entitled “Effects of a low carbohydrate diet on energy expenditure during weight loss maintenance: randomized trial” (Ref 3).
The background and aim
The study was wonderfully simple in its aim: “To determine the effects of diets varying in carbohydrate to fat ratio on total energy expenditure.” The title of the article should be re-read alongside the aim, as this was a study focused on understanding the impact of carbohydrate and fat ratios on weight maintenance, after weight loss, as we will see in “The study” section just below.
This also explains something that you might wonder while reading the paper – why did the study involve weight maintenance instead of trying to contrast weight loss with different carb/fat ratios? Two obvious answers are i) there have been many studies (albeit most not long enough) comparing low carb (albeit most not low enough) with low fat diets and so this would have been less novel and ii) it is known that energy expenditure declines with weight loss, which contributes to weight regain and thus it would be novel to discover that the type of diet consumed after weight loss may have a significant impact on avoiding regain. This would provide a breakthrough in obesity strategies.
I recommend reading the blog written by the senior author, Dr David Ludwig, on Medium where he narrates the background to the study in terms with which regular readers of this newsletter/blog will be familiar... The evidence for long term success with calorie deficit dieting is so lacking you wonder why anyone ever thinks it might succeed. "Decades of research show that, with weight loss, hunger inevitably increases and metabolism slows down" (Ref 4).
Ludwig covers "the carbohydrate-insulin model" (CIM) of obesity (a concept with which readers of Gary Taubes' Good Calories Bad Calories will be familiar) and suggests that we have the direction of causation the wrong way round: "Overeating doesn’t cause weight gain, not over the long term; the process of gaining weight causes us to overeat." According to the CIM, it is the highly processed carbohydrates that have dominated our dietary intake over the past 40 years (since the guidelines changed to advise us to base our meals on such things) that have raised insulin levels, forced fat cells into storage overdrive. "Our rapidly growing fat cells take up too many calories, leaving too few for the rest of the body. That’s why we get hungry. And that’s why metabolism slows down if we force ourselves to eat less."
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