Pamela Malo, MHS, RD, Arivale Coach
[Ed. Note: Please scroll to the bottom of this article for an important update on the study in question.]
It seems like there will never be an end to the low-fat vs. low-carb diet wars – at least as long as headlines in the media continue to proclaim the existence of a “magic bullet” diet for everyone and anyone trying to lose weight and keep it off.
The latest study generating headlines is from a respected group of researchers at Harvard Medicine School and Boston Children’s Hospital. The study enrolled overweight or obese adults into a standard behavioral weight-loss program that restricted their calories and increased their physical activity. To qualify for a second, diet comparison part of the study, participants had to lose 12 percent of their initial body weight in 9 to 10 weeks.
(It’s important to note that losing 12 percent of your body weight that quickly in a behavioral weight-loss program is unusual; losses of 5 to 10 percent over six months are more typical and sustainable. Not surprisingly, the calorie restriction used during the first part of the study was significant: 60 percent of an individual’s estimated calorie needs. It’s also important to note the composition of the diet that produced these significant weight losses was not low-carb; it was a balanced diet of 45 percent carbohydrate, 30 percent fat, and 25 percent protein.)
The second phase of the study, which was 20 weeks of random assignment to high, moderate, or low-carbohydrate diets (60 percent, 40 percent, and 20 percent carbohydrate, respectively), consisted of 164 participants. Protein, added sugars, and saturated fat intake was held constant across all participants. Importantly, researchers adjusted the daily calorie content of the meals to maintain body weight during the maintenance phase. This means weight maintenance was due to calorie adjustment, not any aspects of dietary composition.
The primary outcome of the study was energy expenditure. In the most conservative analysis, the results showed a greater change in total daily energy expenditure in those assigned to the moderate or low-carb diet compared to the high-carb diet – 91 calories per day and 209 calories per day, respectively. Only total daily energy expenditure was impacted – not resting energy expenditure or physical activity energy expenditure – so the research is unclear what exactly changed to drive the increased daily energy expenditure.
Researchers believe their results support the “carbohydrate-insulin model of obesity,” which ties excess weight gain to intake of higher glycemic foods, such as refined carbohydrates.
The study points out – correctly – that keeping weight off after weight loss is hard. It’s a period where intensive lifestyle support is important to preserve success. However, jumping from a study that showed low-carb diets have a small beneficial effect on energy expenditure in people who have lost over 10 percent of their initial body weight to headlines like that run by Today – “For weight loss maintenance, a low-carb diet may be best” – is a definite stretch.
First of all, there have been many randomized clinical trials comparing high-carb and low-carb diets for both weight loss and weight-loss maintenance and showing no difference between the two. While some of these studies have been short (a criticism raised by the authors of this new study), this is not true of all studies, and some followed participants for a year or more and still saw no difference by diet type (here and here, for example).
Furthermore, the largest “real world” study of successful weight-loss maintenance, the National Weight Control Registry of over 10,000 people who have lost weight and kept it off, shows successful “losers” follow carefully planned, portion-controlled balanced diets and engage in daily physical activity.
Clearly, claims by the media that “low carb is better than low fat” are not supported by the bulk of the evidence, including this study, which was only designed to look at the impact of various carb levels on energy expenditure, not weight loss.
In addition, claims from the media that this new study shows “calories don’t matter” (not something the authors claim) are clearly untrue, since it was a significant calorie restriction that caused the weight loss and ongoing calorie adjustments that helped people maintain that weight loss.
Additional important points about the study and the media coverage have been raised by Dr. David Katz, including a key observation that when overall food quality is good (i.e. lots of vegetables, fruits, whole grains, and lean proteins), whether a diet is “low carb” or “low fat” becomes less relevant.
In the end, there is no one-size-fits-all diet solution. It’s important to find out what works for your body, your genetics, and your lifestyle.
- Bad News for Keto? Low-Carb Diets Linked to Increased Mortality
- How to Choose a Diet That Works for You
- How Your Genetics Influence What Foods Make You Gain Weight
Dr. Kevin Hall of the National Institutes of Health has published an important critique of this study.
The gist of Hall’s critique is that the researchers changed the way they analyzed their data in a way that was likely to lead to bigger results. When Hall analyzed the data how researchers originally intended, the results of the study were no longer statistically significant.
The critique also notes that BMJ asked the researchers to report the results using their original plan, but they refused. It’s unclear why BMJ published the study anyway, though it has previously been accused of having an alleged bias toward low-carb diets.
This situation reveals a lot about both the politics of science and the way we are manipulated by the media, which used the results of the study to claim low-carb diets are the best for losing weight and keeping it off (here and here, for example).
Biased analysis of data, biased publication of papers, and, ultimately, biased reporting by the media creates confusion and distrust among consumers. In the end, we’re all the ones who pay the price as US diet quality declines and nutrition-related chronic diseases like obesity, diabetes, and high blood pressure increase.
[Arivale Hot Topics address health stories currently in the news. The Arivale Clinical Team’s commentary on these news articles is not a review of the scientific evidence, nor an endorsement of a specific study, and is not meant as official medical opinion.]