A recent JAMA study comparing the efficacy of low-fat and low-carbohydrate diets found that "people managed to lose weight no matter which of the two diets they followed"—but that "hopeful" finding doesn't provide any new insights on calorie restrictions, limited sugar, or the benefits of whole foods, Aaron Carroll writes for the New York Times' "The Upshot."
For the "phenomenally well-designed study," researchers from Stanford University randomly assigned more than 600 people—ages 18 through 50, who were considered overweight or obese—to either a low-carb diet or a low-fat diet, Carroll writes. Aside from their weight, participants were healthy.
Participants attended a total of 22 instructional sessions over the course of a year and were instructed to minimize their fat or carbohydrate intake—depending on their diet group—for the first eight weeks of the study and then permitted to increase it to the lowest level they felt sustainable in the long term. Participants were also encouraged to reduce the amount of sugar, refined flour, and trans fats in their diets; to focus on minimally-processed foods; and cook at home as much as they could. In addition, participants underwent a glucose tolerance test to measure insulin sensitivity and were genotyped.
The researchers gathered data at the start of the study, after six months, and after 12 months. They found that participants changed their diets, consuming fewer fats or fewer carbs as assigned—but both groups lost similar amounts of weight. The low-carb group lost, on average, just over 13 pounds, while the low-fat group lost just over 11.5 pounds. The researchers also found that neither insulin sensitivity nor genetics made a difference in weight loss.
According to Carroll, some observers have cited the study as proof that maximizing whole foods and cooking at home, all while minimizing processed foods, results in weight loss—but he argues that's not what the study was designed to demonstrate. While the researchers recommended participants follow those guidelines, the researchers did not establish comparable control groups, which means "no conclusions can be made as to the efficacy of these instructions," Carroll writes.
Other observers have said the study shows that counting calories is not the key to weight loss, Carroll adds—but he writes the study wasn't assessing that diet strategy either. Although the researchers did not tell participants to reduce their caloric intake, the participants generally curbed their daily calorie count by between 500 and 600 calories per day. As a result, the "study didn't prove the unimportance of calories," Carroll writes.
And since the study asked all participants—not simply those in the low-carb group—to reduce added sugar, the results can't "really say anything new about added sugars and weight loss," Carroll writes. Similarly, since the study focused on primarily obese individuals, Carroll argues that it provides no insight on what type of diet might work best for people hoping only to lose a couple of pounds—and given that participants "received significant support on both diets," the findings "might not apply to those attempting to lose weight on their own," Carroll adds.
Rather, according to Carroll, the study demonstrates "that people who have staked a claim on one diet's superiority over another don't have as strong a case as they think." It also undermines previous research—often smaller in size and shorter in duration—suggesting a low-carb or low-fat approach might be better for certain people based on their insulin levels or genetics, Carroll writes.
Ultimately, "the best diet for you is still the one you will stick to," Carroll writes. "No one knows better than you what that diet might be. You'll most likely have to figure it out for yourself" (Carroll, "The Upshot," New York Times, 3/26).
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