The following originally appeared on The Upshot (copyright 2018, The New York Times Company).
The endless array of diets that claim to help you shed pounds tend to fall into two camps: low fat or low carbohydrate. Some companies even claim that genetics can tell us which diet is better for which people.
A rigorous recent study sought to settle the debate, and it had results to disappoint both camps. On the hopeful side, as The New York Times noted, people managed to lose weight no matter which of the two diets they followed.
The study is worth a closer look to see what it did and did not prove.
Researchers at Stanford University took more than 600 people (which is huge for a nutrition study) aged 18 to 50 who had a body mass index of 28 to 40 (25-30 is overweight, and 30 and over is obese). The study subjects had to be otherwise healthy. They couldn’t even be on statins, or drugs for Type 2 diabetes or hypertension, which might affect weight or energy expenditure. They were all randomly assigned to a healthful low-fat or a healthful low-carbohydrate diet, and they were clearly not blinded to which group they were in.
All participants attended 22 instructional sessions over one year in groups of about 17 people. The sessions were held weekly at first and were then spaced out so that they were monthly in the last six months. Everyone was encouraged to reduce intake of the avoided nutrient to 20 grams per day over the first eight weeks, then participants slowly added fats or carbohydrates back to their diets until they reached the lowest level of intake they believed could be sustained for the long haul.
Everyone was followed for a year (which is an eternity for a nutrition study). Everyone was encouraged to maximize vegetable intake; to minimize added sugar, refined flour and trans fat intake; and to focus on whole foods that were minimally processed. The subjects were also encouraged to cook at home as much as possible.
All the participants took a glucose tolerance test as a measurement of insulin sensitivity. Some believe that insulin resistance or sensitivity may affect not only how people respond to diets, but also how well they adhere to them. The participants were also genotyped, because some believe that certain genes will make people more sensitive to carbohydrates or fat with respect to weight gain. About 40 percent of participants had a low-fat genotype, and 30 percent had a low-carbohydrate genotype.
Data were gathered at the beginning of the study, at six months and at one year. At three unannounced times, researchers checked on patients to see how closely they were sticking to the instructions.
This was a phenomenally well-designed trial.
People did change their diets according to their group assignment. Those in the low-fat group consumed, on average, 29 percent of their calories from fats, versus 45 percent in the low-carbohydrate group. Those in the low-carbohydrate group consumed 30 percent of their calories from carbohydrates, versus 48 percent in the low-fat group.
They did not, however, lose meaningfully different amounts of weight. At 12 months, the low-carbohydrate group had lost, on average, just over 13 pounds, compared with more than 11.5 pounds in the low-fat group. The difference was not statistically significant.