The back-and-forth (and often heated) debate about which diet is better for weight loss — low-fat or low-carbohydrate — skewed off in a new and conciliatory (perhaps) direction this week.
According to a Stanford University study published Tuesday in the Journal of the American Medical Association (JAMA), both diets work equally well at shedding unwanted pounds — as long as people concentrate on eating plenty of vegetables and whole foods and on cutting back on added sugar, refined grains and highly processed foods.
Oh — people have to stick with the diet, too.
The study also found no evidence that people’s genetic makeup (genotype) or the particular way their body responds to insulin determines which type of diet is most likely to help them lose weight — ideas that have become popular in recent years.
“We don’t really think there is one diet that’s best for everyone,” says lead author Christopher Gardner, director of nutrition studies at the Stanford Prevention Research Center, in a video released with the study. “One of the issues that would really help people is if we could find out which diet is best for whom.”
Past research has suggested that certain biological factors, particularly genetics and insulin levels (which help regulate blood sugar in the body), might explain why some people lose more weight on a low-fat diet than on a low-carb one, and vice versa. Gardner and his Stanford colleagues decided to determine if those assumptions were true by conducting a randomized clinical trial, which is considered the gold standard for testing such hypotheses.
For the study, the researchers recruited 609 adults, aged 18 to 50, from the San Francisco area. The participants were about evenly divided between men and women, and all had a body mass index (BMI) of between 28 and 40. They underwent a variety of tests at the start of the study, including one that measured their body’s response to insulin and one that looked for genetic variants linked to the metabolism of carbohydrates and fats.
The participants were then randomly assigned to either a low-fat or a low-carb diet, which they were asked to follow for one year. During the first eight weeks of the study, the participants in the low-fat group were encouraged to cut down to 20 grams of fat a day (about the amount found in a handful of nuts), and those in the low-carb group were encouraged to cut down to 20 grams of carbs a day (about the amount in 1.5 slices of whole wheat bread).
“Those were huge changes from what they had at baseline,” says Gardner.
After those first two months, the participants were then told they could add back small, incremental amounts of the fat or carbs until they reached a level they felt they could maintain for their rest of their lives. At the end of the yearlong study, the people on the low-fat diet averaged 57 grams of fat daily, while those on the low-carb diet averaged 132 grams of carbohydrates daily — much lower than the 83 grams of fat and 247 grams of carbohydrates that the groups were averaging at the start of the study.
Importantly, however, the participants were also counseled on how to eat a healthy low-fat or low-carb diet.
“[The] point we hit home all the time was quality,” says Gardner. “We want you to go to farmer’s markets. We want you to cook more for yourself. We want you to sit down with your family. [We] don’t want you to snack in front of the TV. [We] don’t want you to eat in the car.”
They also told both groups to eat very little added sugar and refined grains and a lot of vegetables. And they told them to eat in a way that didn’t make them feel hungry or deprived.
At the end of the year, the weight loss for the two groups was very similar: an average of almost 11 pounds for the low-fat group and an average of just over 13 pounds for the low-carb group.
“They lost, collectively, 6,500 pounds,” says Gardner. “Some lost 40, 50, 60 pounds. Some gained 10 or 20 — and everything in between.”
The data also revealed that people with genetic variations linked to a low-fat diet response were not more likely to lose weight on a low-fat diet. Nor were people with genetic variations linked to a low-carb diet response more likely to lose weight on a low-carb diet.
Furthermore, people with a poor insulin response were not, as had been previously thought, more likely to lose weight on a low-carb diet than on a low-fat one.
“At the end of the day, neither of our original hypotheses turned out to be true,” says Gardner.
Limitations and implications
The study has its limitations, as all studies do. Its participants tended to have higher levels of education that the general public, as well as the monetary and other resources that enabled them access to high quality food. Also, the participants were followed for only a year. It’s not known how many were able to maintain their new diet — and their weight loss — after that year ended.
Still, the study was a large, randomized clinical trial, with a relatively small dropout rate. Its findings, therefore, offer some of the best evidence to date for how the “dueling diets” compare when it comes to weight loss.
Gardner is clear about what he thinks is the study’s take-home message: No matter which diet you go on, he says, eat less sugar and less refined grain — “whole” rather than highly processed foods. And eat as many vegetables as you can.
“Those are all enormous challenges in the American diet and many diets globally,” he acknowledges. “And yet we’re battling on points on the fringe of this debate without getting to the core.”
“ I think if we really focused on [decreasing or eliminating] added sugar and refined grain, [and if] we work with some of favorite chefs to make vegetables even more unapologetically delicious, a lot of the debate would go away,” he added.
FMI: You’ll find an abstract of the study on the JAMA website, but the full study is behind a paywall.