When cardiologist Ethan Weiss concluded his 12-week study of weight loss and intermittent fasting, the results surprised him. The randomized trial set up by Weiss, a professor at the University of California, San Francisco, split participants into a control group and another group who ate all their meals between noon and 8 p.m. He expected to find more weight loss in the fasting group. But the results he published in JAMA Internal Medicine last fall showed something else: that those following the time-restricted eating schedule lost roughly the same amount of weight as those in the control group. What’s more, the weight they did lose was lean muscle mass, and not fat.
Randomized controlled trials are considered the gold standard when it comes to scientific studies. Yet when it comes to figuring out how to lose weight, or which sort of diet is best, it’s often the case that different trials yield much different results, even when participants follow similar nutritional plans. (Another 12-week trial of time-restricted eating showed the opposite effect of Weiss’s study: Participants who followed a time-restricted schedule lost weight without losing muscle mass.)
This muddle is enough to leave anyone scratching their head. And it points to a bigger issue in the science of weight loss: There's a lot that researchers have figured out, but there's still no consensus on the single optimal diet or way to lose weight—and there might not ever be.
We Already (Sort of) Know What Works
Teresa Fung, a professor of nutrition at Simmons University, points out that there are plenty of scientific studies focused on diet that determined, to a degree, how to lose weight. “There have been many weight-loss trials that showed quick and effective weight loss is possible," she says.
It’s not hard to find examples of such trials. Some have participants play around with the amount of fat, protein, and carbohydrates they’re eating. Others, like a well-known Weight Loss Maintenance clinical trial, home in on a very specific diet plan. In that trial, conducted from 2003 to 2007, more than 800 participants followed the Dietary Approaches to Hypertension, or DASH, diet. What it found was that people who added more fruits and vegetables to their plate managed to lose weight and keep it off, even 30 months out.
That length of time is key. Weiss says one of the reasons why there tends to be variability in studies gauging what works when it comes to nutrition is because short-term weight loss doesn’t always translate in the long term. “I think you can find almost any diet that has at least one randomized trial that shows weight loss over three months,” he says. “The question is: What is the durability of these impacts?”
Sticking With It
Naturally, the long-term effects of weight-loss plans hinge on someone’s ability to stick with a diet. If science appears unable to sketch out an appropriate eating plan that helps a person lose weight and then maintain, it’s usually a question of adherence—not a question of whether a particular diet is effective. “There are signs that suggest after six months or so on a diet, people get tired of it,” Fung says.
One trial published in the New England Journal of Medicine seems to bear this out. The two-year study followed 322 overweight adults, who were split into three groups with three different diet plans. After one year, the adherence rate to the diets was above 95 percent; by the end of year two, that rate dropped to 85 percent. “I think people assume they can’t stick with a diet,” says Weiss.
Of course, one option—and not necessarily a bad one—is to just avoid diets altogether. The goal, as registered dietitian Wesley McWhorter recently told GQ, is to focus on what you’re eating rather than what you’re restricting, and to ensure that the foods on your plate get more and more healthy over time. That can create a more sustainable pattern of better nutrition.
Weiss says he has a study coming out soon that sheds light on the question of sticking with a diet. In this new study, the best predictor of whether people followed a weight plan for the long-term was something Weiss calls “self-perceived adherence.”
“I think it has a lot to do with how well people feel,” he says. In other words, how well people think they’re doing best predicts how much weight they’ll lose.
Go With Your Gut
How individual people respond to various nutrition plans is one element of weight loss that science is still puzzling out. In the aggregate, a study can demonstrate weight loss across a group of people. But who lost weight, and who did not, might shed more light on the effectiveness of a diet than the simple average.
Eran Elinav, an immunologist at the Weizmann Institute of Science in Israel, is at the forefront of research into this topic. He’s one of the co-authors of a groundbreaking 2015 paper that followed an 800-person group that ate a total of 46,898 meals. Week to week, they monitored participants’ glucose levels, with the thinking being that people who ate foods with identical glycemic-index values would show the same spike in their blood-sugar after eating. What they found, instead, was huge variability, even in response to identical meals.
“What this told us is that this one-size-fits-all diet is probably flawed. We understood that rather than measuring foods and giving them a number, we should measure people instead,” Elinav says. It sounds simple, but people’s bodies react differently to different foods. (This likely has something to do with the gut microbiome, the mix of healthy bacteria that live in the colon and aid in our immune response.)
Given this, a set diet plan that purportedly shows benefits isn't exactly the right approach. In the aggregate, something like the Mediterranean diet shows benefits. At the individual level, the results aren’t always identical. "Trying to nail a one-size-fits-all diet is kind of missing the point,” says Elinav. “It might also be why different diets show different results."
Where the science of dieting is going in the future is toward personalized nutrition. And it’s something many dietitians already intuitively know, that no one diet is going to work for everybody. It’s more likely, instead, that there are going to be a handful of diets that work for some people. The trick is identifying who those people are—and then crafting more precise dietary trials.
“I think there will be more and more well-done studies,” says Weiss. “That, I think, is what we’re headed toward.”
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By Andrew Zaleski