New Study On IF & Weight Loss Has Been Misreported: Here's What The Lead Researcher Says
Last week, the health news world was abuzz with info on a new intermittent fasting (IF) study—but it wasn't positive hype. The study, published in JAMA Internal Medicine looked at time-restricted eating (TRE), a form of intermittent fasting that restricts food intake to, at most, 12 hours a day. And numerous publications highlighted how the study findings suggested the benefits of TRE didn't differ from a traditional eating style, specifically in regards to weight loss outcomes.
For any die-hard fasting fans, the research results were pretty disappointing. But, before you throw in the IF towel, let's dig into the details of the paper. There's a lot to unpack, and it's not as simple as some buzzy headlines would have you believe.
A little background on the research.
The trial was led by cardiologist Ethan Weiss, M.D., and lead author, Dylan Lowe, Ph.D. The choice to study TRE was based on remarkable previous findings in animal models; hints of benefit from small, uncontrolled human trials; and a collection of anecdotal reports touting the benefits of TRE. The results from the mouse experiments were very clear. Mice ate the same amount of calories and food, and there was a significant metabolic benefit to restricting time. So, it was only a matter of time before it was put to the test in a randomized clinical trial, which is exactly what Weiss and the team did.
Leading up to the trial, Weiss was an advocate of TRE. When he appeared on the mindbodygreen podcast in February 2020, he shared that he started following the 16:8 IF style (in which you fast for 16 hours each day and eat during an eight-hour window) because he wasn't hungry in the morning. In the same podcast episode, he excitedly mentioned the current study.
Similarly, when I spoke with Weiss myself, it was clear he'd had high hopes for this study. "TRE had tremendous promise to offer a simple and effective method to lose weight and improve metabolic health," he told me.
How the researchers examined TRE.
The study was designed to test whether the recommendation to restrict food intake to a daily eight-hour window would lead to weight loss in overweight or obese individuals, compared to those who ate meals over the course of the day. What, or how much the participants ate, was not a controlled variable.
They enrolled 116 overweight or obese individuals and randomly assigned them to one of two prescription groups for 12 weeks:
- Time-Restricted Eating (TRE): Eat what and how much you wish (ad libitum) between 12 p.m. and 8 p.m.; noncaloric beverages such as black coffee or tea were allowed within the fasting window.
- Continuous Meal Timing (CMT; served as control): Eat three structured meals each day with snacks permitted between meals. Meal one was eaten between 7 a.m. and 11 a.m., meal two: 11 a.m. and 3 p.m., and meal three: 4 p.m. and 10 p.m.
The 12 p.m.-to-8 p.m. eating window was chosen based on what would best fit into most people's lives, since dinner is typically the most social meal of the day, and breakfast is relatively easy to forgo. (This particular method has since been criticized for falling too late in the day, thus not aligning well with the circadian rhythm of metabolism. That said, it is how many TRE advocates follow the eating style.)
The instructions were straightforward, and that was the researchers' intention: "We wanted to test a real-world and simple prescription," says Weiss. The only recommendation was on the timing of food. Participants were not given advice on diet, nutrition, or physical activity.
According to self-reported data, participants adhered pretty well to the plan. Participants were sent daily adherence surveys through a mobile application, asking, "Did you adhere to your eating plan on the previous day?" to which they could respond "yes" or "no." Of course, there's no way to rigorously measure compliance, so we're just assuming that the data represents the group's adherence.
"This [monitoring compliance] is the pitfall of doing this research in free-living people; it's hard," says Weiss. "But we wanted to test, specifically, what is the effect of the recommendation."
So, what did they find?
The primary outcome of this study was weight loss. Each participant received a digital scale to measure their weight each morning. About half of the participants made in-person visits to the lab to measure secondary outcomes, like blood sugar levels, insulin, and body composition changes, for example.
Both groups lost very little weight, an average of 2 pounds for the TRE group, and 1.5 pounds for the CMT group. That is a negligible amount of weight to lose from a 12-week-long intervention. After analysis, there was no significant difference between the two groups.
Of those who visited the lab for in-person testing, the results showed no differences in terms of insulin, glucose, blood lipids, sleep, activity, resting or total energy expenditure, or fat mass.
In other words, the recommendation to eat within an eight-hour window was not superior to that of eating over a 15-hour eating window when it came to weight loss or other metabolic markers. Everything Weiss expected TRE to improve didn't seem to budge compared to the control.
What can we make of these surprising results?
Let's revisit the fact that researchers don't know what these subjects were eating—they could have been eating junk food, for all we know, just within an eight-hour window. So a null result isn't too surprising, considering one reason TRE can be effective for weight loss is because it can be a means of controlling and/or reducing caloric intake.
Unfortunately, a 16-hour daily fast is probably not enough to protect you from the ill effects of a poor diet. Diet quality matters, and I'm willing to bet most people recommending TRE would agree. Maybe if we teased apart the subset of those who lost weight from those who gained weight, we would find that those in the former were eating a higher-quality diet. That said, all we really know is that the prescription to eat from 12 p.m. to 8 p.m. with no control for what or how much to eat did not lead to improvements in weight or other outcomes, compared to the control.
Though, here's where things get interesting. While there was no significant difference in lean mass (fat-free mass) between groups, the TRE group did lose more lean mass than the CMT group. "We were surprised by this finding," says Weiss.
That said, this was a secondary outcome, meaning it was not exposed to the statistical scrutiny of a primary outcome, so you can probably take it with a grain of salt.
One possible reason is the TRE group ate less protein than the CMT group. The literature supports that increasing protein intake during weight loss (while in a calorie deficit) is a good strategy for both satiety and maintenance of lean mass reasons. So, TRE could have caused these individuals to reduce total protein intake relative to the CMT group.
Also, when you dive into the results, you see that the TRE group had "a significant reduction in daily movement" and "a significant decrease in step count," both of which were not the case for the CMT group. Exercise and movement are also known to help retain lean mass during weight loss, so it is possible that this, too, played a role. Not to mention the influence this may have had on weight loss.
At this point, though, we're getting into the weeds, using an exploratory outcome to explain a secondary outcome. In the science world, this does not hold a lot of power. The study was not designed to rigorously measure these outcomes, but as Weiss emphasized in our interview, it does raise a question that is worth following up on.
The simple takeaway is that a recommendation to eat within the hours of 12 p.m. and 8 p.m. for 12 weeks in an overweight or obese, free-living population may produce minuscule weight loss on average; there are people the method works for, and others it doesn't. Who knows what the results would have looked like if these subjects had been given recommendations on diet and exercise. Ultimately, for what the study was designed to test ("does the recommendation of 12 to 8 p.m. TRE lead to weight loss in overweight or obese individuals?"), it did an excellent job, and this study was well-designed to test their hypothesis. But it is certainly not the IF study to hang your hat on.
So, what now?
It's clear that the interpretation by some large news outlets claiming TRE does not lead to weight loss is false. Still, there was no apparent advantage to TRE over CMT, which was enough evidence to influence Weiss, who tells me he has since begun eating breakfast again and seems to be enjoying his new routine. For Weiss, the results have encouraged him to be more careful about what he recommends to friends, family, and patients.
That said, if you are currently following this style of intermittent fasting and it's working for you, there's nothing wrong with sticking with it. Remember, a portion of the participants did lose weight. And if you're questioning other types of intermittent fasting, Weiss says, "This study shouldn't change the opinions of other regimens of intermittent fasting."
There is still so much more to explore. Who knows if a shorter or earlier TRE protocol would have been more effective or what would happen if TRE was paired with a particular diet, such as low-carb. The intention of this study was to see if recommending TRE as a stand-alone intervention would lead to weight loss in overweight and obese individuals, so we cannot scrutinize the methods for not controlling for calories or macronutrients. All this does is make it difficult to understand the results without knowing what the participants ate, which will be an essential factor to consider in future trials.
The bottom line is that this study does not disprove the efficacy of TRE for weight loss.
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