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 8-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 that 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. to 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.