Time Restricted Eating
My take on the recent study in NEJM
4/28/22 (Read time 8 min)
Time-restricted eating (TRE), the practice of taking in all of one’s calories during a short window during the day, has become increasingly popular over the last several years and has gained a lot
of attention from the mainstream media in the process. A common application of this practice is in the so-called 16/8 format- meaning out of a 24-hour time period, 16 hours are spent fasting (without taking in any calories), leaving the other 8 hour window during which all of the calories for the day are consumed. I am a big fan of this strategy for a lot of reasons, and there are mountains of background data suggesting it can be beneficial. However, clinical trials in humans have shown inconsistent results, and often the studies are too short to provide any meaningful information about TRE. It is not for everyone, but I frequently discuss this eating pattern with my patients as one tool for improving their metabolism.
Published in last week’s New England Journal of Medicine, there was a study comparing 2 different weight loss strategies. The study looked at TRE combined with calorie restriction (TRE + CR) vs calorie restriction alone (CR) in patients with obesity. The main test was to see if TRE helped you lose more weight compared to simply cutting back you calories. The researchers found no statistically significant difference in weight loss between the 2 groups. This led the New York Times to report that “Scientists Find No Benefit to Time-Restricted Eating”. Respectfully, I feel that the New York Times has drastically overstated the findings, and in doing so has really missed the point of what time-restricted eating can offer.
First let’s take a look at the study in a little more detail. It included 139 individuals, all from China, who had obesity with an average BMI of 31-32 at the start of the trial. Half of the participants (CR group) were asked to just cut down their daily calories (to 1500-1800 kcal/day in men and 1200-1500 kcal/day in women). The other half (TRE + CR group) were asked to cut their calories to the same amount but in addition they were asked to follow a 16/8 time restriction. After 12 months, the TRE + CR group lost 17.6 pounds on average, and the CR group lost 13.9 pounds on average. This difference was not statistically significant.
Should this finding challenge how we think about TRE? Well, there are a couple of important limitations to consider when figuring out how to interpret the results of this study:
The population was quite homogenous- all of the study participants were from the same city in China. It is impossible to know how this study would play out in other parts of the world or with a more diverse population.
With enrolling only 139 people, the study was not exactly huge. This matters because it limits the ability to detect subtle differences between 2 groups. In other words, the fewer the number of people being studied, the greater the difference between the groups has to be in order to be statistically significant.
The study authors did not standardize the activity levels between the 2 groups. This is important because if one of the groups was more active than the other it could clearly skew the outcomes. For example, it is not hard to imagine a scenario where the CR group exercised more and thus lost more weight than they otherwise would have- but since it wasn’t measured we really don’t know.
It is also noteworthy that the baseline eating window for both groups prior to the study was about 10.5 hours. This means that both groups were already practicing a milder version of TRE before the study began, i.e. a 13.5/10.5 model. In other words, the 2 groups differed in their eating windows by only 2.5 hours! This certainly impacts the ability to detect a difference.
There was no group in the study that practiced TRE without intentionally cutting calories- again we could imagine a possible outcome where the CR somehow mitigated or lessened the impacts of the TRE. Unless there is a group that is doing TRE but not CR we would not detect this in the results.
Another factor to consider here is that the amount of calorie restriction in both the groups was pretty significant- they were asked to cut their daily calories to only 75% of what they were eating before the study. This tends to be difficult to maintain for any length of time in a “real-world” environment, so it limits the study’s applicability. More importantly though, both groups lost lean body mass in addition to fat mass. Consider that obesity is problematic primarily because of the association with metabolic disease- viewed in this way it could be considered somewhat of a symptom rather than a condition in and of itself. This means that weight loss by any means possible
is not necessarily the goal- rather the goal is to improve the metabolic machinery which will then lead to a reduction in the amount of energy our body feels compelled to store as fat. If we too drastically cut our caloric intake (or if we're not engaging in regular exercise during weight loss), paradoxically our bodies may "choose" to burn muscle tissue to use as energy. This is clearly counterproductive.
These methodological limitations aside, there is a deeper point here- TRE is not intended to simply be a weight loss strategy. Even if it is true that TRE is not better than other methods at inducing weight loss, I think focusing too much on the weight misses the broader point. TRE can be thought of as one tool used for reprogramming an unhealthy metabolism, or to avoid deprogramming a healthy one. While none of the outcomes in this study reached statistical significance (possibly due to the small sample size and/or the other methodological limitations above), I do think it is noteworthy that nearly every secondary outcome measure looking at metabolic health trended in the direction that favored TRE. This is why I believe the New York Times went a little too far with their sweeping, provocative headline. TRE is certainly not a panacea, but it is one tool in the toolbox to promote and maintain metabolic health. I am a firm believer that we need to keep updating our thinking on various practices as more information becomes available. However for the reasons above, this study will not change the role of TRE in my clinical practice. Likewise, the questionable reporting of the study's findings should help remind us to look beyond the dramatic headlines when it comes to interpreting science!