If you’ve researched nutrition and weight loss strategies any time in the past five or so years, you’ve likely come across somebody who’s talking about intermittent fasting or time-restricted eating.
Time-restricted eating (TR) is a version of fasting in which a person restricts the period of the day during which they eat. For instance, I might normally eat breakfast around 8am, and my last meal might be at 7pm, but then I might have some snacks at 9 or 10pm before I go to bed. If I implemented TRE, I might restrict myself to eating between 8am and 4pm in an effort to control my intake of energy, and thus lose weight and improve my health. For years, this strategy has been touted as an effective strategy to improve health and to lose weight.
I’ve prescribed this strategy to my patients, and experts in the field like Dr. Peter Attia, Dr. Ethan Weiss, Dr. Jason Fung, and others have spoken about TRE publicly many times.
This is widely considered to be a useful strategy based on clinical experience, and it’s supported by research in animals as well, with studies in mice showing the most promising results.
Last week, the New York Times published an article with the title "Scientists Find No Benefit to Time-Restricted Eating."
The NYT article was covering a study published in NEJM by the title, “Calorie Restriction with or without Time-Restricted Eating in Weight Loss.”
The study’s conclusion was:
“Among patients with obesity, a regimen of time-restricted eating was not more beneficial with regard to reduction in body weight, body fat, or metabolic risk factors than daily calorie restriction.” (Liu et al., 2022, p. 1495)
At first glance this might seem consistent with the NYT headline, but read closer and you’ll see that the two groups were both undergoing caloric restriction, it’s just that one group also had a restricted eating window.
The NYT article title suggests that time-restricted eating doesn’t work, but the study only tested time-restriction in conjunction with caloric restriction, which in fact, did work. And, it worked as well or better than caloric restriction alone. What’s more, the two groups were hardly different from one another. This fact is only apparent on closer examination of the study, and calls into question any claim that time-restriction isn’t beneficial.
For the rest of this article, I’m going to refer to caloric restriction as CR and time-restricted eating as TR.
The real outcome of this study was that the CR group lost on average 6.3 kg, or roughly 14 pounds and the CR+TR group lost on average 8 kg, or roughly 17.5 pounds. Both groups lost weight. Both groups also improved their health in many other ways including improved blood pressure, lipid measurements, and measures of insulin resistance.
You might be thinking, “wait a minute, the CR+TR group lost more weight, so TR is beneficial!” But, statistically speaking, the two groups were equivalent in terms of weight loss.
So, if the two strategies of CR vs. CR+TR are roughly equivalent, then that seems conclusive. TR doesn’t offer any benefits above and beyond CR. As a result, many experts and physicians have been saying on Twitter (and I’m sure in other forums), that we should stop prescribing TR to our patients. If TR isn’t any better than CR, then we should just prescribe CR.
But, this is wrong.
I’m now going to make several controversial assertions:
- Despite the study’s stated conclusion, I don’t believe the study shows that CR+TR is equivalent to CR alone.
- In the real-world, TR is often prescribed alone, without a specific caloric restriction, and thus this study doesn’t address real-world conditions.
- TR’s main benefit in the real-world may be that it helps facilitate CR by managing hunger.
- The study’s real conclusion should be: under optimal conditions, CR, with or without TR facilitates weight loss, but that TR likely facilitates greater CR than CR alone.
Let’s take one at a time.
1. Despite the study’s stated conclusion, I don’t believe the study shows that CR+TR is equivalent to CR alone.
The reason this conclusion can’t be drawn, in my view, is due to the study’s definition of TR as well as the fact that the two groups were more similar than they were different. What I mean is that this study didn’t really have a group that tested TR.
Review of the supplementary appendix provided with the article on the NEJM website reveals that the CR only group had an eating window of 11 hours (roughly 8am to 7pm). The CR+TR group had an eating window of 8 hours (roughly 8am to 4pm). What’s more, the authors actually go out of their way to mention that in this region of China it’s common for people to take their biggest meal of the day in the middle of the day. If this is true, then both groups consumed most of their calories in a very similar distribution throughout the day.
What’s more, further review of the appendix reveals that the CR+TR group on average ate about 100 fewer calories per day than the CR only group. One possible interpretation is that this group was less hungry due to TR, or that they simply skipped some evening snack due to their TR that the CR only group didn’t skip. Though this number of calories wasn’t statistically significant, it coincides with improved outcome metrics (again not statistically so), for the CR+TR group vs. the CR only group.
I recommend reviewing the study’s table. Nearly every metric was better in the CR+TR group: weight, BMI, waist circumference, abdominal wall fat, visceral fat, LDL-C, HDL-C, Triglycerides, HOMA-IR, etc. In almost every case, the CR+TR group was better than the CR only group.
Overall, these groups are more similar than not. The main difference between them is roughly 100 calories a day fewer in the CR+TR group consumed in a three hour shorter window. This resulted in a non-significant but consistent across the board improvement in weight loss and health metrics for the CR+TR group.
So, how could this be interpreted to mean that the study shows CR+TR is no better than CR alone?
I think it clearly shows that CR+TR is better than CR alone, even if it’s marginal, and that the CR group in this particular study was not different enough to really draw a contrast. These groups are too similar to draw the conclusion that was drawn.
2. In the real-world, TR is often prescribed alone, without a specific caloric restriction, and thus this study doesn’t address real-world conditions.
When I prescribe TR to a new patient, it’s usually without respect to calories. Anyone who’s tried to restrict calories knows that it’s hard to do. We become accustomed to our ways of eating over a period of decades, and simply shrinking our portions or restricting our diets can feel impossible. So, one way to do this is to tell people, “okay, you can eat whatever you want, but you have to eat only between 10am and 4pm.” This is just one example of a TR prescription. But, the point is, the time-window alone is the prescription. There’s no prescribed calorie restriction.
So, a better comparison would have been a group with CR only, and a group with TR only. Then have both groups record calories and see what happens. My bet is the TR group would have had some degree of caloric restriction anyway. It turns out that it’s hard to eat as many calories between 10am and 4pm as you would otherwise eat if allowed to eat from 8am to 9 or 10pm. I can personally vouch for this, as on some days when I’m tired, stressed, and not paying attention, I can easily consume an extra 500-1000 calories before bed just by scavenging around the kitchen. When I’m on a time-restricted plan, those 500-1000 calories are avoided.
3. TR’s main benefit in the real-world may be that it helps facilitate CR by managing hunger.
Time is one variable we can play with to help people manage hunger and restrict calories without requiring counting calories.
Whether or not TR has some magical physiologic benefit over equivalent CR without TR is a different question, but ultimately, this is an academic question that doesn’t matter to people looking to lose weight or improve their health. I’d like to know the answer as to whether TR has a physiologic benefit, but this question is not answerable based on the study in question.
So, how does TR work for people in the absence of prescribed caloric restriction?
It probably works by helping people manage hunger. Most people have experienced the phenomenon of feeling hungry but not having access to food. Just as hunger comes on, if you don’t eat for an hour or two, it goes away. It might stay away for hours. When it comes back, you can do the same thing again, and it’ll go away again. However, if you eat a little bit of food each time you get hungry, it’s easy to go wild and start mainlining everything in sight. But if you just resist that initial hunger pang, the hunger fades into the background and you realize you can go quite a bit longer without eating.
TR allows people to manage hunger. It allows us to be mindful of hunger, and of when we’re eating. When people start paying attention to one aspect of nutrition (in this case, timing), they often become aware of other aspects. It gives a feeling of control where control was previously lacking, and thus tends to invoke all kinds of improved behaviors with respect to nutrition.
This study doesn’t test any of these less measurable aspects of TR, and it doesn’t compare CR to TR, it compares CR to CR+TR. Where it tests TR, it’s still a relatively wide eating window of 8 hours. What would we have seen if the window was smaller, like 4 or 6 hours? The study doesn’t test whether real-world TR works.
4. The study’s real conclusion should be that, under optimal conditions, CR, with or without TR facilitates weight loss, but that TR likely facilitates greater CR than CR alone.
You need to know a few more things about the conditions of the study.
- Participants volunteered after being recruited via public advertisements.
- The participants were given health coaches with whom they communicated regularly.
- Participants were required to weigh, photograph, and keep diaries of everything they ate.
- They received ongoing education about diet and nutrition throughout the study’s duration.
- Participants were given 6 months of free protein shakes.
- The frequency requirements of coaching, education, and tracking were decreased during the latter six months of the study, and in many cases, weight loss slackened or reversed during this period.
These are just a few factors that help interpret the findings of this study a little better. What the study tells us is that CR or CR+TR are both successful under very supportive conditions.
In this study, participants self-selected to participate, and had enormous amounts of support. They were also required to track their nutrition.
Sometimes, in my practice, those are the only two interventions I provide at first. I start with motivated people and I ask them to track what they’re eating. I’m available to them to support their efforts and answer questions and we have regularly scheduled meetings.
In many cases, this is enough to trigger a cascade of behavioral changes that result in massive body recomposition and improvements in health.
Consider this case from my practice.
Coaching is not to be underestimated. It’s hard to sustain nutritional efforts for long periods of time, and coaches absolutely help. When the coaching and tracking requirements were decreased in the second half of this study, weight loss stagnated or reversed in at least some cases.
Overall, with proper instruction, coaching, tracking, and caloric restriction, the study participants were able to lose fat and were able to improve their health. TR added a non-significant, but consistent benefit in nearly every metric measured in the study.
The study proves that volunteers who are interested in losing fat and becoming healthier can achieve this goal with CR or CR+TR along with tracking, coaching, and education.
A final point
A final point is that the study participants were overweight or obese, but did not meet diagnostic criteria for diabetes. So, this study doesn’t include the most metabolically ill people of the population, nor does it include metabolically healthy people who are looking for further advantages in longevity, performance, or just in general health.
The former (diabetics with metabolic syndrome) might have physiologic benefits with TR above and beyond a group who is merely overweight or obese.
The latter (metabolically healthy people) may or may not have improvements in certain markers of health under TR.
This study can’t tell us about either of these other groups, it can only tell us about the group it studied.
Overall, this study is a really great study which proves that people can succeed at losing fat and improving their health using CR or CR+TR in an eight hour feeding window with loads of support.
If you are looking for a near certain way to lose weight, this study gives you a pretty clear template to follow.
Of course, it may not work for everyone, and it’s just one way of doing things.
There is no replacement for a personalized and tailored approach that considers all aspects of health along with an individual’s goals, resources, and values.
Featured Image Credit: Photo by Kenny Eliason on Unsplash