Published on Sept 23, 2024
Many of you will have heard about intermittent fasting, and its supposed health benefits. There are many forms of intermittent fasting, because intermittent fasting simply means that we consciously make an effort to avoid consuming calories for some extended period of time. The most common forms of intermittent fasting are time-restricted eating, of which OMAD or one-meal-a-day is the most extreme version; alternate-day fasting or modified alternate-day fasting; and the 5:2 diet. We’ll talk about all of these at some point, but first, we’ll thoroughly examine time-restricted eating, or TRE.
We are beginning our examination of the health effects of TRE with a discussion of whether TRE can help us lose body weight. We’ll also evaluate whether any weight we lose is mostly fat mass or lean muscle mass. And we’ll look at whether TRE can be a valuable tool to get rid of visceral fat and liver fat, two of the most metabolically harmful types of fat in our bodies.
Most of the social media posts or YouTube videos I see on this topic are based on personal anecdotes or cover one or two specific papers on TRE, but that isn’t the most rigorous approach. To really make well-informed decisions that benefit our health and well-being in the long term, we should not cherry-pick one or two appealing studies but instead look at all of the evidence, appropriately weighed by the strengths and limitations of each study. We also call this the cumulative evidence, or the totality of the evidence. So I have carefully reviewed several dozen studies for you, and condensed their findings into this blog post.
Let’s get started with a very brief description of what TRE is.
What is Time-Restricted Eating?
In time-restricted eating, we make a conscious effort to limit our food or, more specifically, our calorie intake to a short window of time during the day. The most common forms of TRE use an eating window somewhere between 4 and 10 hours, which means that then we fast for the remaining 14 to 20 hours. The most popular form is probably 16:8, where we eat in an 8-hour window and fast for 16 hours. Food intake always occurs during the day, and we do differentiate between early TRE, where all of our food intake occurs in the first few hours of the light cycle, and late TRE, where we may skip breakfast and eat all of our food in the afternoon and early evening.
Several mechanisms have been proposed to explain why limiting our food intake to a relatively short eating window during the day may be beneficial.
For one, TRE could help us align our circadian rhythm to the light-dark cycle of our natural environment and avoid a disruption of our natural circadian rhythm, which has been linked to numerous health problems.
Second, there may be benefits to fasting for an extended period of time overnight.
And third, it may be easier to prevent overeating and weight gain if we have fewer opportunities to consume food while we are awake.
Well, let’s get started by looking at the beginnings of this line of research.
The Impact of Time-Restricted Eating on Body Weight in Mice
It is interesting to start with one of the early studies in mice that got the whole world excited about TRE. In this study, researchers observed that if they feed mice a high-fat diet, they overeat and become pretty obese in just a few weeks. However, they also found that these mice liked the fatty food so much that if they had access to it 24 hours a day, they would constantly feed, even waking up frequently to do so. So they asked whether they could prevent this overeating and weight gain if they restricted access to the food to 8 hours per day. And indeed, the mice that could feed only for 8 hours a day did not gain nearly as much weight as those mice that had access to the high-fat food all day and night. Interestingly, though, that was not because they ate less. The mice learned very quickly that the yummy food would be taken away from them, and so they just ate way more during the 8 hours they had access to it. And to the researchers’ surprise, they consumed as many calories as their neighbors, who could feed 24 hours per day. So why did they not gain as much weight? Well, the researchers monitored the mice very carefully, and found that the energy expenditure was a lot higher in the mice on 8-hour TRE, and that explained why they didn’t gain as much weight as their friends even though they ate just as much food.
So, the natural question is whether humans respond similarly to time-restricted eating. In other words, do humans also gain less weight, or lose weight, on TRE even if they don’t consume fewer calories? That leads us to the first question we want to answer.
Does Time-Restricted Eating Lower Body Weight Independent of Reduced Calorie Intake?
Fortunately, we do have data from two high-quality trials in which participants were randomized to either TRE or a control group in which they ate in their normal eating window, AND in which all foods were provided, with a set calorie intake to keep body weight stable.
Sutton and colleagues published results from a small study, but it was very well-controlled and also conducted as a cross-over design trial where each participant completed two dietary phases. In one phase, participants consumed all of their foods in a 12-hour window for five weeks. This was the control phase, and is already a bit of a shorter eating window than how most people eat, but this is fairly close to normal. The neat thing about this study was that all foods were provided by the study team, and all foods were consumed under supervision in the clinic. In other words, the researchers had exact control over WHEN participants ate, WHAT they ate, and HOW MUCH they ate.
Participants also completed a second diet phase, which also lasted five weeks. This was the TRE phase during which they received the exact same amount and type of foods as in the other study period, but now they were asked to eat breakfast, lunch, and dinner in this narrow 6-hour eating window early in the day. They then fasted for 18 hours until breakfast the next morning. Each participant completed both diet phases, in a random order.
And even though researchers had planned to give participants enough food to keep them weight stable in both diet phases, participants lost a little bit of weight, 1.4 kg or 3 pounds on the TRE diet, and 1 kg or 2.2 pounds on the control diet (see table below). Now, this is an important point to make: one thing that every single study in participants with overweight or obesity shows is that what motivates them to participate in a dietary research study is usually, at least to some degree, the hope that they will lose some weight. And therefore, we very often see participants lose weight no matter what the intervention is, even in the control group that doesn’t receive a specific intervention. This is important because in this field of research into the effects of TRE, there are many studies that did not include a control group, and without control group, almost any intervention will look like it causes weight loss. Therefore, very importantly, we always need to compare the change in weight in the intervention group with the change in weight in an adequate control group. In this case here, that difference was not statistically significant.
Well, you may say, but people in the TRE group seemed to lose slightly more weight, and maybe the study was just underpowered to detect this as statistically significant. That is correct, because only 8 participants completed both diet phases. The weight loss data, by themselves, are a bit inconclusive. It is possible that this additional weight loss of 0.4 kg, almost a pound, over 5 weeks seen in the TRE diet phase could reflect greater energy expenditure, as was seen in the TRE-few mice. Fortunately, the investigators actually published a separate paper in which they reported energy expenditure data of participants on these two diets, and the number was almost exactly the same. In other words, there was no evidence whatsoever that TRE affects energy expenditure in any way.
This is consistent with data from a second study conducted by Maruthur and colleagues that was somewhat similar but larger. This study enrolled 41 participants and randomized them to one of two diets, each of which was consumed for 12 weeks. One was a 8-hour TRE diet, and the other a 12-hour control diet. So this was a parallel-design study, and every participant consumed only one of the two diets. Again, all foods were provided to participants, and food and calorie intake were experimentally controlled. The way this was done is that the investigators calculated a number of calories that would be expected to keep every participant weight stable, based on a formula that uses their body size and physical activity level. They used the same formula in both diet groups, and participants had to consume all of the provided food every day, either in a time-restricted window or, in the control group, more spread out over the entire day.
Again, as in the first study, participants lost some weight, but it was about the same amount of weight on both diet arms.
From these two well-controlled trials, we can fairly confidently conclude that TRE does not have an impact on energy expenditure, or at least not one that is clinically relevant. With just two studies, the evidence is a bit limited, but I think we can already conclude that TRE certainly does not have a huge effect on energy expenditure and body weight that is independent of calorie intake, as was seen in the original mouse studies.
Ok, does this mean that TRE does not affect body weight at all? No, not so fast. It only means that TRE doesn’t seem to affect energy expenditure in humans, at least not in a major way, as it seems to do in mice.
Next, let’s take a look at what happens to body weight in people who adopt TRE if their calorie intake is not experimentally kept stable.
Does Time-Restricted Eating Trigger Weight Loss?
The first studies I’d like to discuss are randomized controlled trials in which participants were simply asked to eat as they usually would (control group) or eat only in a specific restricted eating window (TRE group). What all of these studies had in common is that no one was TOLD to restrict their calorie intake, all they were told is to either follow TRE or not. The table below shows those studies that I considered. I only considered studies with an intervention duration of at least 6 weeks, because effects on body weight are difficult to detect in shorter studies, and I also only considered studies that compared changes in body weight in a TRE group to an adequate control group. There are actually many more studies that I gauged as methodologically too limited to consider here. See the end of this blog post for a critical discussion of those scientific publications that were not considered here, and why they were not considered.
These studies varied widely in how much they restricted the eating window, from 4-hour to about 10-hour TRE. But what they all had in common, without exception, is that following TRE made people spontaneously eat less food and fewer calories and lose weight. The weight losses were consistently and significantly greater in the TRE groups than in the control groups.
That’s the second conclusion we can draw here. Following TRE does help people lose weight, but not by increasing energy expenditure but primarily by helping them reduce their calorie intake.
OK, so, if TRE helps us lose weight by putting us into a caloric deficit, how does it stack up to the traditional approach to weight loss, an imposed caloric restriction? In other words, what do studies tell us in which people were randomized to either TRE with no calorie restriction imposed, or a control group in which people continued eating in their normal much longer eating window, but they were asked to count calories and only consume a limited number of calories.
Well, we have two such studies of reasonable quality (see table above), and they both show similar weight loss resulting from TRE without imposed caloric restriction when compared to a control group with calorie restriction. In one of the two studies, the TRE group lost a lot more weight, but weight loss was highly variable in both groups and so the difference was not statistically significant. Also, average weight loss was higher in the control group in that second study, so I’d say that the best evidence we have right now suggests that over 6 or 12 months, just adopting a time-restricted eating pattern without paying any attention to calorie intake has about the same effect on body weight as an imposed caloric restriction. That’s actually really good news, because it gives us options. I do think that, for many people, TRE is probably a more appealing option to control calorie intake and body weight in the long term than continuous calorie counting and calorie restriction, which is hard for most people in the long run.
OK, so that leads us to the obvious next question, and that is whether adding TRE to a calorie-restricted diet can help us lose more weight than with calorie restriction alone.
The table below shows key data from clinical trials of sufficient quality in which participants were randomized to a TRE diet in which they ate in a restricted eating window and where they also actively restricted their calorie intake, or a control group in which they ate in their normal eating window while also actively restricting their calorie intake.
Here, we do get a bit of a mixed picture. Two trials detected significantly greater weight loss with calorie restriction plus TRE compared to calorie restriction alone, while the other four detected no differential effect. However, let’s take a closer look at some of the details. For one, one could argue that in many of these studies, the participants in the control groups actually exercised a mild form of TRE. Note that the average eating window in the control groups was around 11 hours or less in several studies, which is time-restricted compared to the average eating window in the United States, for example, which is around 15 hours per day. So this alone may have limited the ability of these studies to detect an additional effect of TRE. Further, in the study by Thomas and colleagues, the difference in the eating window between the TRE group and controls was barely more than an hour, which may simply have been too small. Despite these limitations, the absolute average weight loss was actually greater in every single one of these studies, so it also stands to reason that some of these studies were simply underpowered to detect the additive effect of TRE on weight loss in populations that were already losing weight due to the imposed calorie restriction. I have discussed this point in much detail in a previous blog post about that last trial listed here by Liu and collegues. The key point here is that putting participants on a diet that is calorically restricted makes them lose weight, but not everyone loses the same amount of weight. Thus, weight change will be highly variable, and that variability will create a lot more noise, making it more difficult to detect the independent effects of another intervention (in this case, TRE).
Taken together, I actually would cautiously interpret the available evidence as suggesting that adding TRE to calorie restriction might modestly enhance weight loss. My best guess is that TRE may simply make it a bit easier for people to maintain a caloric deficit over an extended period of time.
To conclude this part, just restricting food intake to a window of 4-10 hours per day consistently leads to spontaneously lower calorie intake and weight loss, whereas it does not seem to affect energy expenditure, at least not meaningfully. That effect of TRE on calorie intake and weight loss is similar to that of an imposed calorie restriction, and there may be a modestly greater weight loss when TRE is added on top of an actively calorie-restricted diet.
With that, let’s move to the next part, and that is …
Does Time-Restricted Eating (Primarily) Reduce Fat Mass or Lean Mass?
What I am interested in here specifically is if people lose weight on a TRE diet, do they lose mostly fat mass, which is what we want, or do they preferentially lose lean mass, meaning muscle mass. There have been some rumors that TRE causes more muscle loss because of the extended fasting period, and I agree it’s important to understand whether the weight we lose on any intervention is actually body fat or muscle.
Let’s look at the available studies again that measured changes in fat and lean mass, in the same categories as before.
We’ll start with those studies in which participants were randomized to either TRE or control without imposed caloric restriction in either group. By and large, what most of these studies show is that people who follow a time-restricted eating plan and spontaneously lose weight lose mostly fat mass (see table below). That’s what we want, which is why I have shaded these studies in green. For example, look at these longer-term studies at the bottom of the table. In Pavlou et al., participants lost an average of 4.5 kg over 6 months, which is almost 10 pounds, and about two-thirds of that weight loss was body fat. Even better are the data from Lin et al. In this 12-month study, participants lost almost only body fat and barely any lean mass. Basically, in all of these studies, the percentage of weight loss that was lost as fat mass was similar or better in the TRE group compared to the control group.
However, there are three studies in which participants experienced a substantial loss of lean mass as they lost weight on TRE (see table below). An extreme example is the study by Brady and colleagues in which participants lost a very modest amount of weight, and most of that was lean mass. But even in the other two studies, there was about as much loss of lean mass as of body fat. That’s not particularly desirable.
OK, next, we have two studies in which TRE was compared to a calorie restriction control group. These data suggest that the degree to which we are losing fat vs. lean mass with TRE is similar to what we see with a calorie-restricted diet.
And lastly, let’s look at those studies that investigated the impact of TRE when added to an already calorie-restricted diet. So this is calorie restriction compared with calorie restriction plus TRE. And in these six trials (see table below), there was not a single one that suggested a more significant loss of lean mass relative to the total weight loss in the TRE group compared to the control group. In all studies, fat mass accounted for at least two-thirds of the total body weight lost, which was similar or better than what was observed in the control group that followed calorie restriction only.
I’d like to highlight two particular studies that are relevant in this context. For one, the RCT by Stote et al.. These investigators had participants complete one 8-week period where they just ate one meal a day, which is nowadays often called OMAD. They gave them 4 hours to complete this meal, so participants were fasting for at least 20 hours per day. This was a cross-over design study, and they had each participant also complete an 8-week control period where they ate in their normal eating window. This design is particularly powerful because it allows us to compare each participant to themselves. An additional strength of this study was that all foods were provided and partly consumed in the clinic, under supervision, so this was a well-controlled study. And these data show a very substantial loss of body fat, 2.1 kg or 4.6 pounds, whereas lean mass increased quite substantially by 1.5 kg. I am honestly not sure why lean mass increased here, as there was no exercise program included in either phase, but the main point I wanted to make is that TRE, even an extreme form of TRE such as OMAD, does not automatically lead to a loss of lean mass.
The second study is an RCT by Moro and colleagues. This was a parallel-design study in which participants were randomized to TRE or a control diet where they ate in their normal eating window. In addition, all participants completed regular resistance training. What we see in this trial is that while the overall weight loss was modest (an average of -1.0 kg in the TRE group compared to +0.2 kg in the control group), participants in the TRE group lost more body fat than that (-1.6 kg) and tended to gain lean mass (+0.6 kg). This increase in lean mass was the same in the control group, which is not too surprising given that both groups exercised, but the key point here is that, as in any weight loss program, engaging in exercise, particularly resistance training, could be a very effective way to make sure you hang on to your muscle mass when losing weight on TRE.
Taken together, if we consider all of the available evidence, we can conclude that about two-thirds of weight lost on TRE is body fat in most studies, which is similar to what we see in calorie-restricted diets. We do NOT see any evidence that TRE somehow leads to a greater reduction in lean mass, which is reassuring. Nevertheless, a recommendation would be that if you follow any form of TRE, or any other weight loss program for that matter, that you engage in some form of resistance training regularly and keep your protein intake high to minimize any loss of muscle mass.
A short note here: most studies measure changes in body composition by dual-energy x-ray absorptiometry (DEXA) scans. These do not directly measure muscle or lean mass, but instead allow us only to differentiate between body fat and fat-free mass as well as bone. Fat-free mass includes muscle and organ tissues, as well as the body’s water content. This is important to be clear about because, in short-term studies, significant shifts in the fat-free mass are often related to a reduction in body water and not muscle content.
With that, let’s move on to the last portion of the blog post, where I’d like to take a look at what TYPE of body fat is lost in TRE, and whether there is a particularly strong effect on the types of body fat that matter the most for our long-term health, visceral or intraabdominal fat and liver fat, a type of ectopic fat.
Does Time-Restricted Eating Lower Visceral Fat and Liver Fat?
I’d like to start by sharing data from a high-quality trial that looked at the impact of TRE on visceral and liver fat. This trial here by Wei and colleagues was published last year. They enrolled 88 participants who suffered from both obesity and non-alcoholic fatty liver disease, or NAFLD, and randomized them to either restrict their calorie intake every day for an entire year (control group), or to restrict their calorie intake and ALSO follow 8h-early TRE, with food intake only between 8 AM and 4 PM.
One of the primary factors that is linked to elevated visceral and liver fat is excess body weight and overall fat mass, so let’s first take a look at what happened to body weight and fat mass in this study. Given that participants in both study groups were actively restricting their calorie intake, we would expect them to lose weight. And indeed, in both groups, participants lost about 10 kg or 22 pounds at six months and about 8 kg or 17.6 pounds at 12 months. And that degree of weight loss was very similar in the two groups, with no additional weight loss in those who also followed TRE. Most of that weight loss was due to a loss of body fat in both groups, and in this particular study, we didn’t see any hint of an additional benefit from following TRE in the context of a diet that was already calorically restricted. So, both groups lost similar amount of weight and body fat.
How about visceral fat? As we see in the table above, both groups lost visceral fat, and the reduction in visceral fat was highly significant in each group. See how the 95% confidence interval does not include zero in either group. However, in this study design, the more important comparison is whether the change from baseline to the 6 and 12 month time points differed between the two groups, and this was not the case for either time point. In other words, just like the reduction in body weight and overall fat mass, the amount of visceral fat lost was similar in the two groups. Meaning it was mostly driven by the caloric restriction and there was no additional benefit from following TRE.
And what happened to liver fat content?
The liver fat content of these participants at baseline was about 13.5%. In healthy people, the fat content in the liver should be 0%, and whenever it is 5% or higher, we call this fatty liver disease. So, imagine now that at baseline 13.5% of these livers consisted of fat. That’s a lot, and not good. Definitely a risk factor for liver disease, and usually associated with insulin resistance in the liver and also with elevated fasting triglyceride levels.
The graph above shows the absolute changes in liver fat in both groups as they restricted their daily calorie intake with or without TRE. In orange are the data from the TRE plus caloric restriction group, and the data of the control group that only followed daily caloric restriction (DCR) is shown in blue. We see that 6 months into the intervention, the liver fat content is reduced by an average of about 8 percentage points, meaning it was reduced from an average of 13.5% to around 5.5%. And that reduction was very similar in the two groups, meaning that whether participants followed TRE had no impact. That would strongly suggest that the reduction in liver fat was primarily driven by the calorie deficit and the weight loss. Also compare this with how the weight dropped mostly in the first 6 months of the trial, and then stabilized or even rebounded a little bit. We see this exact pattern mirrored in the liver fat data, which is another indication that the reduction in liver fat content was mostly driven by the reduction in calorie intake and body weight.
This is just one trial, but it’s a good one, and these data would strongly suggest that both visceral fat and liver fat are primarily responsive to caloric restriction and weight loss, and that TRE does not offer any benefit beyond that.
For liver fat, this conclusion is supported by another crossover design study by Andriessen and colleagues of 10h-TRE vs. control in patients with type 2 diabetes who averaged about 9% liver fat at baseline. In that study, participants were experimentally kept weight stable, and in that setting, liver fat content did not change.
For visceral fat, there are a few other trials that give us some insight. In general, what they suggest is that significant reductions in visceral fat are driven by overall weight loss, but that TRE does not lead to additional reductions in visceral fat beyond its effect on weight loss. For example, Liu et al. (2022) found similar weight loss in participants on calorie-restricted diets, irrespective of whether they also followed 8h-TRE, and that was associated with a similar loss of visceral fat in both groups. Jamshed et al. (2022) found a greater loss of overall body weight in participants who followed TRE plus a caloric restriction compared to caloric restriction alone. Yet, the reductions in visceral fat seen in both groups were similar and did not differ from one another statistically. Pavlou et al. (2023) found no significant changes in visceral fat mass in participants following 8h-TRE, in spite of significant weight loss. Similarly, Cienfuegos et al. (2020) found no significant change in visceral fat in people following 4h- or 6h-TRE compared to control, even though body weight was decreased significantly in both TRE intervention arms. It is possible that these studies may have been underpowered to detect intervention effects on visceral fat mass, which was a secondary or exploratory endpoint in all studies. Still, none of the studies suggest that TRE somehow promotes a greater loss in visceral fat mass than can be explained by the loss in overall body weight. The only two studies that detected a greater loss of visceral fat on TRE compared to controls were those by Domaszewski et al. (2023) and Chow et al. (2020). However, this reduction in visceral fat was seen in the context of also greater weight loss in the TRE groups in these studies, which is again consistent with the idea that calorie restriction and weight loss are the primary drivers of a reduction in visceral fat, and that TRE does not independently contribute to lower visceral fat mass.
To conclude, pretty comprehensive evidence for visceral fat and limited evidence for liver fat suggests that reductions in these metabolically very damaging fat compartments are driven entirely or almost entirely by changes in the overall body weight. In other words, TRE doesn’t seem to have any impact on visceral fat and liver fat, independent of any impact it has on overall calorie intake and body weight. Therefore, it would be my assumption that if you had excess liver and visceral fat, and you adopted TRE and it helped you lose some weight that both liver fat and visceral fat would be reduced. And the more weight you lose, the more visceral fat and liver fat you will likely lose.
Which form of Time-Restricted Eating is Best for Weight Loss?
We can categorize TRE based on the length of the daily eating window, and also based on when in the day that eating window is. The question is, do these things matter?
In terms of the length of the eating window, most studies use windows from 4 hours to 10 hours. We don’t have a lot of studies that have systematically compared, say, 4h-TRE vs. 8h-TRE, but from those studies that we do have and just generally looking at effect sizes across studies, I would say that it doesn’t matter too much as long as you reduce your eating window meaningfully from what you are currently doing. As in, if the current eating window is 11 hours, I wouldn’t expect much of an effect from adopting 10h-TRE.
In TRE, we also differentiate between early TRE and late TRE. What we mean by that is if you do, say, 16:8 TRE where you eat in an 8 hour window and fast the remaining 16 hours, you could be eating between 8 AM and 4 PM, or between 12 Noon and 8 PM. One is commonly called early, the other late TRE. Does it matter when it comes to how much weight or fat mass we lose? As I was reviewing the data, I couldn’t detect a specific pattern. Two studies by Hutchison and colleagues and Deng and colleagues that actively compared early vs. late TRE did not detect any differential effect on body weight, fat mass, visceral fat, and liver fat. This was consistent with a recent meta-analysis that came to the same conclusion.
My general suggestion would be that if you want to try TRE, pick something you can happily sustain for an extended period. Because if you try, say, 8h-early TRE for half a year and lose some weight, you are probably going to regain that weight should you stop TRE and adopt your typical longer eating window again. Similar to when you go off a calorie-restricted diet. So in the interest of giving yourself a chance to reduce your body weight and improve your health over the long term, pick the form of TRE that is feasible for you and that you enjoy enough to do it over the long term. I, for example, have loosely followed 10h-TRE for probably 7 or 8 years. I don’t stress out about it, and will make exceptions if there’s a party or if I am really hungry, but I would say that I roughly eat in a 10-hour window most of the time. And to me, that’s an almost effortless way to prevent overeating and weight gain.
Scientific Studies Not Considered
Many more studies have been published that reported on the effects of TRE on body weight or body composition, but that were not considered because they did not meet standards for scientific rigor. Of note, our approach here was to review the study design and reporting first, and to not let the scientific findings of the study impact on whether the study was considered further.
Several studies were excluded from consideration because they did not include an adequate control group. These included the studies by Gabel et al. (2018), Lowe et al. (2020), Prasad et al. 2021, Przylj et al. (2021), Wilkinson et al. (2020), and He et al. (2022). An adequate control group is important because of what is often called hidden time effects that may occur in study participants that have little or nothing to do with the nature of the intervention. Specifically, participants enrolled in nutrition studies often want to lose body weight, and a common observation is that they lose body weight even if they are randomized to a control group. It is therefore imperative that changes in an intervention group are compared to an adequate control group. Lowe et al. (2020) did include a control group. However, the study design made it possible for control participants to also follow TRE and still be compliant, and we, therefore, made the decision not to consider this trial. Similarly, He et al. (2022) compared TRE vs. a low-carbohydrate diet vs. TRE plus a low-carbohydrate diet. Unfortunately, without a control group that did not receive any intervention, the results from this trial are difficult to interpret. What we can say is that 6h-TRE, when added to a low-carbohydrate diet, leads to a greater loss of body weight and visceral fat than a low-carbohydrate diet alone, which is consistent with the data discussed above that TRE leads to a reduction in calorie intake and body weight.
Three other studies did include a control group, but the intervention periods were too short to adequately gauge impacts in body weight and composition. There included the trials by LeCheminant et al. (2013), Jamshed et al. (2019), and Andriessen et al. (2022).
The study by Schroder et al. (2021) was not considered because participant were not randomized, but volunteered for the TRE intervention.
Two studies by Pureza et al. (2021) and Phillips et al. (2021) were not considered because they tested a version of TRE that had a very long eating window of 12 hours, which is similar to the control eating window in many studies and similar to the normal eating window of many people.
Lastly, two studies were excluded because of the insufficient or intrinsically inconsistent reporting of study findings or statistical analyses. These included the studies by Feehan et al. (2023) and Meessen et al. (2022).
A review of the principal findings of these excluded studies did not suggest that their inclusion would have meaningfully changed the overall conclusions from this analysis.
Summary & Conclusions
OK, let’s summarize everything we’ve learned in this blog post.
First, in humans, TRE does not meaningfully increase energy expenditure. In other words, without reducing calorie intake, TRE does not meaningfully promote weight loss.
Second, restricting the eating window leads to a spontaneous reduction in calorie intake and weight loss, without calorie counting and an active restriction in calorie intakes.
Third, the weight loss achieved with TRE is similar to that typically seen in people who are actively restricting their caloric intake.
Fourth, adding TRE to a diet that is already actively calorically restricted may lead to a modestly greater weight loss.
Fifth, TRE-driven weight loss is not associated with a greater loss of lean mass than other forms of weight loss.
Sixth, TRE does not seem to offer any additional benefits for reducing body fat, visceral fat, or liver fat beyond its impact on calorie intake and weight loss.
And Seventh, with regard to weight loss, the specific type of TRE, that is, early or late TRE or how long the eating window actually is, does not seem to matter too much as long as the individual eating window is sufficiently reduced and somewhere in the 4-10 hour range.
OK, that’s it for this blog post. I hope this will provide valuable guidance if you want to try TRE. In our very next blog post, we will cover the impact of TRE on cardiometabolic health, including blood sugar regulation and insulin resistance, the fasting lipid profile, and blood pressure.
Take care.
References
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- Sutton et al.; Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metabolism 2018; 27: 1212-21.
- Ravussin et al.; Early time-restricted feeding reduces appetite and increases fat oxidation but does not affect energy expenditure in humans. Obesity 2019; 27: 1244-54.
- Maruthur et al.; Effect of isocaloric, time-restricted eating on body weight in adults with obesity. Annals of Internal Medicine 2024; 177: 549-559.
- Domaszerwski et al.; Effect of a six-week intermittent fasting intervention program on the composition of the human body in women over 60 years of age. International Journal of Environmental Research and Public Health 2020; 17: 4138.
- Domaszewski et al.; Comparison of the effects of six-week time-restricted eating on weight loss, body composition, and visceral fat in overweight older men and women. Experimental Gerontology 2023; 174: 112116.
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One Response
Thanks for this thorough analysis and write-up – this will definitely inform my future actions and expectations. And what a great list of references – I am especially interested in the ones like #2 which investigate effects on insulin sensitivity.