Physiology Friday Issue #10: Time-Restricted Eating is “No Better” Than Calorie Restriction for Weight Loss
Reducing the number of calories consumed rather than the timing of eating seems to be the primary driver of metabolic improvements and weight loss for individuals with obesity.
Hello friends,
Happy Physiology Friday!
Intermittent fasting has been hailed as the modern-day panacea for metabolic health, circadian rhythm function, weight loss, and performance. There are many well-known and proven benefits of fasting (though many studies have been conducted in rodents) but there is also a lot of hype and misinformation.
Among fasting regimens, time-restricted eating, or TRE for short, is perhaps the most popular. TRE is simple — limit your daily eating window to a certain amount of hours and fast for the rest of the day. The most popular TRE variants involve the 16:8 and 18:6 fast, though any iteration of fasting: eating is permitted (and doesn’t have to involve even numbers!)
TRE is popular because of its convenience and easy-to-follow framework. But whether TRE is superior to other forms of dieting like traditional calorie restriction (CR) is not known. Some studies have found that if similar calories are consumed, TRE may have unique metabolic benefits due to the extended fasting window or (and especially) because calories are limited to the early part of the day — something known as early time-restricted eating (eTRE or eTRF).
This may be the case, but many have argued (with data) that TRE is no more than a means to achieve calorie restriction. This is the subject of a new study that compared whether TRE combined with calorie restriction would prove to be superior to calorie restriction without TRE for metabolic health and weight loss.
What did this study do?
139 participants with obesity were randomly assigned into one of two groups: a time-restricted eating group with calorie restriction or a calorie restriction only group.
Both groups were given instructions to follow a diet that included 1500 to 1900 calories per day or 1200 to 1500 calories per day for men and women, respectively. This intake represented approximately 75% of participants’ normal daily caloric intake — i.e. they reduced their daily calories by around 25%. This is a pretty substantial reduction.
While no specifics are given on what participants ate, their dietary macronutrient composition looked something like this: 40–55% carbohydrates, 15–20% protein, and 20–30% fat. A pretty “standard” diet.
The main difference between the groups was when the day’s calories were consumed. The TRE group ate within an 8-hour window each day — between 8:00 am and 4:00 pm — while the calorie restriction-only group had no limitations on the timing of their food intake.
What did they find?
Bodyweight
Both groups lost a (clinically) significant amount of weight compared to baseline. Specifically, the time-restricted eating group lost an average of 8 kg, and the calorie restriction-only group lost an average of 6.3 kg over 12 months. However, there were no significant differences between groups, meaning that TRE and daily calorie restriction resulted in similar amounts of weight loss.
The amount of weight lost between groups was also similar even when participants were divided into subgroups based on their sex, baseline body mass index (BMI), insulin sensitivity, and adherence to the diet. Similar to weight, waist circumference and BMI were improved in both groups, with no between-group differences.
Body composition
Time-restricted eating plus calorie restriction led to an average loss in fat mass of 5.9 kg, while calorie restriction alone reduced body fat mass by an average of 4.5 kg. This loss in fat mass was similar between the groups. Similar findings were observed for lean body mass, visceral and subcutaneous fat, liver fat, trunk fat, and lean mass — all of which improved in both groups but showed no differences between TRE and calorie restriction.
Cardiometabolic risk factors
Both of the diet regimens led to significant improvements in systolic and diastolic blood pressure, fasting glucose, glucose tolerance, and blood lipids. However, the benefits were not different when comparing time-restricted eating to calorie restriction alone.
Bottom line
- Time-restricted eating with calorie restriction and calorie restriction alone similarly improve weight, body composition, and cardiometabolic risk factors
- ~80% adherence to both diets implies that long-term time-restricted eating regimens and calorie restriction are feasible weight-loss strategies, at least up to 12 months
Thoughts and musings
Several media stories on this study have cited it as further proof that there is nothing special about time-restricted eating. While this study might suggest so, I think it is a bit premature to conclude that TRE is useless, for a few reasons.
For one, the degree of TRE compared to participants’ baseline eating window was quite modest — they went from eating 10 hours each day to just around 8 hours — only a ~2-hour reduction in the daily eating window. Though weight loss and other metabolic improvements still occurred, it would be interesting to observe what would happen if someone’s eating window were cut in half or more. This would likely be the case if the same TRE regimen were implemented in the US (this study was conducted in China) where meal timing and eating windows are likely quite different.
Second, this was a tightly-controlled study which may limit the generalizability of the findings. While participants achieved a 25% reduction in calories during the trial period, in free-living conditions, reducing one’s caloric intake is much more difficult, that is, unless one ascribes to a daily eating window.
Many have reported (and studies have confirmed) that time-restricted eating under free-living conditions leads to a natural reduction in calories — an “unintentional” calorie restriction. With less time to eat during the day, some people end up eating less. Without a time restriction, it’s much easier to overconsume calories and thus, harder to lose weight.
In an accompanying editorial, researchers iterate that:
“From a public health point of view, time-restricted eating may turn out to be an approach to accomplish calorie restriction and improve metabolic health without the resource-intensive approach of intentional calorie restriction.”
That’s the crux of the debate. If eating within a 6- or 8-hour window helps you limit your food intake or achieve your health and performance goals, then it’s the optimal strategy for you. Some people find that a restricted eating window limits their social eating opportunities and therefore avoid it, and that’s ok too. TRE isn’t for everyone, nor should it be — it’s not a panacea.
None of this is also to say that TRE isn’t superior to other forms of dieting in other ways. There is good evidence that eating within a restricted window — particularly early in the day — may be optimal for circadian rhythm and hormonal health. There are many aspects to health other than weight, and we must remember that when dissecting and criticizing these interventions.
A proper conclusion may be: “time-restricted eating is not superior to calorie restriction…in this study!”
I tend to practice a daily 16:8 TRE regimen with two large meals — eating between noon and 8 pm each day — though I adhere somewhat loosely to this window. I do so less for the fasting piece and more for the convenience, finding that I can power through the morning with focus before my first meal and also extend my window late enough to afford an evening out if I wish. Sometimes I break my fast at 11, and sometimes I’ll eat something well past 8 pm. Flexibility is often warranted.
Do you practice a specific TRE regimen or another strategy to help with weight loss or improve metabolic health? Drop a comment below!
As always, thanks for reading. See you next Friday.
Brady
Studies cited
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