In recent years, one of the most popular diet interventions has been fasting, in a variety of different forms. These have included intermittent fasting diets in which you’re supposed to fast for a few days per week, such as the 5:2 diet, or a few days per month. They’ve also included various forms of time restricted eating, such as the 16:8 diet, where you’re supposed to get all your calories within an eight hour window each day, and the more extreme warrior diet, in which you’re supposed to get all your calories in a four hour window. But there is still little clarity on how effective these modifications are in terms of weight loss. And up to this point, pretty much all the evidence in support of fasting comes from animal studies, which are notoriously unreliable.
A study has just been published in JAMA (the Journal of the American Medical Association) that looks at the 16:8 diet in particular. It recruited 141 people and followed them over a twelve week period, during which they were randomized to either time restricted eating in the form of a 16:8 diet or a control diet. The authors of the study have financial ties to companies that promote ketogenic diets and fasting, so they were probably hoping for a positive study, which is something to be aware of before we get in to the data.
The 16:8 group was supposed to consume all its calories between 12:00 pm and 20:00 pm, and was not allowed to consume anything containing calories outside this window. The control group was instructed to eat three square meals per day, but was also allowed to snack as much as desired between meals (presumably the reason the researchers didn’t just tell them to do whatever they wanted was so they wouldn’t realize that they were in the control group).
Neither group had any restrictions placed on what they could eat or on the total number of calories they were allowed to consume, which is good, because we want to know if the time restriction in itself results in weight loss, even without further restrictions.
All participants received a bluetooth connected scale with which they were supposed to weigh themselves at home on a daily basis, and they downloaded an app which provided daily reminders in order to help them stick with their prescribed diet. They also filled in whether they were successfully following the diet on a daily basis in the app.
Of 141 randomized participants, 105 followed through to the end of the 12 week intervention. Of the 36 individuals who dropped out, 25 never took part in the study in any way other than signing up for it. Since no weight measurements were ever recorded for these 25 individuals, they weren’t included in the subsequent analysis, which I think is reasonable. So there were actually 116 individuals who provided at least one weight measurement, and were included in the analysis.
Of the 116 participants, 59 were randomized to time restricted eating, while 57 were randomized to the control group. 40% of participants were female, and 60% were male. The average age of the participants was 47 years. The average weight was 99 kilograms (218 pounds) and the average BMI (Body Mass Index) was 33 (the cut-off for overweight is 25, and for obesity is 30).
So, what were the results?
Participants in the control group adhered to the prescribed diet of three solid meals per day (with freedom to snack whenever desired) 92% of the time. Participants in the time restricted eating group adhered to the 16:8 requirement 84% of the time. It makes sense that the time restricted group would have a harder time sticking to their diet than the control group, since they were being expected to fast for a significant portion of the day. But 83% adherence is still pretty good, suggesting that most people are able to stick with a 16 hour fast most of the time without too much difficulty, at least over a 12 week period.
In the control group, the average weight loss over 12 weeks was 0,68 kilograms (1,5 pounds). In the time restricted eating group, the average weight loss was 0,94 kilograms (2,1 pounds). This was an unimpressive 0,26 kilogram (0,6 pound) difference in favor of time restricted eating. And the difference wasn’t anywhere close to being statistically significant.
Unfortunately, things gets worse. The researchers gathered a lot of additional data from a subgroup of 50 participants (25 from each group) that lived close enough to the research lab in San Francisco to come and have measurements taken in person. One of the things they looked at was muscle mass in the arms and legs (a measure known as “appendicular lean mass”).
In the sub-groups, the control group lost 0,17 kilograms (0,4 pounds) of arm and leg muscle out of a total weight loss of 0,6 kilograms (1,3 pounds). The time restricted eating group lost 0,64 kilograms (1,4 pounds) of arm and leg muscle out of a total weight loss of 1,7 kilograms (3,7 pounds). While the total weight loss was not statistically significant between the groups, there was a statistically significant greater loss of muscle mass in the time restricted eating group.
That’s not good. Of the small amount of weight lost in the time restricted eating group, 37% was muscle in the arms and legs. In the control group, the proportion of weight loss that was was arm and leg muscle was 28% .
So, what conclusions can we draw?
The 16:8 diet does not seem to work as a method for losing weight. The weight loss over twelve weeks was marginal. Of course, there is a big weakness with this study, and that is that it only followed participants for twelve weeks. It is possible that a statistically significant difference would have been seen between time restricted eating and control if the study had gone on for a full year.
However, people who start a new diet usually see the biggest effect in the first few months. Even if we were to assume a linear weight loss over the course of a full year, the difference between the time restricted eating group and the control group would still only be 1,1 kilograms (2.4 pounds), which is tiny.
And what is worse is that that an unusually large proportion of the weight loss in the time restricted eating group came from muscle mass, and in particular skeletal muscle in the arms and legs. These muscles are critical to our ability to stay independent as we age, and we definitely don’t want to lose them any faster than necessary.
Of course, this is only one study, and a hundred odd patients isn’t that many, and they weren’t followed for very long, so we shouldn’t draw too far reaching conclusions. But the evidence from this study is going in the wrong direction when it comes to both weight loss and muscle loss. Based on that, I wouldn’t recommend time restricted eating to anyone.
As mentioned earlier, the authors had ties to companies that promote ketogenic diets and fasting. Based on that information, I would assume that they wanted the study to show a positive effect. If they wanted a positive study and got a negative one, then that suggests we can trust these results.
It’s possible that an even more extreme restriction, like 20:4 (“the warrior diet”) would still work, and I hope that is investigated in future studies, just as I hope we will see more studies on fasting in general and for longer time periods, so that we can truly find out what works and what doesn’t. When it comes to fasting interventions, there is a lot of guesswork at the moment, and a lot of recommendations are made based on weak evidence. It would be great if that could change in the near future.
You might also enjoy my article about whether exercise is an effective way to lose weight or my article about whether a low carb or low fat diet is better for weight loss.
If you haven’t already done so, I strongly recommend that you read my guide to scientific method in medical and health science.