New study states intermittent fasting doesn’t work — but is that true?

A new study, published in the Journal of the American Medical Association (JAMA), shows no benefit from intermittent fasting. But, after taking a close look, it’s revealed that we should be cautious about how we interpret this conclusion.
The study suggests time-restricted eating does not lead to greater weight loss or metabolic benefits. Even though this was a well-designed randomized controlled trial, we are left with more questions than answers.
But I will admit, I was shocked when I initially saw the results of this study. I have written about the benefits of time-restricted eating before — and I am personally a big believer in intermittent fasting. I, myself, utilize this practice, recommend it to my patients, and write encouraging posts on the topic.
However, these new findings led me to ask: What does this mean for those who follow time-restricted eating, which has become one of the most popular weight loss interventions?
Additionally, I questioned whether this new study would disprove the entire concept of time-restricted eating? But, as it goes with any trial, there are many details and caveats worth exploring before making that call.
For reference, the senior author is Dr. Ethan Weiss, a prominent preventive cardiologist, and researcher at the University of California, San Francisco. He was also a podcast guest in episode #56 of the Diet Doctor Podcast, which comes out on October 5. He is someone that I have a great deal of respect for — both personally and professionally.
In Dr. Weiss’ study, the authors randomized 116 overweight individuals to either the consistent meal timing (CMT) group or the time-restricted eating (TRE) group. The CMT group ate three meals per day — plus snacks if hungry. The TRE group was instructed to eat all of their calories between 12 pm and 8 pm.
There was no advice or restrictions about what to eat or how much to eat. Participants were only advised when to eat.
Of participants, 92% of the CMT group complied with the control diet, whereas only 83% of the TRE group complied with their diet. After 12 weeks, there was no significant difference in weight loss or metabolic markers (glucose, insulin, hemoglobin A1c (HbA1c), blood pressure, triglycerides, low-density lipoprotein (LDL) cholesterol) between the two groups.
Within the TRE group, there was a small decrease in lean mass. But I wouldn’t put too much stock in that as a secondary variable, especially since there are studies that show the opposite effect. This question is far from settled, especially given some of my other questions about the study.
Because of this result, this conclusion is already being spread by various media outlets, stating that time-restricted eating doesn’t work — and we should all go back to eating three meals a day.
But hold on a minute. It isn’t quite that cut and dried.
Let’s take a minute to discuss why TRE has been proposed to have health benefits. First, it is an effective means of reducing daily calories. And second, it allows insulin to remain low for longer — thereby, allowing for greater fat oxidation.
The current trial failed at the first point. The TRE group ate the same calories as the control group. In fact, at the end of the trial, there was a trend towards greater calorie intake.
One of my main rules for any patient I start with TRE — and a big point we make in Diet Doctor’s soon-to-be-released coaching program on intermittent fasting — is that you shouldn’t “make up for lost calories.” Instead, you should eat your meals as you usually would, or at a minimum add a little extra protein, and simply take one meal out. It is one of the easiest ways to reduce your calorie intake.
This trial didn’t achieve that. I can only assume it was because participants weren’t coached on the strategy of not making up calories. Right away, we can see that this study tested equal calorie diets for TRE, eliminating one of the most significant potential benefits of TRE.
And so, this does not exclude TRE as being an effective means of safe and sustainable calorie reduction when participants are properly coached.
But what about the second point? Did this study disprove that there is anything special about keeping insulin levels lower for a longer period? Maybe.
Sixteen hours may not be enough time to see that benefit. Would 18, 20, or even 24 hours be enough? Or, what if the eating window was cut off earlier to around 5 pm when the body is more insulin sensitive? These are interesting questions to which we don’t yet have answers.
What may even be more important to consider is that we don’t know what the subjects ate during their eating window.
I’ll admit, I jumped on the bandwagon after Drs. Pam Taub and Satchin Panda published their non-randomized trial, showing metabolic benefits within 14 hours of fasting and no regard to baseline diet.
However, even though I believed the science, I always coached my clients that what you eat still matters. Why would we want to spend the time lowering our insulin for 16 hours only to eat nutrient-poor, high-carb foods over eight hours that will only keep our glucose and insulin levels rising higher and higher?
We wouldn’t. That just doesn’t make sense.
Is that what happened in this trial? We don’t know. I didn’t see a report on diet quality or macronutrient breakdown in this trial. But interestingly, there was a considerable variation in the waterfall plot, showing many participants lost weight and some gained weight. Why the difference? Could it have been related to their underlying diet? Again, we don’t know.
It’s true that fasting, even as short as 16 hours, will trigger hunger and cravings in some people, leading to increased snacking, more overall calories, and worse food quality. I’ve seen it many times in my practice. Those individuals clearly should not fast.
But for those who can fast, reduce their overall calories, and continue to eat nutritious food that doesn’t cause insulin and glucose “spikes,” it is likely another story. Those are the individuals who will likely continue to benefit from time-restricted eating.
As with many trials, the study answers one question — and then we are left with many more questions.
Don’t get me wrong. This was a well-run and designed study by Dr. Weiss and colleagues to answer their questions. I don’t begrudge them for the trial they conducted.
We have to be careful, however, with how we interpret the results.
The interpretation should not be that “TRE doesn’t work.”
Instead, the interpretation is that 16 hours of TRE starting at 8 pm, with no control for diet quality and no reduction in calories, does not lead to weight loss or metabolic benefits for the majority of people.
Aside from that, there are still many questions and many potential benefits for TRE.
We have to be open to the idea that TRE may only be a means of calorie reduction with no other potential benefits. But that remains to be proven.
I will continue to recommend TRE, along with a high-quality, low-carb diet, as a means of reducing calories and lowering insulin, aiding weight loss, and improving metabolic health. I look forward to more studies examining this specific approach, and for now, I see no need to abandon this practice.
Thanks for reading,
Bret Scher, MD FACC
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59 comments
If you read the whole article,you'll see that Dr Sher wrote a more balanced and accurate
conclusion, according to the data in this study. There is no misrepresentation on his part.
Personally, I have been doing 23 hrs a day (OMAD) since 2012, along with a low carb diet and occasional 48 hour fasts. 16 hours a day was inadequate for weight loss for me too, in the beginning. I started losing weight significantly only after I touched 20 hours and more.
For myself, I do 16:8 routinely, eating between noon and 7 p.m. or so. When I feel the need to eat earlier, I use some of the food I would have eaten later, not extra; usually, this means I'll cook a couple of eggs and then not put eggs in my salad at noon. My weight is stable, a bit higher than I'd like, but within the normal range, at at my age (71), apparently I won't be as skinny as I was at 21.
In my personal experience for the fast to make any difference it needs to be at least 48hrs. I would like to welcome any study into this as the experts in the fasting field share different opinions regarding health benefits if any of a longer fast.
If I want to loose let say 5kg I eat one good meal every three days for as long as necessary. That may be for two three weeks.
To me, the Dr.Scher's criticism is not well founded, I am afraid. If you want to study whether the timing of the meals matter, which is after all what TRE fundamentally claims, evidently you try and keep as many other parameters the same, including the total calories -- otherwise you'll never know whether there's a causal relationship between the weight loss and the TRE (or one of the other factors, e.g. the calories). You simply have to reduce the number of variables. The whole point of TRE is the claim that it adds something beyond calorie reduction (like increased fat oxidation due to lowered insulin). If TRE on the same calories produces the same result as eating e.g. 5 times a day, then that is pretty good evidence that the timing of the calories does not matter. Personally, I do alright on TRE, but I am not a sufficient statistic and don't claim to be one.
I followed the link and read the study. Weight loss: TRE group lost a statistically significant amount and CMT group did not - over 12 weeks. Since there was no statistical significance between the the groups weight loss then the weight loss was irrelevant...? It was only 12 weeks - in general (sorry I do not have specific research links to give), previous research has shown that most diet strategies are effective in the short term. Where was the control group that matched the subject pool that made no changes to their eating? What would comparisons to a true control group have shown? Why did the researcher choose short term model vs long term and publish results at defined intervals to see what happens? What about using the methodology of using the subjects as their own control group 12 weeks following one plan then rest period and 12 weeks following the other plan? Regardless, my biggest issue with the study is the idea that IF/TRE is being viewed as a short term weight loss protocol. In ALL my 5 years of reading, studying, learning and following IF/TRE (anecdotally and with or without low carb), I have only seen it recommended as a way of life change long term not as a short term diet protocol; therefore, to me, the results may be reliable and valid but I do not view them as relevant. Of course, media write ups will be ... well - trash - and that is my heavily biased opinion on media write ups. Questions: Do fasting glucose/ fasting insulin tests show indications of increased or decreased insulin sensitivity? Is the statistically significant change in the diastolic pressure for the TRE group a positive or negative change? Thank you.
Honestly, letting people eat what they want doesn't sound like a very controlled study because it leaves too many variables open as to why each person ended with the result that they did. Just like Vitamin D and calcium need each other. Maybe IF is more impactful if done with a LCHF diet. Also, 12 weeks is not enough to determine the long term effects like liver function, and the likes.
I believe there is more studies to be done to rule it out or make a claim that it's not effective.
Thanks!
I know people who are firmly convinced that they are intermittent fasting while chowing down on doughnuts, cake, pie and all manner of high carb food for what seems to be 8 hours of non-stop eating.
I expect that if the designers of the study had implemented a controlled list of allowed foods in recommended quantities for both groups, we might have had a better analysis one way or the other.
Everything I've read about IF protocols assumes a keto/low carb diet. So is it the IF that people are having such success with? Or is it the keto/low carb that is the real money maker? Or is it a combination of both?
What if you don't want to be keto/low carb for the rest of your life? If you don't have a blood sugar issue or PCOS - why do it in the first place if all gains would be reversed once you began consuming carbs again?
If you just did IF and consumed a diet that included carbs, but didn't eat more than usual during eating periods, then it stands to reason that weight loss would happen just based on caloric deficit.
I have no answers, only questions.
As always Diet Doctor does a great job at pointing out the obvious problems with the study. No coaching on eating until full, no diet suggestions, stopping the window at 8 am, etc. If these folks were eating the SAD, then their glucose and insulin where not only spiking but probably remaining high well after 8 am. Unless they went to bed at 11 they didn't have the 3-4 hour window before bedtime. Also, eating SAD with 16 hours of fasting will probably not change your blood markers for the better. It bothers me that people continue to try to prove that low carb is dangerous or bad for people. The data (N=1,s) says otherwise. If people don't believe the results from thousands and thousand of folks then don't do low carb/keto and TRE.
You do you and I'll do me. Thanks as always.
I believe in it too and I'm very glad you've raised these thoughtful and incisive questions.
For me, I didn't know that I was doing intermittent fasting until someone told me that was what I was doing. I simply listened to my body and ate when I was hungry. When I did eat I try to stick to whole foods, consuming as little as was necessary for satiation. Turns out I was eating a keto diet without trying. Except for having a whole milk latte in the morning, I usually don't feel hungry until early afternoon. I usually eat two meals before 9pm and sometimes have a little nuts or cheese as a snack before midnight. I go to sleep around 1am.
Type 2 dietetic and fighting to be in remission.
Ultimately, we are all n=1's. If my experience is that IF works for me, then it works for me.
1. There was a fairly high drop-out rate in the study (about 25%).
2. The fasting window used in this study was noon-8 p.m. We're learning that earlier eating windows are likely more effective.
3. No food diaries were kept so we don't know how the fasting patients (or the control group for that matter) actually ate. It's possible that the two groups ultimately ate very similar diets. It's also possible that the fasting group "overate" during their eating window blunting the benefits of the fasting.
4. The fasting group appears to have been significantly less active than the control group. Not clear why. This lower activity may have accounted for the loss in lean mass and may also have blunted some of the benefits of the fasting.
5. The fasting group appears to have had worse sleep than the control group. We know from Matthew Walker's work that sleep has a big impact on fasting and the variables studied here. Thus, that could have impacted the results.
What can we conclude from this study? Without complete nutritional data, activity data, sleep data, etc., we can't make any conclusions. Further studies are needed.