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EventsNew 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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- P DIs it true that this study was funded my medifast, accompanied that makes prepackaged diet food and encourages a five or six meals a day?
- NadiaI have been doing IF 16:8 daily since August 10th of this year and I have lost about 6 pounds. What confuses me is how much I can eat.. the literature says, eat until full but what does that look like if I only eat 2 meals a day? I am at a standstill with my weight and I cannot seem to figure out HOW MUCH of my low carb vegetarian meals I can consume. The meal plans provided by Diet Doctor’s app does not tell us how much of the serving size we can eat during the feeding window. Thoughts?
- Peter ZielinskiI am fat adapted. I do not need to eat more than twice a day. Otherwise I would eat too much. Regardless of any study about IF.
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. - John NicklinThe study just proves that there is little or no difference between the two eating methods studied when subjects are allowed unrestricted and uncontrolled intake of virtually any food. That should not be a big surprise. It should also not be a big surprise that a certain group of people on social media would seize on such a study to 'definitively prove that intermittent fasting does not work and furthermore, is dangerous to your health.'
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.
- SueJoanne I’m interested to hear more about how & why you change your diet with your cycle. I battle terribly with water retention on a cyclical basis and it seems worse when I do TRE. I’ve done low carb16/8 or 18/6. I don’t know if I’m doing something wrong or TRE is not for my body, but putting on 2-3kg for most of the month then losing it in a day or 2 does my head in!! Any advice from anyone would be appreciated.
- AliAll I know is is doing a 16 hour fast with 8 hour eating window is the only way I can maintain my weight loss. I eat 800 calories a day in three small meals and I am still losing weight. Two meals a day makes my HBCA level go up. I eat low carb and healthy. Later next year I will have to increase the calories which my dietitian will show me how to do. Taken four years to shed the weight I have and still obese but 3 stone lighter and 6 and 1/2 stone lighter than in 2012. I wish I had known earlier about this diet. All metabolic indicators are better. I hope I can lose more weight another 4 stone to go....Sadly my insulin cells produce little had the creatine blood test done, I wonder if thats because my diet is restricted and less insulin is being produced. TRE works for me and I have nearly 4 years of blood works to prove it.
Type 2 dietetic and fighting to be in remission. - Mike RothfussTo lose weight on any diet you have to be focused on weight loss. You can't eat from the time you wake up until the time you go to bed and be your ideal weight. You can't then eat 5-6-7 meals within an 8 hour time period and be your ideal weight. If you eat in the 8 hour window the beauty of that is you "should be" eating fewer meals. If you also make those meals only nutrient rich foods you'll be even better. People not focused on losing weight won't do this. An example of this is when you see an overweight vegetarian. Doesn't make sense. Vegetarian doughnuts? Lastly another great benefit of fasting is autophagy -- https://www.dietdoctor.com/renew-body-fasting-autophagy -- it isn't just about weight loss.
- GillAlso an interesting theory on the benefits of allowing the digestive system a rest from food https://l.facebook.com/l.php?u=https%3A%2F%2Fthefatemperor.com%2Fep95...
- ReinholdReading "Breakfast is a dangerous meal" convinced me that only two meals a day is enough. I lost 1-2 stone in weight (fat, hopefully?) easily but now progress is much slower but I find compliance very easy still. It is part of my new life style and I am happy.
- LauraYou zealots are having to eat your words. There is nothing wrong with the study: it shows that there is nothing magical about intermittent fasting and that CALORIES, yes, CALORIES are still king, so that if time-restricted feeding "works" - no better than anything else that people might prefer - it's only or principally as a means of calorie reduction. On your recipes on this very website, you recommend NOT counting calories. You recommend huge amounts of fat in a single meal. Following these recommendations, many people DO eat more than they need or at least, aren't in a caloric deficit, and complain they aren't seeing any weight loss even though they "feel great" on keto. The message that these are just methods of reducing calories and not beneficial for weight loss in themselves should be made MUCH clearer by you and the zealots you promote. Jason Fung, the biggest and most deceitful promoter of fasting, supported by this website and promoted here, is notorious for his sneering comments at anyone who suggests calories matter. He'll go through mental gymnastics to insist you just have to eat "real food" (vague, vague, poorly defined) and just think about when you eat. That contradicts your claim that you guys always preach food quality (not accessible to most people by the way) and proper nutrition. Fung also contradicts himself, first claiming that overweight people can eat fat without worry because it doesn't spike insulin, but later claiming that overweight people cannot overeat fat either without putting on weight. Of course, he NEVER admits it could be about the extra calories in dense fatty foods. There is more than one way to lose weight, more than one intervention, and you zealots have NO proof that fasting and low-carb are better. Instead of fear-mongering and trying to force everyone into a lifestyle they hate and that won't be sustainable, be fair and reasonable in considering the mounting evidence that shows that your preferred diet doesn't have to be for everyone and is NO BETTER THAN ANYTHING ELSE.
- LauraJason Fung and Ken Berry, MD, both of whom you love to promote here, often mock fat doctors and ask why we should take advice from them. I can ask them the same. Why are you not lean, Jason Fung, if insulin is all that matters? Don't you fast? Why could you, Ken Berry, stand to lose more than a few pounds despite eating a strict carnivore diet? Why are several vocal low-carb advocates, including some who work for this site, still not at a lean body weight if health is all about keeping carbs low? Yeah, I'll take advice from people who have credentials other than "writer" and who have themselves become lean and fit through exercise and appropriate energy intake, not gorging on bacon and other fat while sitting on their ass claiming exercise is useless because it suits their indulgent and lazy tendencies.
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- Michael B.It's all interesting, all data to be considered. Re-evaluation based on evidence is a good thing. For myself, I find that IF supports lower calories. I usually have a coffee in the morning (with 1 Tbs heavy cream) and nothing until lunch. On those mornings when I gave into hunger and ate something before lunch, I found myself eating more the rest of the day as well – both at lunch, dinner and a large snack in-between. I do think that my diet (low-carb, decent amount of protein) does give me better satiety and makes it easier to last until lunch.
Ultimately, we are all n=1's. If my experience is that IF works for me, then it works for me.
- Sugar MouseVery interesting study and nice analysis by Dr. Scher. The key issues are:
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.
- SherylThe whole point of intermittent fasting is that you can eat whatever you want in a restricted period. Personally, this was a very well designed study. Dr. Weiss himself did 16:8 and was shocked at the results of his study. In a NY Times interview he said he now eats an early breakfast! I too just keto/low carb cycle now. No more dreaded eating windows. I have gone back to mindful eating and eating slowly. So many studies out there about how eating slowly naturally decreases calories, but they are now being ignored in favor of the fasting fad! Love Diet Doctor and will NEVER give up my keto/low carb cycling, but I'm done with eating windows. Long overdue. I hope Diet Doctor continues to sing the benefits of low carb eating, with fasting now taking a back seat.
- Ginger RoseI almost bought into the hype of the study until I went on Dr. Weiss’ twitter feed to get more info, kept reading...and I know this may sound weird, but his slamming, derogatory political prejudice against a party he doesn’t agree with, made me question more about him and his study. I have been dong 16:8 and low carbing for a while. It is life changing. Weight loss, better sleep, more energy, sharper mentally. I too read the NYTimes article, and also the almost 400 comments from people who relayed how much fasting has helped them. We need more studies. 16:8 works.
I lost 5 pounds in a week, just by cutting out the snacks. I was sold.
In stages, I pushed my breakfast to a later time, then made it into brunch once I started not really wanting lunch. Brunch is a substantial meal, usually about 10:30-11AM, and about 90% of the time, I don't have anything else until dinner at 5:30 PM. I'm done eating about 6PM. giving me a 16.5-hour fasting window. I've continued to lose weight; in fact, my "COVID 19" is weight lost, not gained (actually as of today it's 21 pounds gone since the lockdown in late March). I honestly don't think I'm eating substantially fewer calories, and my weight is definitely still coming down, and although slow compared to some, I'm peri-menopausal and have PCOS, so for me, it's a significant loss.
I do agree that we need to know what the patients on TRE were eating, and how often. I suspect that, like many of my friends who are trying it, they are snacking often or eating more meals within their "eating window." It's hard for people if they haven't already been eating low-carb, which is why I advise people who want to try it to go moderately low-carb first. Once you're not hungry all the time, it's much easier to do time-restricted eating without being tempted to eat at short intervals.