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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