Will intermittent fasting help you lose weight?
Intermittent fasting has broken through the realm of fad diet into a permanent part of the zeitgeist. It is the number 1 Google diet search trend, it’s been written about in mainstream publications like the New York Times, and even made its way into the New England Journal of Medicine.
Intermittent fasting (IF) basically means that there are intermittent times when you aren’t consuming calories. IF can be a confusing term that means different things to different people since there isn’t a formal definition.
Let’s chat about IF and look at whether there’s real evidence it helps with weight loss.*
*scroll down to the heading on weight loss if you want to skip all the other stuff
Can you describe intermittent fasting in a bit more detail?
That’s a good place to start.
As I mentioned before, IF means that you choose to avoid eating for various periods of time.
Everyone technically has a period of intermittent fasting each night while sleeping (with few exceptions), but advocates of IF recommend extending this period of not-eating longer than just a night of sleep.
The most common way to practice IF is a 16:8 breakdown, where you don’t eat for 16 hours a day and consume all of your meals in an 8 hour window.
IF is used interchangeably with time restricted feeding (TRF). As commonly practiced, with a 16:8 window, TRF is a more accurate term than IF, since it almost certainly takes much more than 16 hours to get the true biologic benefit of a fast (more on this later).
But I don’t think that we need to spend too much time on the nomenclature here, especially since different people will use different definitions. The take home point is that for most people IF = TRF = a compressed window of eating with a longer window of not eating each day.
What are the benefits of IF?
It depends who you ask here.
If you start reading about IF, you’ll encounter a lot of discussion about the way that intermittent fasting impacts your biology. You’ll read about things like autophagy, oxidative stress, and the mammalian target of rapamycin (spoiler alert: you’ll get bored quickly if you Google these terms).
Proponents of IF tend to argue that there’s something special about the restricted eating window that leads to weight loss, metabolic benefits apart from weight loss, and disease prevention.
That’s certainly what the New England Journal of Medicine review would have you believe.
But after actually digging into the research cited here, I come away with the conclusion that these benefits - including weight loss - are exaggerated by IF proponents.
Why do you say they’re exaggerated?
Let’s ignore weight loss here for a moment, since we’ll address it later on.
You’ll see a lot of articles about IF activating autophagy, which is how the other benefits of IF are supposed to accrue.
Autophagy is the biological housekeeping process of your cells. Dysfunctional autophagy increases risk of Alzheimer’s and cancer. Turning on autophagy may protect against the chronic diseases of aging.
Unfortunately, I think that the benefits here are exaggerated because 16 hours isn’t enough time to turn on the protective biological mechanisms that are supposed to occur.
The data regarding intermittent fasting turning on autophagy are extrapolations from studies that don’t actually test intermittent fasting in humans, but instead assume that animal research and non-IF human studies can make our case.
Sure, you may see these benefits in mice who are fed every other day, but alternate day feeding for a mouse is like a human fasting for a week. And just because you see benefits from alternate day calorie restriction, you can’t say that means anything about the impact of intermittent fasting.
So if someone tells you that IF turns on autophagy and thus protects you from disease, that person is some combination of exaggerating, extrapolating, assuming, and/or lying.
Ok, enough about the research. Does IF help with weight loss?
This is probably all anyone cares about here.
There’s certainly a lot of anecdotal evidence here - I’ve heard from patients and read accounts of people who have effortlessly lost weight by eating for a shorter period of time each day.
We have one randomized clinical trial - the TREAT trial - that tried to answer this question.
The TREAT trial divided people up into two groups, one group that was instructed to eat 3 meals a day, and one group that was told to eat only from noon to 8pm. Neither group was given specific instructions on what to eat or how much to eat.
“The intervention only included recommendations to the timing of food intake (no recommendation for calorie and macronutrient intake or physical activity), and participants received daily reminders about their eating windows through the app.”
No restrictions on snacking.
No restrictions on calories.
No restrictions on dietary choices.
What do you think they found?
At the end of a 12 week trial, there was no significant difference in weight between the groups. So the conclusion is that intermittent fasting doesn’t “work” for weight loss.
But there was a huge amount of variability among participants in the groups. Look at these graphs below. Each bar represents a person in the study. The vertical axis is percent weight change. CMT is the 3 meals a day group and TRE is the time restricted eating group. Look at the variation from person to person:
There’s a huge amount of variation from person to person when it came to responses to a diet change.
Some people gained weight with a restricted eating window. Some people lost weight. On average, there was no different between the groups.
It’s worth noting that the intermittent fasting group actually lost some muscle mass compared to the control group, so you could argue that intermittent fasting may have some risks associated with it.
What’s the bottom line?
Just because intermittent fasting didn’t lead to weight loss in the one, small, randomized trial doesn’t mean that it can’t work for you.
I look at the tremendous individual variability in that trial as an echo of my clinical experiences, and what I’ve written about in this newsletter before: you need to self experiment to see what works for you.
The other important take home is not to believe the miraculous promises from IF proponents as being other than an extrapolation or exaggeration of the scientific reality as we understand it.
If you’re interested in learning more about the TREAT trial, take a look at this Twitter thread from Ethan Weiss, the principle investigator in the trial:
Thank you for reading! If you’re enjoying my newsletter, please consider sharing with your friends and family and encouraging them to subscribe!
I always appreciate any feedback or thoughts you might have. You can reply directly to this email to reach me directly.