If you said to me, “Hey Wendy, what’s the question you get asked most often as a dietitian?” I would not hesitate. It’s “what do you think of intermittent fasting?”
Intermittent fasting is one of those topics that, after it gained popularity years ago, I thought would disappear. I didn’t quite see how the idea of not eating at all could continue to be appealing to people, but I underestimated the hunger for structure that humans have.
So after being asked over and over again for years about my thoughts on the topic, I figured I’d answer the burning question once and for all! With some research sprinkled on top.
Buckle up!
What is intermittent fasting?
Intermittent fasting is an eating protocol otherwise known as time-restricted eating. There are different fasting patterns or eating windows to choose from. Before it went mainstream, intermittent fasting really meant mixing entire fasting days (no food at all) in with feeding days (where you’d eat normally). Sound appealing to you? No?
Because that sounds awful, I’m guessing most people squeeze their eating into a daily time restricted feeding window – something like fasting for 16 hours and eating within 8 hours.
That being said, no research has shown one method (fasting/fed days vs. eating window days) to be superior in comparison so we can treat them the same.
How does intermittent fasting work?
The idea behind intermittent fasting is that your body will access stored forms of energy (hi, body fat) when no food is available and this will allow you to lose weight. This hasn’t been explained in science. Other explanations would be that skipping meals or reducing the number of hours you can eat during the day will naturally nudge you toward a calorie deficit. Which brings us to our next question:
Will you lose weight intermittent fasting?
You might. And you might not. If you aren’t tracking calories, then it’s impossible to know if you are eating at, above, or below your maintenance level and calories are king when it comes to weight loss.
Here are some research summaries: one study found that people who skipped breakfast did eat less calories per day than those who ate breakfast. They didn’t lose weight though because they were so tired their physical activity plummeted. So they weren’t in a deficit anymore. Of course you could keep your activity up and lose weight if you skipped a meal.
In contrast, there was a study that gave participants three meals a day or one meal a day. Both eating plans were at maintenance level, which means no weight loss was intended. But it turned out the group eating one meal did lose weight in the end. Another study also looked at maintenance calories for two goups (one time restricted and one not) with an added exercise protocol and found more fat loss in the time restricted group despite lack of a calorie deficit. This study relied on participants self-reporting their intake, though. Important, because there could be errors.
Another study did basically the same thing except made sure the participants were eating at maintenance, and ended up finding no difference between groups at the end. Hmm. Interesting!
One factor that can’t be studied is genetics. One of the reasons nutrition research is so hard to nail down is that people respond to food differently and have different body compositions decidedly wholly by genetics and not diet. So it’s fair to say that some people might do great on an intermittent fasting diet while others might see no results at all.
Who should try intermittent fasting?
If you want to lose weight and are struggling to maintain a calorie deficit on a normal day, it might be worth it to try implementing an intermittent fasting protocol. Think of it this way – if you sleep 7-9 hours a night then you’re already basically halfway there.
If you are asleep from 10 pm – 6 am, trying to reduce your eating window by stopping at 6 pm and starting the next day at 10 am might be doable because it’s really just 4 hours on either waking side without food. You can adjust that window to whatever works for you as long as it’s 8 continuous hours.
This might not work though if your schedule doesn’t make it easy. If you are up very early but typically don’t eat dinner until 7-8 pm, it might be difficult for you to wait several (not just a few) hours in the morning to eat.
Also take into consideration your exercise habits. If you want to wait until noon to eat but you work out at 6:30 am, I promise you it will be very difficult (and probably not so great for your muscle mass) to wait that long. You might be too hungry by the time your eating window kicks in and end up overeating.
If you work shifts or nights it also might be difficult depending on how that lines up with your sleep schedule. Especially if you don’t have the same schedule weekly.
My main point is – it’s completely possible to achieve weight loss or a better body composition without needing to stick to a fasting window. It’s just that for some, the structure of the window is hugely beneficial.
What are your thoughts on intermittent fasting? Have you tried it before and had success? Or tried it and hated it? Curious about it but haven’t tried it yet? I’d love to hear from you!