A dietitian weighs in on the trendy eating approach, plus the common mistakes people make.

By Brierley Horton
April 02, 2019

The concept of intermittent fasting (commonly referred to as “IF”) seems fairly straightforward. You abstain from eating in intervals—somewhere between 16 or 20 hours a day—or heavily restrict your intake and eat a very low-calorie diet a couple days a week. There are also some IF followers who eat just one meal a day (also called OMAD).

There’s quite a bit of research proving that IF works for weight loss—and improves things like blood sugar control and cholesterol, which are markers for chronic diseases. Some studies have even found that IF may boost people’s energy and help them sleep better.

I’ve been a dietitian for nearly 15 years, so I’ve read my share of research on IF, I’ve written about it a handful of times, and I’ve even tried intermittent fasting out for myself. Here’s my take: it works really well for some people regardless of what “type” of IF they follow, but one common denominator is that plenty of folks out there are doing it wrong.

There happens to be a lot of misinformation floating around the internet (both about intermittent fasting and dieting as a whole category), so I culled a list of some of the top comments and questions on Reddit’s Intermittent Fasting thread and answered them the best I could. Here are some common questions I found about IF, plus some common mistakes to avoid when trying it.

Mistake #1: You’re Not Sure What Counts

“So my wife and I are trying to start on the IF journey (16:8). It’s harder than we thought. … How can she do coffee (other than straight black) in the mornings without throwing off the benefits of the fast? She doesn’t care for black coffee, but the chokehold of caffeine has her in its stimulant grip, and I don’t want to die in that ditch today.”

I, too, live and die by that a.m. cup of coffee. But this person is overthinking it. The best thing to do is simplify your routine. Have your morning cup of coffee exactly as you like it and break your fast then. For example, if you’re doing 16:8 (where you fast for 16 hours and eat during an 8-hour window), have your coffee at 8 a.m. and your last meal at 4 p.m.

Mistake #2: You’re “Eating” Without Realizing It

"I'm trying to cut myself down to one meal a day, so while I'm not actively trying to do IF, I think I'm kind of doing it by proxy (definitely been doing something akin to 16:8 lately). But does this mean no calorie intake whatsoever outside of the time I normally eat? Or just no actual food? Because I have coffee every morning and I've also been drinking a cup of soy or almond milk for "breakfast” and I don't know if it's going to mess with my progress."

OK, this is intermittent fasting. Period. Even though this person says he or she isn’t “actively” doing IF, they are. There’s nothing “sinful” about IF, so no need to deny it. But to reap the benefits, you can’t have that quote-unquote breakfast. Fasting means no calories (which also means no food) during that fasting time period.

More on intermittent fasting:

Mistake #3: You’re Eating Too Many Calories

"Does anyone else eat like crazy right when the fast is over and is it normal to have a huge appetite during the feeding period? It is hard for me to get full after a 20 hour fast and I just eat the whole 4 hours. LOL."

I’d venture to guess that this person is eating more calories than what’s needed in that 4-hour window. So instead of laughing your way through a marathon all-you-can-eat session, plan for how you’ll break your fast. Stock up on high-protein foods (like meats and seafood) and/or high-fiber foods (like fruits, vegetables, beans, and most whole grains). They’ll not only fill you up, but will keep you feeling full.

"If I do a 20:4 fast then I should consume 1500-2000 calories within four hours? Am I understanding this correctly?"

Technically, yes. But depending on a person’s body size, eating 2,000 calories in a 4-hour window might not yield any weight loss. I don’t know this person’s size, though. Now, most people lose weight on 1,500 calories, but one of the pros of IF is that it’s hard to eat a ton of calories in a short window of time. For some, following IF is an easier way to cut calories and lose weight than simply following a traditional calorie-restricted diet. So if you can’t hit the 1,500- or 2,000-calorie mark in 4 hours every day, it’s OK. If falling below 1,200 calories a day becomes a regular habit, though, reconsider your diet plan. If you’re not sure how many calories you’re consuming, track them in a free app like MyFitnessPal.   

Mistake #4: You’re Overanalyzing

"Does IF mean no food outside meal times, or no calories?"

Um, they are one in the same, no? Does anyone reading this article know of foods that have zero calories? If so, please share! This person’s assumption is correct, though—no food and no calories outside of the “feeding window.”

“Will eating ______ break my fast?”

Does anyone else feel like this question comes up about a thousand times a day? Short answer: Yes. Eating anything with calories breaks your fast. Exceptions to this rule would be black coffee, unsweetened and milk-free tea, water, and diet soda (though research says diet soda could actually increase your appetite, which might make it hard to stick to your fast.)

Mistake #5: You’re Pushing Yourself Too Hard

“I’ve been doing IF almost 2 months, mostly OMAD, sometimes 48/72 hours extended fasts. The last 3 or 4 days whenever I break my fast I feel a great regret. I always feel like I could push the fast a little longer. What should I do?”

Extending a fast doesn’t supercharge the powers of IF. If this sounds familiar to you, please find yourself a counselor who specializes in eating disorders. I’m not saying you, or this person here, has an eating disorder, but food should not induce feelings of remorse or regret. Left untreated, this could develop into a larger problem. And also, huge kudos to this person for so bravely speaking up and sharing their food feelings!

The Bottom Line

When done correctly, intermittent fasting can be a great tool to help you lose weight. If you’re curious about how it works, check out our beginner's guide to IF here. And, as always, talk to your doctor before trying anything new.

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