Is Intermittent Fasting Healthy?

As a fitness expert for decades, it’s no surprise we aren’t into fad diets or crazy concoctions that claim they’ll help you lose weight. I recommend the 80/20 philosophy—aim to eat right and make good choices about 80% of the time, but leave 20% room for indulgences and your favorite treats. So when we heard that people have been trying intermittent fasting as a way to lose weight, we were curious. Does this actually work? Is it safe? We researched whether or not intermittent fasting is healthy so we can tell you our honest opinion.

What Is Intermittent Fasting?

The concept of fasting is nothing new—people have been fasting for religious reasons for centuries. Fasting itself simply means to go a stretch of time without eating. We all naturally fast when we sleep every night. Intermittent fasting does not require you to go more than 24 hours without eating. Instead, you have a few options:

• Fasting Window. You can choose a fasting window of which to not eat during the day, typically around 16 hours. Because you spend roughly 8 hours per night asleep (naturally fasting,) you’re then only required to tack on an additional 8 hours of fasting during the day. Many people choose to fast 4 hours before they go to bed, and 4 hours after they wake up—skipping breakfast. You can still drink water, coffee, and tea during your fasting hours.
• Full 24-Hour Fast. You can do a full 24 hour fast once or twice a week, so from 7 pm Friday to 7pm Saturday, for example. You can once again still drink water, tea, or coffee during your fasting period. You return to normal eating after the fast is over, meaning you aim to consume the same amount of calories you normally would, not more, after you’re done fasting.

Related: The Science of Skipping Breakfast

Why Do People Do Intermittent Fasting?

From our research, we read many accounts of people who have tried intermittent fasting for different reasons, both relating to physical and mental well-being. But here are the three biggest reasons for intermittent fasting that we’ve come across.

1) A Belief That It Burns More Fat

Some people believe that by fasting intermittently, you are able to reset your digestion, helping your body burn fat more effectively and ultimately lose weight. It takes about 6-8 hours to digest a meal. Some people think that by fasting for that amount of time, you then force your body to use stored fat—and not your food—as energy. The belief is that this burns up the fat instead of the food.

2) To Create a Calorie Deficit

By eliminating one meal out of your day, it’s suggested that you’ll create a calorie deficit. While it’s true that you need a calorie deficit (fewer calories going in than the amount of calories burned) in order to lose weight, if you are too hungry from fasting you can overeat at your next meals and end up eating just as many calories—if not more—than you would if you ate throughout the day.

3) A Desire To Retrain Your Appetite

Proponents of intermittent fasting will tell you that by doing this, you train your brain to only eat when you’re truly hungry, not by arbitrary times of the day (breakfast, lunch, and dinner). Some people claim that this can help people who tend to over-eat or binge-eat better identify when they are actually hungry. However, as noted above and as we’ll discuss in further detail below, fasting can actually cause “rebound overeating” in some cases.

Negative Side Effects of Intermittent Fasting 

• Rebound Overeating

  This is perhaps one of the biggest drawbacks to intermittent fasting: it makes you overly hungry so you end up overeating and consuming more calories than you normally would at your next meal.

• Mood swings

  Some people are particularly sensitive to drops in blood sugar. (Have you ever heard the term “hangry?”) Intermittent fasting can cause anxiety, irritability, and mood swings.

• Poor sleep

  Going to bed when your stomach’s growling is never a good idea. Being hungry can wake you up in the middle of the night and keep you from getting a solid night’s sleep.

• Weakness or fatigue

  It can be harder to work out when you feel hungry or unsatiated, and you can experience muscle weakness, headaches, or just overall fatigue.

• Adrenal stress

  You can place undue stress on your adrenals by making your blood sugar spike and drop so significantly.

• Lack of nutritional guidance

  Fasting doesn’t require that you eat certain foods during the time you’re actually eating. Because of this, you can technically eat whatever you please when you’re not fasting…and that lack of direction won’t help someone lose weight who consumes unhealthy foods for the rest of their day.

What We Think About Intermittent Fasting

There is no one-size fits all approach to nutrition. Our bodies have different needs. Some of us are sensitive to dairy or gluten, while others may be deficient in things like iron or magnesium. Some people are comfortable going long stretches without eating, while others feel the effects of low blood sugar a few hours after a meal. Finding the nutritional strategy that works for you is all about identifying what makes your body feel good and operate most efficiently.

As a trainer and health coach, however, the concept of fasting does not make me think “fat loss.” What proponents of intermittent fasting might not realize is that fasting doesn’t cause your body to break down only fat. When you don’t consume carbohydrates after about six hours, your body will begin to convert some lean tissue (meaning muscle) into carbohydrate as well. Your metabolism needs fuel in order to work efficiently. When you starve your body, your metabolism shuts down to conserve energy. In some fasting cases, you’ll burn less fat than if you actually fueled your body with healthy food regularly, because glucose must be present for body fat to burn.

Here’s what I suggest for people to stay healthy and burn fat:

• Eat a healthy diet of sensibly portioned meals comprised of as many plant-based foods as possible. Fill up on veggies, fruits, nuts, seeds and healthy fats. Add in lean protein, whole grains and dairy in moderation if you like it.
• Incorporate both cardio and strength training into your workout routine. Strength training builds muscle, and muscle is a more metabolic tissue than body fat. This means that the more muscle you have, the more calories you’ll burn daily, even at rest.

This is what we want for both fat burn and long-term health: a metabolism that runs efficiently—not one that’s constantly going into starvation mode. Have you tried intermittent fasting? What works best for you? Let us know in the comments below!