It’s no surprise that intermittent fasting is one of the most popular types of eating plans. You don’t need to measure out food or buy any prepackaged shakes. There are no required weigh-ins or calorie counting. All you really have to do is not eat during certain hours. It’s pretty simple.
There are different ways to go about it, of course. Most people do the 16:8 diet, in which you fast for 16 hours and then eat within an eight-hour window. There’s also the 5:2 diet, where you drastically cut back on calories just two days a week, and there are 24-hour fasts, where you don’t eat anything one day each month.
Regardless of the method, significantly restricting when you eat can throw your body for a loop and cause a handful of odd side effects. Intermittent fasting may not be suitable for everyone. (People with a history of disordered eating, for example, should definitely avoid it.)
It’s important to know what to expect before you jump into any new eating habit. Here’s what happens to you — mentally, physically and emotionally — when you’re fasting intermittently.
You might lose weight.
Many health experts, including personal trainer Jillian Michaels, say that intermittent fasting actually isn’t that great for weight loss. That’s because you’re not necessarily eating less or cutting back on calories. There are just longer gaps in your day when you’re not eating at all.
That said, many people do lose weight because they consume fewer calories during those restricted food hours.
Eating for only eight hours a day also makes it less likely that you’re having a big meal right before bedtime. Our metabolism goes down when we sleep and we burn fewer calories. Nighttime eating has been linked to both obesity and diabetes.
Intermittent fasting “really does keep you from doing some really bad things, which is to eat a big meal before you go to bed,” said Dr. John Morton, a bariatric surgeon with Yale Medicine. Big meals before bed are “probably the worst thing you can do when it comes to weight loss,” he added.
You could get super hungry.
A lot of people who fast experience hunger pangs, mainly when they start the program. That’s because our bodies are accustomed to using glucose — a sugar that comes from the food we eat — for fuel throughout the day. When it’s deprived of food (and, therefore, glucose), the body will essentially send signals saying, “Hello, aren’t you forgetting something here?”
Once your body gets into the groove of fasting, it will start burning stored body fat for energy rather than glucose. And as you spend more time in a fasted state, your body will get increasingly efficient at burning fat for energy.
In short, those hunger pangs should dissipate and your appetite will level out, Morton said. He added that fasters will ultimately have fewer cravings and hunger pangs the more consistently they fast.
In the meantime, that hungry feeling may drive some people to overeat. “The natural tendency is when you haven’t eaten breakfast, you go, ‘Since I didn’t eat breakfast, I’m going to eat more [for lunch],’” Morton noted.
If the hunger pains are bad enough to interfere with your daily life, get something to eat. The idea is not to starve yourself.
Your energy levels and moods will fluctuate.
Research has shown that fasting can cause some people to feel fatigued, dizzy, irritable and depressed.
“In the beginning, your energy levels might be low because you’re not getting the proper nutrients that you need,” said Sharon Zarabi, a registered dietitian and bariatric program director at Lenox Hill Hospital in New York.
As your body gets used to intermittent fasting, your energy levels will pick back up. “Your body becomes more efficient at using energy and this helps improve mood, mental ability and long-term performance,” Zarabi said.
There’s even some evidence that suggests intermittent fasting can ultimately help fight depression and anxiety. The body releases a hormone called ghrelin when you’re hungry or fasting, which — in high amounts — has been associated with an elevated mood.
Your gut health may improve.
Many people who partake in intermittent fasting note improved gut health. Fasting gives your gut a chance to rest and reset as your digestive system doesn’t have to deal with uncomfortable effects of eating like gas, diarrhea and bloating.
“Anytime you fast, you’re giving your body a break from trying to metabolize what you just ate,” Zarabi said. “By fasting, we let the gut microbiome refresh, which in turn improves our overall digestive pathway.”
You could cut your risk for chronic diseases.
Intermittent fasting has been linked to a lower risk of chronic diseases like diabetes and cardiovascular disease.
According to recent research from Mount Sinai, this is because fasting reduces inflammation ― and reducing inflammation helps our bodies battle various chronic inflammatory diseases like diabetes, heart disease, cancer and inflammatory bowel diseases. Researchers are still working to figure out how and why this happens, but the evidence so far suggests that the fasting body produces fewer of the subset of monocytes, a kind of blood cell, that are known to damage tissue and trigger inflammation.
This is a big reason why people who fast intermittently may live longer and stay healthier.
Your heart health could improve.
Intermittent fasting can help lower your blood pressure, cholesterol and triglycerides — the type of fat in our blood that’s associated with heart disease. That is, if you lose weight in the process.
“As long as you’re losing weight, you’re going to improve all those things,” Morton said.
Before you start an intermittent fasting program, health experts recommend meeting with a dietitian or physician. There’s a critical distinction between fasting and starving, and if you ignore that, you could wreck your organs and immune system.
The bottom line: pay attention to your body and eat in a way that works best for you.