Intermittent fasting has become the buzzy diet of the moment among celebrities, and plenty swear by it. Jenna Jameson loves it, Vanessa Hudgens says it makes her feel “healthier,” and Halle Berry says she usually eats just two meals a day on her intermittent fasting diet.
But intermittent fasting isn’t for everyone. After all, it involves fasting (yup, going without food) for a set period of time. Clearly, that’s not going to feel good for everyone.
While you probably get the gist of fasting, you might be a little fuzzy on the details of what it means to be on an intermittent no-eating plan. Here’s how it works, plus what you can actually eat when you’re on it.
So…what is intermittent fasting, exactly?
Intermittent fasting centers around a pattern of eating and fasting periods, that is, times when you don’t eat. “Intermittent fasting is when you allow yourself to eat only during a specified window of time each day,” explains Alissa Rumsey, RD, a NYC-based dietitian and owner of Alissa Rumsey Nutrition and Wellness. It usually involves fasting for a certain number of hours or even days that are spaced out during the week, and there’s no “right” way to do this, says Sonya Angelone, RD, a spokeswoman for the Academy of Nutrition and Dietetics.
“There are so many different kinds of fasting now,” Rumsey says. The 5:2 diet is one. On this diet, women eat less than 500 calories (for men, it’s less than 600) for two non-consecutive days a week. So, you might have a 500 calorie day on Tuesday and Thursday, and then eat normally on Monday, Wednesday, Friday, Saturday, and Sunday. With the 5:2 diet, you don’t need to eat certain foods—you just limit how much you eat during certain periods of time.
Another popular type of intermittent fasting is the 16:8 diet. With this diet, you only eat during eight hours of the day. You can technically choose when you want your eight-hour period to be, but most people tend to stop eating at a certain time in the evening, like 6 p.m., and then wait to eat again until 16 hours later (in this case, it would be at 10 a.m.). That way, you’re sleeping for part of your fast instead of sitting around, thinking about the food you’re not eating for a good part of the day.
There’s also something known as the 24-hour fast (or Complete Alternate Day Fasting). “That requires fasting from all food for 24 hours once or twice per week,” Angelone explains.
BTW: You’re not expected to survive on just air during fasting periods. People usually will have water, tea, coffee, and other very low calorie or non-caloric drinks during this time.
Why did intermittent fasting diets become so popular?
Back in 2012, BBC journalist Michael Mosley released a TV documentary called Eat Fast, Live Longer, followed by a book, The Fast Diet, which brought the idea of intermittent fasting to the cultural forefront. In 2013, journalist Kate Harrison published her book The 5:2 Diet and in 2016, Jason Fung, MD, had a bestseller with The Obesity Code, a book that spelled out how to use intermittent fasting to combat insulin resistance and reach a healthy weight.
Along with the books came celebrity endorsements of the diet, and that certainly hasn’t hurt the intermittent fasting trend, either. (I mean, have you seen Halle Berry?) “Many influential people have talked about their experience with intermittent fasting so it has gotten quite a bit of spotlight and exposure on social media platforms,” Rumsey says.
People also like intermittent fasting because it doesn’t take a lot of thought. “It doesn’t require counting calories, macros, or measuring ketones,” Angelone points out. “You can eat most anything you want between a specific window of time, although most programs recommend eating healthfully when you do eat.”
Can you lose weight with intermittent fasting?
Research has found a link between intermittent fasting and weight loss, but there isn’t much research to prove that intermittent fasting is a better weight loss method than other diets. For example: a meta-analysis published in the JBI Database of Systematic Reviews and Implementation Reports in 2018 found that intermittent fasting had similar weight loss results to a traditional calorie-restricted diet.
In some cases, even when intermittent fasting did lead to weight loss, it wasn’t the most sustainable diet. One randomized clinical trial of 100 metabolically healthy, obese adults (published in JAMA Internal Medicine in 2017) found that those who fasted every other day for a year had only slightly more weight loss than those who ate a restricted-calorie diet on a daily basis. The difference in weight loss was so slight that it wasn’t considered clinically significant, and 38 percent of people in the alternate-day fasting group had trouble sticking with the diet. Whomp whomp.
Basically, research has found that you can lose weight on an intermittent fasting diet, but you can also lose weight by watching what you eat. It’s also worth pointing out that weight loss due to intermittent fasting isn’t guaranteed to last. “Some people may experience weight loss in the short-term, but many people eventually gain that weight back,” Rumsey says.
And, most intermittent fasting research has only been done on people who are obese. “There is almost no credible research that shows intermittent fasting is good for people at a healthy weight,” says Scott Keatley, RD, of Keatley Medical Nutrition Therapy. If you’re curious about intermittent fasting but don’t have much weight to lose, it might not be the best diet for you.
Are there any long-term health benefits of intermittent fasting?
There is a growing body of research that suggests that intermittent fasting does have health benefits outside of weight loss. One study published in Cell Metabolism in 2018 linked intermittent fasting and lower insulin levels and blood pressure. Researchers followed a small group of obese men with prediabetes—some were put on a 16:8 intermittent fasting diet, while others ate over a period of 12 hours. Both groups didn’t gain or lose weight. But after five weeks, the men in the 16:8 group had much lower insulin levels and improved insulin sensitivity. They also significantly lowered their blood pressure and said they had decreased appetites. They weren’t as hungry as they were before—even though they were fasting.
It might seem counterintuitive, but appetite control is a big benefit of fasting. A recent study in the journal Obesity showed that people who ate only during a six-hour window, compared to following a normal eating schedule, felt less hungry than the control group, even though both groups ate the same amount of calories. Intermittent fasting has also been linked to increased neuroplasticity, or the brain’s ability to form new synaptic connections and fight injury.
Unfortunately, there hasn’t been a ton of research on humans that follows the effects of intermittent fasting over the long term. While some studies have followed participants for a year, that’s about as long as most go. It’s a slightly different story when it comes to rats. “In rodents, intermittent fasting has been shown to prevent age-related diseases including tumors, heart disease, diabetes, dementia, and even extend lifespan,” says Keatley. But, he adds, “there is a big difference between rats and humans and the research does not show these benefits enough to actually recommend it as the fountain of youth.”
Will I have to avoid any foods on an intermittent fasting diet?
This is a huge perk of intermittent fasting: You technically don’t have to alter what you eat—you just have to eat within a certain window of time.
That said, plenty of people have paired intermittent fasting with another type of diet, like keto. (Jenna, Halle, and Vanessa have all said that they do this.) Again, it’s not a requirement, but doing intermittent fasting while also altering your diet could help you get results faster.
Does intermittent fasting have any side effects?
There are a few things that can happen when you follow an intermittent fasting diet. One thing to note is that fasting can interfere with your hunger cues. “Since people are only allowing themselves to eat in a certain window of time, they are completely ignoring their internal cues of hunger,” Rumsey says. “It causes people to disregard hunger cues, which then means once they are ‘allowed’ to eat, they are starving and it can be hard to stop eating.” This could lead to an unhealthy obsession with food for some people, Rumsey says.
Some fasters find that the diet makes them feel sharper and more alert, but alternately, fasting for long periods can also lead to mental fogginess, disrupted sleep, and decreased alertness, Rumsey says. And, she adds, fasting for extended periods of time “takes a large toll on your blood sugar levels,” causing you to flip-flop between low blood sugar and a spike when you eat again.
Should I try intermittent fasting?
It really depends. If you feel like you have a healthy relationship with food and you do well with parameters, it could be a good fit for you. It’s certainly not an easy diet if you hate the feeling of being hungry, but the fasting periods do get easier with time, Angelone says.
While anyone can try intermittent fasting, ultimately, people who are overweight and/or have high blood pressure or high cholesterol (but are otherwise healthy) are considered the best fits for the diet, Angelone says.
How do I start?
There’s really no magic plan to follow with this, but you could try to figure out a form of intermittent fasting that makes the most sense for you. The 5:2 diet can be tricky, given that it involves calorie restriction for an entire day at a time, so it might be better to start with something like the 16:8 diet and progress from there if you feel good.
You also want to define what you’re hoping to get out of intermittent fasting, and it doesn’t hurt to check in with an expert, like your doctor or a dietician, before you dive in. “Work with a professional who understands your goals and can help you determine what you need to eat for a diet like this to achieve,” says Keatley. Then, he recommends shooting for “small changes and small gains as these are most likely to do the least harm and provide the most long-term benefits.”
Above all, listen to your body. If you feel like you’re hungry all the time and are kind of miserable on an intermittent fasting diet, it probably isn’t the right fit for you. If you love the simplicity and you’re seeing the weight loss or health results you want, fast away.
Korin Miller is a freelance writer specializing in general wellness, sexual health and relationships, and lifestyle trends, with work appearing in Men’s Health, Women’s Health, Self, Glamour, and more.