If you even passively follow wellness influencers, there’s a good chance you’ve heard of intermittent fasting (IF). The concept is simple: During certain hours of the day, you eat. During others, you don’t. The buzzed-about benefits include weight loss, lowered disease risk, more energy, and even slower aging.
But does intermittent fasting actually work for losing weight and improving your health? And even if it works — since when is not eating a good idea? Here’s what you need to know about this trendy eating style including benefits, drawbacks, and what to keep in mind before you try it.
Intermittent fasting’s rise to popularity
Though it came into the spotlight more recently, people have been practicing intermittent fasting much longer (think about it: you technically fast when you’re asleep). “A British journalist produced a documentary about IF in 2012, and shortly after that, wrote a book about it,” explains Katherine Basbaum, an R.D. with UVA Health. “IF made its way to the U.S., and then more or less snowballed with more media coverage, books, and celebrity endorsements.”
Once it became popularized, word of mouth helped IF spread. “In my practice, I see people participating in intermittent fasting as a result of family, friends, or colleagues who have successfully lost weight, as well as social media influencers who promote this approach, particularly within the fitness industry or at a local gym,” says Maya Bach, R.D.N. It also doesn’t hurt that celebrities sing its praises, Jennifer Aniston and Halle Berry among them.
And while experts don’t necessarily think IF’s popularity is a bad thing, they do agree it’s important to understand the pros and cons before diving in.
Intermittent fasting benefits
IF is often touted for its weight loss benefits — and those are real. The reason it works is surprisingly simple: Some people find it easier to eat less when they have fewer hours to eat during the day. “With only a short window to eat, you will probably eat a meal and feel full until the next time to eat,” explains Natalie Rizzo, R.D. “That means you won’t mindlessly graze on foods you don’t need.”
But importantly, the reason it’s effective for weight loss is not much different from the main reason why other diets help you lose weight. “IF creates a calorie deficit,” Basbaum says. In other words, IF helps people eat fewer calories than they’re burning, which results in weight loss. The only catch? “It’s only really and truly effective for the individual that can stick to it long-term,” Busbaum explains. “If not, then it’s the wrong diet for them and the weight will not stay off.”
Overall, doctors and dietitians emphasize that the research on IF is still relatively early. Still, there are some health conditions it seems to help with. Some doctors who treat type 2 diabetes use IF as one of their primary interventions, most notably Jason Fung, M.D., author of The Obesity Code. While his views are controversial, Dr. Fung believes that fasting (and reducing refined carbohydrates) can help type 2 diabetes patients with insulin management and weight loss. But as for whether IF would work better for type 2 diabetes than any other weight loss approach, the evidence is scant.
Advocates of IF also often cite studies suggesting that fasting and/or caloric restriction may help people live longer, improve heart health, reduce cancer risk, and even encourage a healthier microbiome. By giving your digestive system a break, it could also lead to better digestion, particularly for those with existing digestive issues, some say.
The only problem? Most of the research on these topics has been done in animals. “Animal models of fasting suggest that it may have a role in healthy aging and reducing cardiovascular disease and cancer risk,” says Nate Favini, M.D., medical lead at Forward. “I suspect this will be true in humans too, but it’s too soon to say.”
Because of the lack of human research, many experts are still skeptical as to whether these benefits are real, and if they are, to what extent. “This is not to say that positive results from animal studies are insignificant, just that it is too soon to confidently extrapolate those positive effects to humans,” Basbaum notes.
Another positive: IF can be combined with any eating style. Whether you’re Paleo, vegetarian, or follow the Mediterranean diet, you can easily do both. Followers of the keto diet in particular tend to favor intermittent fasting, as they believe it may help them reach ketosis, a state of burning fat for fuel, faster. Fasting is known to switch the body’s fuel source from carbohydrates to fat, although it takes between 10 and 14 hours for this to happen, depending on the individual.
Lastly, some people simply feel better when making IF part of their daily routine. “Anecdotally, many people tell me that fasting improves their energy and focus,” Dr. Favini says. “I’ve personally noted improvements in my energy when I’m fasting and a deepened sense of appreciation and connectedness to the world around me.”
The drawbacks to consider
First and foremost, the long-term benefits of intermittent fasting have not been evaluated, points out Stella Volpe, Ph.D., an R.D. and chair of the department of nutrition sciences at Drexel University. While IF appears to be effective for weight loss, there isn’t high-quality research available that goes beyond one-year outcomes. “Studies that go beyond a year are needed to ascertain if intermittent fasting can help maintain weight loss,” Dr. Volpe adds.
What’s more, IF may not have the special “edge” for weight loss that some advocates claim it does. “Some research also has shown that intermittent fasting and a generally lower caloric intake both led to the same amount of weight loss, and individuals tended to not feel as hungry compared to intermittent fasting,” Dr. Volpe says. If you’ve ever dieted, you know that feeling hungry can make it harder to stick with your program.
What’s more, some people find themselves overeating during their eating hours, especially after dealing with cravings during their fasting hours, which could cancel out any potential weight loss, experts say. “You can definitely eat more calories than you need in an eight-hour window if you don’t choose the right foods,” Rizzo explains.
IF isn’t for everyone
It’s also important to note that IF isn’t suitable for everyone, especially people with certain health issues. “This includes people who need food intake for medication, people who are underweight, people with conditions that make them prone to electrolyte abnormalities (such as kidney disease), or people who are pregnant, breastfeeding, or trying to get pregnant,” Dr. Favini says. And while intermittent fasting may be helpful to those trying to manage type 2 diabetes, those with type 1 diabetes should avoid intermittent fasting because there is risk of very low glucose levels.
Dr. Favini also stresses that intermittent fasting can be dangerous for people with eating disorders or a history of eating disorders. “In general, I’d be very cautious about this approach if that’s something you’ve struggled with in the past.”
Lastly, it can be challenging socially. “For those who enjoy a lot of socializing with friends or family, an IF regimen may be challenging,” Basbaum says. “It may require having to choose between skipping outings that involve food if they take place during a fasting period, or going to the event and not eating while everyone else is.” To be fair, other diets that restrict what and when you eat may have similar effects, but that’s why so many nutrition pros advocate for an inclusive approach to eating.
How to try intermittent fasting
For what it’s worth, nearly every expert in this article recommends speaking to a doctor or dietitian before trying IF. They’ll evaluate your medical history, current dietary habits, and goals to determine how well IF might work for you. Most people skip this step, but it can help you understand if IF really is a good fit, or if you might see results from making different types of changes instead.
If you do decide that IF is right for you, here’s what to keep in mind as you get started.
Consider what other changes you could make first.
Losing weight or eating healthier doesn’t necessarily have to be extreme. “I always recommend people first evaluate their diet, making healthier swaps and changes, before they implement something like intermittent fasting,” says Samantha Presicci, lead R.D. at Snap Kitchen.
For those eating diets higher in refined carbs and sugar, Presicci recommends making dietary changes to minimize these food groups first. If you still find yourself wanting to try intermittent fasting after that, go for it.
Pick your intermittent fasting plan.
There are several different eating schedules to choose from, also known as intermittent fasting protocols. One of the reasons experts recommend working with a professional is to help determine which plan fits best with your schedule, lifestyle, and goals, Dr. Favini says. “There’s no one-size-fits-all recommendation,” he adds.
- 16:8 — This protocol, also sometimes called “leangains,” tends to be the most popular. You fast for 16 hours a day, and eat for eight. For example, many people will fast from 8 p.m. to 12 p.m., eating between the hours of 12 p.m. and 8 p.m. on any given day. This is also known as an intermittent calorie restriction regimen.
- 5:2 — Here, the ratio applies to days instead of hours. Five days per week, you eat normally. Two non-consecutive days per week, you eat a very low calorie intake, usually around 500 or 600 calories. This is also known as an alternate day fasting regimen.
- Eat-stop-eat: This is also known as the 24-hour fast, and involves fasting for a full day, then resuming eating as normal. Usually, this is done between one and three times per week.
- The Warrior Diet or 20:4: Modeled after the supposed eating regimen of ancient warriors, this protocol involves fasting for 20 hours, then eating a large meal, usually at night.
Build up to longer fasts.
As with most lifestyle changes, it’s best to ease into intermittent fasting. “I always tell patients that slow and steady wins the race,” Basbaum says. “Because in most cases, the faster the results occur, the faster they will likely disappear.”
If the standard fasting regimens seem too difficult or extreme at first, experts recommend slowly building towards your goal. “For example, for a 16:8 plan, you may want to start from 12 hours then slowly increase the fasting interval until you reach 16 hours,” Dr. Favini says.
Prepare for irritability.
“Most people will feel more irritable and hungry when they first start fasting,” Dr. Favini says. “This is normal since your body is adjusting to lower blood glucose levels and it can take up to two to four weeks for the feeling to go away.” Half the battle here is being prepared.
Hydrate — even when you’re fasting.
One of the biggest fasting mistakes experts cite is not drinking fluids during the fasting period. While anything with calories will break the fast (including alcohol), it’s still important to avoid dehydration. “Make sure to consume plenty of water during your fast and consider adding electrolytes if your fast is longer than 16 hours, or if advised by your doctor,” Dr. Favini says. Aim to focus on non-caloric beverages like water, tea, and coffee (without sugar or milk) during your fast.
Plan how you’ll break your fast.
And prioritize nutrient-dense foods, not junk food. “Often times, people think they can eat anything and as many calories as they want within the eating time frame,” Volpe says. But if you load up on junk food or overeat during your eating hours, you’re unlikely to see the results you’re after. To avoid bingeing or letting your appetite get the best of you, Dr. Favini recommends planning your post-fast meal ahead of time.
Pay attention to how you feel.
In some cases, IF simply isn’t worth it. “Be careful to not push the fasting too far,” Dr. Favini says. Common side effects of IF include fatigue, hunger, cravings, and irritability. Some people may be able to deal with these effects better than others. Dr. Favini’s advice? “If you’re noticing dramatic decreases in your energy, discuss it with your doctor before continuing.”