Touting benefits like faster weight loss, improved metabolic health, lower inflammation, and reduced risk of cardiovascular disease (and more), it’s no surprise that intermittent fasting (IF) has become so hot in recent years.
Though IF always involves restricting the time during which you eat, there’s no one way to put it into practice. In fact, a number of different types of intermittent fasting have become popular.
So, should you fast for 18 hours every day or go completely without food for a day or two per week? This guide breaks down five of the most popular (and most-researched) types of intermittent fasting. It’ll help you understand the pros and cons behind each—and identify which might work best for you.
1. 5:2 Fasting
The fasting style known as “5:2” involves eating as normally for five days of the week and dropping your calories down to just about 500 on two non-consecutive days.
In addition to being one of the more straightforward approaches, “the 5:2 approach is a good way to ease into intermittent fasting,” says Victoria Hahn, R.D.N., a dietitian with L-Nutra.
Sound manageable? “Research suggests that this type of diet may be just as effective as a continuous caloric restriction when it comes to weight loss,” Hahn says. One small 2011 study published in the International Journal of Obesity, for instance, found that the 5:2 method produced similar weight loss and insulin regulation benefits to other forms of dieting.
If giving it a try, Dr. Anna Cabeca, D.O., author of The Hormone Fix, recommends boosting your fat intake the evening before low-calorie days. In addition to keeping you feeling fuller for longer, eating more fat can help your body make the switch from burning carbohydrates to fat (read: body fat) more easily, she says.
Still, you can’t go bananas on non-fasting days if you want to see results. “Over-eating (especially junk foods) on non-fasting days may not do much to promote fat loss, and could even result in weight gain,” says Dr. Josh Axe, D.N.M., C.N.S., D.C., founder of Ancient Nutrition and member of The Vitamin Shoppe Wellness Council.
2. 16:8 Fasting
One of the most common types of intermittent fasting, the 16:8 method involved fasting for 16 consecutive hours every day and eating normally during the other eight hours. “Typically, this simply involves not eating anything after dinner and skipping breakfast the next morning,” says Axe.
More flexible and less disruptive to someone’s overall lifestyle than other methods, Axe recommends this approach for women and athletes.
“While other, more-extreme fasting approaches can potentially disrupt women’s hormonal balance and affect their menstrual cycles, this approach is a bit gentler and therefore less likely to create unwanted side effects,” he explains.
It also works well for athletes, because it allows for plenty of opportunities to consume adequate fuel and refuel after exercise, every single day. The biggest risk of the other methods is that an energy-deprived body may break down muscle, Axe explains. The 16:8 method, though, minimizes this risk. In fact, one 2016 study published in the Journal of Translational Medicine found that men who followed a 16:8 approach while resistance training for eight weeks lost fat mass without impacting their muscle mass.
3. Overnight Fasting
Similar to the 16:8 approach, the overnight approach to fasting involves eating only within a 12-hour window each day. You might, for example, choose to eat breakfast at 8 a.m. and finish up dinner by 8 p.m.
The main benefits of this super-gentle approach: It’s easy to implement and doesn’t require counting calories or skipping meals. At most, you’ll have to pass on late-night snacking.
However, this approach is likely the least conducive to weight loss, according to Hanh. Research shows that the body takes between 12 and 24 hours to utilize the carbohydrates stored in the liver before burning stored fat for energy. (Of course, this depends on factors like exercise, muscle mass, and metabolism.) So, this method potentially leaves the body with little time to tap into those fat reserves.
For this reason, Hahn recommends it for folks with very specific goals, like nixing mindless snacking at night.
Still, one 2018 study published in Cell Metabolism linked five weeks of daily 12-hour fasting to improved insulin sensitivity in pre-diabetic men, suggesting that some of the health benefits still stand.
4. Weekly 24-Hour Fast
Also known as the Eat-Stop-Eat method, this IF approach involves fasting for one 24-hour period per week. On the other six days per week, you’ll eat normally.
Because it involves a full day without food, “the eat-stop-eat method decreases overall carbohydrate intake,” says Cabeca. “The decrease in carbohydrates can help lower your blood sugar low and decrease insulin resistance.”
Since people generally become more insulin resistant as they get older, she suggests people over 50 may benefit most (as long as they’ve cleared it with their healthcare provider). In fact, one study out of Salt Lake City’s Intermountain Healthcare Heart Institute linked just one 24-hour fast per month with longer lifespans in patients with cardiac catheters.
The drawback of this option? It’s not easy. Fatigue, headache, and irritability are all common side effects, according to Axe.
5. Alternate-Day Fasting
One of the most intense types of intermittent fasting, alternate-day fasting entails fasting every other day. While some don’t eat at all on fasting days, others eat a very small amount (typically around 500 calories), says Axe.
The potential payoff: One 2017 JAMA International Medication study found this method effective at helping patients decrease body weight. Beyond weight loss, a 2019 Cell Metabolism study suggests alternate-day fasters achieved improved cardiovascular markers, reduced fat mass, and improved body composition in just four weeks.
The downside of this method: Data shows that alternate-day fasting is (understandably) hard for people to stick to long-term.
That said, “if you’re able to eat very limited amounts on fasting days without feeling very weak or in a bad mood, it could be a good fit,” says Axe. If you notice you feel unwell when fasting, though, it’s a no-go.