Intermittent fasting (IF) has been making the rounds in the health and wellness world since the early 2000s. Advocates claim the wellness hack can lower blood pressure, reduce inflammation, and lead to weight loss. There are a few different versions of the diet regimen, but the most popular is a time-restricted fast, which involves eating only during a prescribed time window. Other versions of IF call for full-day fasts or calorie-restricted weeks, but time-restricted fasting is the most approachable option for athletes because it allows them to fuel their training on a daily basis.
There is some preliminary research to back up the benefits of IF. One study published in August suggests that time-restricted eating led to a decrease in body fat while also lowering blood pressure in obese adults, and another 2018 study from UC San Diego showed a decrease in metabolic diseases in mice. Another pilot study published in the Journal of Nutritional Science found that simply moving breakfast and dinner three hours closer together led to a reduction in body fat despite no change in caloric intake. But experts have pointed out that most IF studies have small sample sizes of specific groups—clinically obese patients, athletic men, or mice—so the results may not easily translate to the general population. “This is the problem with a lot of studies,” says nutrition scientist Stacy Sims. “In the fitness industry, people see things that work in clinical populations and think, ‘Hey, we can use this to get leaner and fitter,’ without really looking at the implications. Then something like IF gets media attention and buzz, and then the science starts to catch up and shows that there are a lot of implications that don’t get discussed.”
Recently, IF has become a topic of discussion in the athletic performance world. As an athlete, IF may be worth considering, but it ne to be practiced carefully and intelligently. Journal of the International Society of Sports Nutrition published a position paper in 2017 stating that time-restricted IF combined with training can help athletes lose fat while maintaining strength, but it cautions that athletes need to carefully plan their nutrition timing for optimal athletic performance.
“I just chastised one of my athletes for fasting during his ride earlier today!” says Frank Overton, founder of FasCat Coaching. “Endurance athletes need to think about themselves like Ferraris: You need to keep the gas tank full if you want to race it.”
Because of this, Overton and Sims don’t recommend any type of fasting for elite athletes in the middle of an intense training block. Sims encourages clients to simply take downtime from eating—after dinner, don’t eat anything else, and then start eating again in the morning. It’s a less intensive version of the eight-hour eating window, but not eating for 12 hours—say, from 7 p.m. to 7 a.m.—offers similar benefits with less risk. “That’s the way to do it. The positive benefit from IF is not having calories for a certain amount of time, when your body can repair. Not eating for that window around when you sleep is great because your metabolic rate is already low, so you don’t get into a compromised state of not having enough energy,” Sims says.
According to Robb Wolf, author of Wired to Eat and The Paleo Solution, one of the greatest benefits of IF comes from eating in line with our circadian rhythm and shutting down food consumption several hours before bed, though most of the specific research has focused on mice. Wolf adds that with time-restricted fasting, people are eating during times when they’re more metabolically flexible and insulin sensitive—meaning your body can switch between carbs and fat for fuel and more efficiently process foods, particularly sugar—which can aid in fat burning as well as reducing your risk of diabetes. If you’re trying to lose weight, IF isn’t a magic bullet, but it may help promote healthier eating habits. “I deal with some people who refuse to cut out bread or other stuff like that, but they’re willing to eat in a smaller window. During that period of time, someone would have to make a huge effort to overeat,” Wolf says. (Wolf is quick to note that if you already have a history of disordered eating, IF may not be right for you.)
Know Your Why: “Think about the potential rewards and potential downsides. That’s not sexy, but it’s really important,” Wolf says. So, with IF, maybe you’re hoping to kick-start healthier eating habits by seeing a reduction in body fat or to give yourself a rule that forces you to skip that late-night snack. But the potential downsides—more limitations around when you can train hard, for example—may outweigh the rewards.
Time Your Training: If you want to train at a high intensity or for a long time, Overton says it’s important to remember that you absolutely need fuel. If you train in the morning, you may want to start your eating window earlier to ensure you’re properly fueled for your workout and that you’re able to eat a recovery meal afterward. If you work out on an empty stomach, you risk compromising your muscles’ ability to recover and adapt to your training.
Improve Your Food Quality: Fasting doesn’t give you license to eat tons of junk food during your feeding window. Stick to basic nutrition principles: Eat plenty of veggies, plus healthy proteins and fats.
Open Your Window: You don’t have to fast for 16 hours of your day, Sims says. As long as your body is getting plenty of downtime from eating, where it can digest and rest, you’re on the right track. Depending on your work, family, or training lifestyle, you might want to consider a ten- or 12-hour hour window. You’ll still get many of the benefits but without the added stress.
Don’t Aim for Calorie Reduction: It might happen naturally, but Sims notes that it’s crucial to keep eating enough to fuel your day and overall training. If you start focusing on tallying calories in addition to shortening your eating window, you can quickly slip into unhealthy habits.
Skip the Coffee: Wolf and Sims both warn fasting enthusiasts that coffee will stimulate your liver in the morning, causing it to release hormones into your bloodstream. Since the point of IF is to give your body a break from processes like this, that makes coffee a no-go.
Pay Attention to Stress Cues: If you’re a serious athlete, you may not want to add stress to your life by living under strict meal-timing rules. “I saw this a lot with CrossFit athletes,” Wolf says. “They ended up with problems like fatigue and thyroid issues, sleep issues, and hormonal disregulation.” Stay in touch with your body and scale back on training intensity or dietary restrictions as needed.
Don’t Do Too Much, Too Soon: Start with a wider eating window, like 14 hours, and slowly whittle down to 12, ten, or even eight. Avoid making other extreme food changes at the same time. For instance, Wolf has noticed people starting a ketogenic diet at the same time as time-restricted eating, which she considers a recipe for disaster. Stick to one intervention at a time.