Should Runners Be Intermittent Fasting?

Molly Hurford
by Molly Hurford
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Should Runners Be Intermittent Fasting?

In recent years, intermittent fasting has become a trendy buzzword. Studies have come out showing it can improve metabolic efficiency and digestive issues, but for runners, especially those with busy lives in addition to tough training schedules, the answer isn’t black and white. In a study on mice, intermittent fasting for full 24-hour periods interspersed with two days of feeding resulted in better metabolic health and weight loss, while another showed intermittent fasting (in humans) lowered diabetes risk and improved cardiovascular health.

So there are plenty of potential benefits to fasting, but athletes haven’t always benefitted. One study looked at runners fasting during Ramadan and showed it negatively impacted their performance, and another showed that it also was “accompanied by significant metabolic, hormonal and inflammatory changes.” Now, Ramadan fasting is more strict than many intermittent fasts, but it allows for daily eating, whereas some versions of intermittent fasting involve full 24-hour fasts.


Stacy Sims, PhD, and author of ROAR, has a lot to say about intermittent fasting for athletes. “Intermittent fasting has come about to try to combat things like diabetes and metabolic issues, and people have taken it into daily life, thinking it would work for them if it works for diabetics,” she says. “But if you take a step back and ask why someone would need to do intermittent fasting in the first place, it’s mostly because people are eating at all kinds of weird times during the day in the first place. You get up early and have breakfast, drive to work, have more food, eat throughout the day and eat all the way up until bedtime.” Even if you’re sticking to a certain number of calories per day, the eating all the time approach still wreaks havoc on your digestive system by not giving it chance to rest.


Sims recommends a more moderate version of intermittent fasting — one that should be considered ‘normal eating,’ rather than a diet protocol. Simply put, she wants people to take a prolonged break, around 10–12 hours, from eating every 24 hours. Most of that happens while we’re sleeping, so we just have to tweak our timing to hit those numbers.” A lot of research is showing people who don’t eat after dinner and eat again at breakfast have metabolic rates that are absolutely good,” she adds. “Their body composition is fine. Even those trying to lose weight who skip eating that last snack before bed, but eat normally all day and have that 10–12 hour fast, see results. It’s so much better for the body than trying to do the more extreme 5:2 style fasts.”


Sims shares some advice for getting that done efficiently, without feeling any added stress.


“Back when our parents were growing up, you had three square meals, maybe a snack, but there wasn’t food after dinner,” she says. “Your body was intermittent fasting during the day like that: You finished eating at 7 at night and didn’t eat again until 7:30 the next morning. So you had this big period of fasting, and that was where your body reset. Back then, there wasn’t as much of an issue with metabolic control, but now that people are eating all day and at all times, there is no long set period of fast, so you don’t reset. The easiest way to intermittent fast is to eat normally, but not have a snack before bed.” To get in the rhythm of shorter intermittent fasts, make sure you’re eating three balanced meals daily, maybe with a snack after a tough workout.


Some people swear by fasted-state training, with shorter, easier workouts that can be done in the morning before breakfast. But the training must be truly stress-free. Any longer forms of fasting really aren’t good when you’re adding the stress of life and training, Sims believes. “Part of it is tied to energy availability: People are trying to lose weight with exercise, and they’re stressed, and they might skip a meal here and there. Then they bring in intermittent fasting and are skipping even more meals and can get into this low energy availability state, where you don’t have enough calories to support your resting metabolic rate, and you don’t have enough calories to support your immune system, your endocrine system — all these things that allow your body to respond to stress. So then you’re starting to see people putting on more belly fat and feeling flat or fatigued and trying to fight through it.”


Rather than doing fasted-state workouts or going full days without eating, Sims says, “If you fuel your body for your training and you tend to front-load your calories so you have enough for training, the rest you can play around with. You can do intermittent fasting if you’re fueling in and around your stress and it won’t negatively affect your body.” That means trying to eat more calories around your most active part of the day, and ‘active’ includes life stress as well as workout stress. So aim for a bigger breakfast and lunch and a lighter dinner, if possible.


If you are planning to do a light run or short walk before breaking your fast in the morning, don’t hit the espresso machine on your way out the door. “As soon as you have coffee, you start to affect the liver, which will release free fatty acids and change a bunch of hormones, which defeats the whole fasting idea,” Sims says. Let your body get the full 10–12 hours without anything other than water for a true reset.


“A lot of people have a sweet tooth that kicks in at night because when you’re tired, you’re more prone to carbohydrate cravings, so it can be hard to kick that habit,” says Sims. Her solution is to have something sweet before dinner and settle your brain that way or modify and have something healthy. She likes Greek yogurt with fruit and a bit of maple syrup right after dinner. “If you really want sweet stuff, really crave it, don’t wait until 10 p.m. to give in,” she adds. “Have something small close to your meal so you don’t interrupt the fasting aspect.”


If you want something really sweet, ask yourself if you’ve been having enough protein during the day. Sims says that if you’re craving sugar every single night, it’s an indicator that something might be missing in your diet — and many people, especially those training regularly — are consistently skimping on protein. (Aim for around 20 grams in each meal, plus an extra 20 following your workouts.)


Put the time formerly spent snacking after dinner to good use by adding extra sleep. Not only does it feel like breakfast comes sooner, but sleep helps you recover faster so you’re ready for tomorrow’s run.

About the Author

Molly Hurford
Molly Hurford

Molly is an outdoor adventurer and professional nomad obsessed with all things running, nutrition, cycling and movement-related. When not outside, she’s writing and podcasting about being outside, training and health. You can follow along with her adventures on Instagram at @mollyjhurford.


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