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Intermittent Fasting and Workouts


Are you a gym rat who’s nervous about how intermittent fasting will affect your workouts? Or the opposite, an intermittent fasting fan who wants to try a gym membership but doesn’t know how to swing it? Here’s a research-based guide to maintaining a regular workout schedule – and actually maintaining performance – when you eat in a restricted feeding window. The short version is: it’s very possible, research shows it probably won’t be a huge problem for strength or endurance, and there are a lot of options for making the schedule work.

For the purposes of this post, we’ll assume that your fasts are 24 hours or less in duration – if you’re fasting for longer than a day, it’s probably a good idea not to do intense workouts during the fast.

Is it safe to work out on IF? Doesn’t it tank your workouts?

Yes to the first question; no to the second! Research shows that most people can do pretty well working out while intermittent fasting, especially if they’re smart about hydration, electrolytes, and taking in enough total calories.

This study looked at women doing resistance training – one group ate normally, while the other group at only between noon and 8 pm. They followed these diets for 8 weeks. Both groups also got supplemental whey protein. At the end of the 8 weeks, the researchers found that the women ate about the same amount of food (even though they ate at different times, the IF group didn’t eat less), and got the same amount of muscle growth in both groups. Women in both groups also showed improved performance, with no difference between the groups. This suggests that as long as you eat enough, you can still get stronger with IF.

Here, researchers divided people into three groups:

The third group, the ones who combined alternate-day fasting with exercise, lost the most weight and didn’t struggle overly much with the exercise component of the program.

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Amazing top secret performance-enhancing tip from these studies: eat enough food! Whether you eat it in 12 hours or 6 hours is less important.

For the runners of the world, this study found that intermittent fasting didn’t have any noticeable effect on aerobic performance either way. To quote the conclusion:

“IF [Intermittent Fasting] induced no changes that aided in aerobic exercise performance compared to those who were not IF. IF had no ill effects on exercise performance.”

There’s also research on Ramadan, the Muslim holy month when healthy adults fast from sunup to sundown. Ramadan fasting isn’t totally equivalent to a typical intermittent fasting program for health, but the studies are still interesting. This study looked at male Judo athletes during Ramadan; the researchers found that the men had higher fatigue after Ramadan, and that they lost a little weight, but that overall their performance was mostly the same. In power athletes (like wrestlers and sprinters) another group found similar results.

Ultimately, all of these studies suggest that it’s totally fine to combine intermittent fasting with exercise, as long as you…

How to Combine Workouts and Intermittent Fasting

Coordinating the timing

Timing can be a big practical challenge – but maybe not as big as you think. Too many people get tied up in knots about needing to eat before or after workouts. In fact…

Bearing that in mind, here’s a look at some options:

If you need to work out first thing in the morning

If you need to work out in the afternoon

Fast until noon or 1pm, then eat a medium-sized meal for lunch. Work out halfway through your feeding window, after your first meal but before you’re done eating for the day.

If you can work out whenever you want

College student? Work from home? Retired? Non-traditional schedule? Pick any of the above – or you could fast until 12 or 1pm and work out at the very end of your fasting window, just before your first meal. Towards the end of their fasting window, some people can start to feel weak, sluggish, and generally not up for a big workout, but if that works for you, go for it!

Yes, you can work out and IF!

It’s very possible, even if it might take a couple tries to get the timing and logistics set up the way you like them. Human bodies are super adaptable and there’s no reason to stress over the perfect timing of meals and workouts. As long as you feel strong and ready for your workouts, and you’re eating enough to recover from them in between each session, go for it!

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Hi I’m Ashley, I’m an ADAPT Certified Functional Health Coach

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