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:
- Alternate-day fasting only (every other day, the subjects ate one tiny meal containing 400-500 calories; on non-fasting days, they ate normally)
- Exercise only (stationary bike three times a week)
- Alternate-day fasting + exercise
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.
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…
- Drink enough water
- Get enough electrolytes
- Eat enough food in total to support your workouts. If you need 3,000 calories on a hard training day, it’s fine to spread them all out between 7 am and 8 pm, and it’s also fine to eat them all between noon and 6 pm, but either way, you have to eat them.
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…
- It’s totally fine to work out fasted – research shows that it’s not perfectly optimal for prolonged aerobic exercise (e.g., a long run), but for other types of exercise, it’s fine and it may even spark some valuable metabolic adaptations.
- It’s also fine to fast for a while after working out. Most of us have heard that we need to eat immediately after exercise (especially strength training) to support recovery and make sure our muscles have enough protein to rebuild themselves. But actually, research shows that it’s fine to fast for a while after you work out before you eat, as long as – you guessed it – you get enough calories in the day overall. If you’re an athlete in the top 1% of your sport and trying to qualify for the Olympics, there might be a noticeable difference in performance from optimizing nutrition timing, but for most of us, the simple barometer of “I feel fine when I do this” is probably enough.
Bearing that in mind, here’s a look at some options:
If you need to work out first thing in the morning
- Work out first thing in the morning and then fast for a few hours afterwards (your muscles won’t atrophy away, promise!)
- Work out first thing in the morning, eat breakfast and lunch, and then stop eating in the early afternoon.
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!