How to Exercise While on an Intermittent Fasting Diet

How to Exercise While on an Intermittent Fasting Diet

So you’ve taken up intermittent fasting (IF), and you’re wondering how to fit exercise into your IF lifestyle.

Admittedly, intermittent fasting and exercise is a tricky proposition. For optimal athletic performance, conventional wisdom suggests you consume carbs before your workout to fuel your muscles and protein afterward to repair them.

If you’re fasting, that can be a tall order. But it’s not impossible.

“Getting the most from an IF diet, and an exercise program, is a question of timing your workouts properly around your fasting periods,” says Angelo Poli, ISSA, owner of Whole Body Fitness in Chico, CA. Here’s the best way to combine intermittent fasting and exercise to reap the maximal rewards from each.

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Can You Exercise While Fasting?

It’s possible to perform both aerobic activities (like running and walking), as well as anaerobic activities (like strength training) while fasted. As long as you exercise sensibly, you won’t starve, waste away, or pass out.

But if you want to get optimal results from intermittent fasting and exercise, it’s best to schedule your exercise sessions carefully in relation to your eating windows. That’s because different forms of training rely on different types of fuel.


When is the Best Time to Work Out When Fasting?

fasted workout - woman getting ready to workout

Aerobic training

You can perform low-intensity aerobic training (easy cycling, light jogging, walking), as well as moderate-intensity exercises like steady-state running, before, during, or after your feeding windows.

  • During these activities, a high percentage of calories burned are coming from fat, which is plentiful in most people’s bodies, even if you’re very lean.
  • You’re unlikely to lose any more fat doing fasted cardio than you would if you exercised after eating, but you will improve your body’s ability to burn fat for fuel — a key component of long-term metabolic health.

Anaerobic training

You should perform anaerobic training (all forms of strength and power training, high-intensity intervals, sprints, CrossFit WODS) during your feeding window. Ideally, you’ll exercise about 60-120 minutes after a full meal, or about 30 minutes after a protein- and carb-rich snack.

  • Anaerobic training is fueled by glucose and glycogen, so you’ll perform better with some carbs in your system.
  • And protein helps repair and rebuild muscle tissue after your workout.
  • For best results, have another carb and protein-containing meal or snack soon after this type of training as well.


How to Effectively Work Out While in a Fasted State?

As long as you stick to the parameters above — especially when it comes to performing high-intensity exercise during your feeding windows — you can run, bike, lift, take exercise classes, or use exercise apps as you usually would.

Remember, though, the goal of IF is fat loss, not muscle gain. So don’t lift like a linebacker while following this diet.

“Stick with 2-3 lifting sessions per week, exercising the whole body each session,” says Poli.

These are guidelines for intermittent fasting and exercise. Periodically lifting on an empty stomach or running on a full one won’t hurt you. You just won’t get quite as much out of those workouts as you would if you’d followed these guidelines.


Tips For Working Out When Fasted

fasted workout - plate with clock on it fasting

1. Keep it Aerobic

The easiest mistake to make trying to follow an exercise program on an IF diet is training too hard in a fasted state.

“Don’t let that low-intensity session turn anaerobic by doing a lot of hill sprints or sled pushing,” says Poli.

Once you get rolling in your cardio session, it’s easy to push the intensity into anaerobic territory.

2. One Diet at a Time

Unless you have a medical reason for it, there is no need to restrict carbohydrates, gluten, fat, or any other wholesome, healthy food while following an IF diet.

Intermittent fasting facilitates fat loss without additional interventions.

3. Lift to Keep Your Muscle

“An IF diet—like any other diet that restricts calories below maintenance level–can lead to loss of muscle mass if you’re not careful,” says Poli.

The antidote? Strength training. 2-3 sessions a week ensures that you’ll preserve your muscle mass while stripping away fat. Just be sure you’re getting enough protein, and don’t let your calorie intake dip too low.


How Intermittent Fasting Works

fasted training - woman checking watch

One of the benefits of intermittent fasting is its simplicity.

You don’t count calories, macronutrients, or grams of anything — all you do is watch the clock.

Depending on the approach you choose, you might, for example, fast for sixteen hours each day and eat normally for eight, eat normally for five days and fast for two, or any number of other feed-fast combinations. Different approaches work better for different people.

The primary reason most people practice intermittent fasting is to lose weight. That’s because IF limits your calorie consumption. You don’t consume as many calories when you restrict yourself to eating just eight out of every 24 hours, or five days out of every seven. And eating less, as your 7th-grade Nutrition teacher told you, leads to weight loss.

So intermittent fasting is not for everyone. If you are trying to maximize muscle growth, for example, IF isn’t your best bet.

“Fasting periods compromise the ability to maximize muscle protein synthesis,” says nutrition coach and researcher Alan Aragon, MS.

Trying to add size while in a calorie deficit — like the one produced by IF — is like trying to build a brick wall without bricks.

So before you commit to intermittent fasting, be sure it aligns with your goals. Assuming you get all the nutrients you need during your eating windows, IF can be a tool for staying lean or losing fat. But if you’re trying to pack on muscle mass, choose another approach.

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