AFTER LEARNING TO count your macros, spending your time and money on grocery hauls and meal prep, and going all-in on protein, you might be looking for a different way to tweak your nutrition plan to meet your fitness goals. You’ll probably land on one of the most popular practices for physique-minded folks in recent years: intermittent fasting.

The practice has come a long way since its humble beginnings: In the early 1940s, scientists were testing the effect of daily fasting on the lifespan of rats. A daily practice of fasting appeared to correlate to a longer lifespan in the animals—springing the idea for the now popular diet strategy. The process is spelled out in the name: You’ll eat only for a small window of time during each day. By only allowing for such a small period for intake and eliminating opportunities for mindless eating, you’ll create a calorie deficit. You’ll gain control over your consumption by simply sticking to a timer—no more logging meals or purging your kitchen of foods some restrictive diets don’t allow.

Of course, there are some caveats to the practice. Food is fuel—and we need that for our workouts. If you’re the kind of person that struggles putting forth your best effort in the gym unless you’ve had your pre-workout snack, you might find it difficult to determine when to fit your workout into your day while you’re intermittent fasting.

This is important for more than just your energy levels. Our bodies utilize glucose stores for energy during exercise. Fasting diminishes those stores, lowering our blood sugar levels. Low blood sugar levels may cause several symptoms including elevated heart rate, shaking, confusion, and dizziness—increasing your risk of injury in the gym.

That doesn’t mean you can’t work out when you’re using intermittent fasting. It’s a matter of proceeding safely in a way that will still optimizes your training. A few small changes in your schedule and practices and you can continue your quest for that dream physique safely..

What Is Intermittent Fasting?

Let’s back up for a moment. First, it’s important to understand the intermittent fasting schedule before you consider how to implement the strategy within a workout plan.

The most common type of intermittent fasting is the daily, time-restricting format. Typically, there’s a 16-hour fast, followed by an eight hour eating window. For example, someone could make their eating period between 12 p.m. and 8 p.m., only consuming lunch and dinner.

The time schedule can vary depending on your day. If you’re a person that needs more fuel in the morning, you may run on a 9 a.m. to 5 p.m. schedule instead. The time of day doesn’t really matter, as long as the consumption window sits at eight hours.

What Are the Benefits of Intermittent Fasting?

If you frequently succumb to the temptation of a nighttime walk to the fridge, IF might just be the thing you need to keep your snacking in check and lose that excess weight.

“Intermittent fasting puts a cap on eating and can eliminate or minimize overconsumption at night. This can decrease calorie intake, can help with better sleep as the stomach won’t be having a fiesta when it is time for a siesta,” says Leslie Bonaci, R.D., M.P.H, sports dietician for the Kansas City Chiefs.

Weight loss isn’t the only benefit to the practice, though. There’s some evidence to suggest that intermittent fasting can reduce the risk of diabetes, lower blood pressure, and help maintain muscle mass.

That said, intermittent fasting is by no means the perfect diet for everyone, or a magic bullet for weight loss. For some, it’s a simple practice that may help eliminate those extra trips to the fridge. But for others, eating in a small window of time is just not sustainable— and the best diet is always one that keeps you consistent.

If fasting is that diet for you, exercise is vital to keep your heart healthy and muscles strong while dieting. Take these tips to schedule your workouts appropriately with your feeding hours.

How to Work Out When You’re Intermittent Fasting

Don’t Try to Build Muscle While Fasting

Whether it’s a 5/2 protocol (eat for five days, fast for two) a 16/8 (fast for 16 hours, eat for eight), or any other version of IF, most people on a fasting diet wind up losing weight. That’s because it’s much harder to overeat if all your daily calories are crammed into an eight-hour window than if you can spread them out over 15 to 16 hours. That’s what makes IF such an effective weight-loss tool: by restricting the time frame in which you can eat, you effectively restrict the number of calories you take in as well.

But if your main goal is maximizing muscle, fasting isn’t a great idea. “Unless you’re a real novice, you can’t build appreciable muscle in a caloric deficit,” says Angelo Poli, ISSA, co-owner of Whole Body Fitness in Chico, CA, creator of the MetPro diet and exercise app. A pound here and there? Maybe. But you won’t build anywhere near as much as you would if you consume a few hundred extra calories above and beyond what your body needs each day. So don’t try. Your primary goal while fasting should losing fat. To build muscle, you need fuel.

You Should Train While Fasting

Even if your main goal is losing fat, you still need to lift, which prevents your body from burning through muscle to fuel your daily activities. You won’t gain much muscle if you’re fasting, but if you lift, you won’t lose it, either. “The same activities that build muscle when you’re fueled help preserve it when you’re in a caloric deficit,” says Poli.

Since you’re only trying to maintain the muscle you have—not pack on additional beef—you can get away with a fairly infrequent lifting schedule—2 to 3 times per week, exercising your whole body each workout (try this routine).

Eat BEFORE You Lift Weights

Lifting weights, sprinting, doing CrossFit WODS, and other high-intensity activities all depend on carbohydrates for fuel, explains Poli. If you perform any of these activities during (or worse, at the end of) your fast, your performance will suffer. Instead of getting stronger and faster, you may well get weaker and slower.

What to do? “If you’re a big guy with a lot of weight to lose, no big deal,” says Poli. “Go ahead and lift on an empty stomach. You might lose a little bit of muscle, but you’ll burn fat, too—and that’s your main goal.”

If you’re a slimmer guy worried about your muscle definition, no need to worry— we have a solution for you too.

Sandwich Your Workout

It’s vital that you eat before you workout so that you can get the most out of your lift, but depending on your build, it might be good to ensure a little something afterwards. “Eliminating the post-workout refueling may delay recovery as well as muscle protein and muscle glycogen resynthesis,” says Bonaci.

So, if you’re on the slimmer side and are worried about losing some of your muscle definition, it’s probably better to schedule your lifting workouts during your feeding window. So if you eat from noon to 8 p.m. each day, try to hit the gym around 5, then go home and eat a high-protein meal to ensure adequate recovery from your workout.

Bonaci suggests sandwiching your workout into your meals if you’re limiting calories. Meaning, eat one third of your meal before the gym, and the other two thirds of the meal afterwards. That way, you get some in before to power the workout, and the rest after to recover. “Ideally, food and fluid would parenthesize the workout,” she says.

Overall, it’s best to do your best to fit your workout into your eating period so that you can have something before and after. If your schedule just doesn’t allow for it, it might be time to adjust the structure of your eating period— either by extending it, or changing the start and end times. “The window of eating may need to be longer to accommodate physical activity,” says Bonaci.

Fast Before Cardio

Many bodybuilders and other physique athletes swear by “fasted cardio”—jumping on a treadmill or bike for 30 minutes or more before breakfast—as a muscle-chiseling tool. Research is equivocal on whether this practice burns more fat than hitting the pavement after a meal or two. But Poli says it can’t hurt. “As long as you keep that cardio session low-intensity, you may well burn more fat in a fasted state,” he says.

Regardless, it’s less essential that you fuel up with carbs when you do lower-intensity work than it is when you lift or perform other high-intensity activities. Reason? “Slow cardio and other low-intensity activities run primarily on fat,” says Poli. “Even very lean athletes have plenty of fat on their bodies to power them through a long workout,” he says (think of lean ultra-runners who race for hours at a time without a bite to eat.)

While fasted lifting is a big fat mistake, fasted cardio is fine, and may help you burn additional fat. So for best results, schedule those lifting sessions during or after your feeding windows, and schedule cardio before them.

The Bottom Line

The key to exercising safety while using IF is to listen to your body. If you’re starting to feel light-headed after hopping onto the treadmill for some fasted cardio, be smart about the situation and stop and grab some food and water until you feel okay to start again. Carry a snack in your gym bag, for example, just in case.

To ensure you’re following a program that fits best to your body, speak with your doctor before starting up any new diet or workout program. They will be able to help you determine what will be safe and effective for you.

Headshot of Andrew Heffernan, C.S.C.S.

Andrew Heffernan, CSCS is a health, fitness, and Feldenkrais coach, and an award-winning health and fitness writer. His writing has been featured in Men’s Health, Experience Life, Onnit.comand Openfit, among other outlets. An omnivorous athlete, Andrew is black belt in karate, a devoted weight lifter, and a frequent high finisher in triathlon and Spartan races. He lives in Los Angeles with his wife and their two children. 

Headshot of Cori Ritchey

Cori Ritchey, NASM-CPT is an Associate Health & Fitness Editor at Men’s Health and a certified personal trainer and group fitness instructor. You can find more of her work in HealthCentral, Livestrong, Self, and others.