INTERMITTENT FASTING HAS become a nutritional strategy staple in the fitness world, up there with Keto and the Atkins diet.
The strategy is relatively simple, and it’s right there in the name. Practitioners only eat during a preset period of time, and fast for the remainder of the day. The idea is by limiting the amount of time you have to eat, you may also limit your overall calorie intake. That way, you’ll put yourself into a calorie deficit and lose 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 that late night trip 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 you’re already living the fasting life, or if you think this type of strategy sounds like a viable diet option for you, there’s one thing to consider: How do you formulate your workouts around fasting?
Weaving your exercise plan into your eating schedule is crucial, given the nature of intermittent fasting. Food is your body’s fuel, and limiting your intake might bring you up short on energy at the gym. No one should be passing out because they’re maxing out deadlifts after a 16-hour fast. Then, there’s the question of protein. You should always get some protein in after a lift, right?
Needless to say, it can be confusing deciding how to strategize within this framework. We asked the experts what the best practices are when it comes to working out while intermittent fasting
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.
Time does matter, however, when you’re working out and fasting. Here are a few ways to still make the most of your training while intermittent fasting.
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.
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.com, and 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.
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.
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