Intermittent fasting: Should you exercise on empty?

Food is fuel. So what would happen to your exercise regimen if you weren’t eating your standard three meals (plus snacks) per day?

Intermittent fasting, which can involve anything from fasting for 14 hours at a time to forgoing food a couple days a week, is all about restricting food consumption during specified periods. You consume little to no calories during “fasting” periods, and eat pretty normally during “feasting” times.

The end result is impressive weight loss, according to many devotees of this trendy diet. Some intermittent fasting methods even claim that this on-and-off eating plan can help regulate blood sugar, prevent diabetes, lower the risk of heart disease and slow the aging process.

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Here’s the catch: The timing of when you do (or don’t) eat can have a major impact on your workout. Is it risky to exercise on an empty stomach? Read on to find out.

Do intermittent fasting and exercising mesh?

Whether you’re pounding the pavement or cranking out squats, your body primarily uses glycogen, or stored carbohydrates, to fuel exercise.

The exception occurs when your glycogen reserves are depleted, which could happen if you haven’t eaten in a while, says Kelly Pritchett, Ph.D., R.D. and a board-certified specialist in sports dietetics and an assistant professor in nutrition and exercise science at Central Washington University. When that’s the case, your body is forced to find and burn other energy sources, such as fat. That’s why, in one British Journal of Nutrition study, men who ran before eating breakfast burned up to 20% more fat than those who fueled up before their run.

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Before you get too excited, consider this: “When glycogen is in short supply, your body also reverts to breaking down protein – your muscles’ building blocks – for fuel,” Pritchett says. So, while you may shed more fat when exercising on an intermittent fasting diet, you may lose more muscle, too. If you’re heading out on a long run, but haven’t eaten any carbs, your body might start burning protein within a couple of hours.

That won’t just thwart how much weight you can bench press or how toned your butt looks. It will also slow your metabolism, which can make losing weight more difficult in the long run. In an effort to prevent starvation, your body adapts to the number calories you give it. So if you’re frequently making drastic cuts to your calorie intake, your body will eventually adjust, burning fewer calories per day to ensure you have enough energy left to stay upright, breathing and healthy, Pritchett says.

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In one small study from the Pennington Biomedical Research Center, after a group of men and women fasted every other day for 22 days, their resting metabolic rates (how many calories they burned each day by simply living), had dropped by 5%, or 83 calories. That’s not exactly ideal for any exercise plan that’s supposed to end in weight loss.

Plus, if you’ve ever tried to power through a tough workout with a growling stomach, you know that working out on empty is just plain hard. If your glycogen or blood sugar levels are low, you will feel weak. And if you don’t have enough energy to really go after it during workouts, your fat-burning and muscle-building results will suffer, says Jim White, R.D., owner of Jim White Fitness and Nutrition Studios in Virginia.

How to sweat smart when fasting

Intermittent fasting enthusiasts don’t need to throw in the towel on tough workouts just yet, though. Maintaining a consistent exercise routine is important for your health, both physical and mental. So if you’re following an intermittent plan, here are the best ways to structure your workouts so you can still get great results:

1. Keep cardio low-intensity if you’ve been fasting.A good gauge of intensity is your breathing: You should be able to carry on a conversation relatively easily if you’re exercising mid-fast.

“If you are going out for a light jog or stint on the elliptical, you probably aren’t going to have an issue,” says White. But it’s important to listen to your body, and stop exercising, if you feel light-headed or dizzy. If you push your exercise intensity or duration too high, your workout will become a struggle.

2. Go high-intensity only after you’ve eaten: Intermittent fasting programs such as LeanGains have strict rules about scheduling meals around workouts to maximize fat loss while still staying fueled. In general, the closer you schedule any moderate to intense sessions to your last meal, the better. That way, you’ll still have some glycogen (aka leftover carbs) available to fuel your workout, and you’ll reduce your risk of low blood sugar levels, he says.

Try to follow high-intensity workouts with a carb-rich snack, since your glycogen-tapped muscles will be hungry for more.

3. “Feast” on high-protein meals: If you’re looking to build serious muscle, you’ll need to eat before and after lifting. While a preworkout snack can help you fuel, regular protein consumption is vital to muscle synthesis both throughout the day and right after your strength workout, when your muscles are craving amino acids to repair themselves and grow, Pritchett says.

To maximize muscle growth, the Academy of Nutrition and Dietetics recommends consuming 20 to 30 grams of high-quality protein every four hours while you are awake, including after training. On a fasting plan, timing is key: Schedule your strength training workouts so that they’re sandwiched between two meals, or at least two snacks. And make sure to use your “feast” meals to meet your protein needs.

4. Remember: Snacks are your friend: Some intermittent plans allow dieters to eat snacks and meals during their feast periods, so take advantage of that flexibility.

A meal or snack consumed three to four hours before your workout (or one to two hours before, if you’re prone to low blood sugar) will help ensure you have the energy to power through those reps.

Aim for a meal that combines fast-acting carbohydrates with a blood sugar-stabilizing protein (such as toast topped with peanut butter and banana slices). Within two hours of your last rep, chow down on a post-workout snack containing about 20 grams of protein and 20 grams of carbohydrates to promote muscle growth and help replete your glycogen stores so you stay energized, White says.