Whether you're running, lifting, or doing yoga, working out can burn hundreds of kilojoules. That means you might be hungrier than normal and need to consume more food to replenish your energy. But if you're following intermittent fasting (IF), should you be concerned about exercising on an empty stomach? The short answer is it depends on the type of fasting diet you're following, the way you time your workouts, and the fitness goals you want to achieve.
While most IF diets allow you to determine your own eating and fasting periods based on your lifestyle, you need to be smart about properly fueling your body before and after a fast so that it doesn't negatively impact your workouts. Here's what you can do to ensure you're working out safely while intermittent fasting.
Is it safe to work out while fasting?
First note that there are many different methods for IF, including the 5:2 program, which involves restricting your kilojoule consumption to 25% of your kilojoule needs two days a week and eating normally the rest of the days. On the 16:8 diet, you eat during an eight-hour window and fast for the remaining 16 hours. While you're fasting, you can drink water, black coffee, and tea, but everything else is off limits.
"It is safe to follow IF and be physically active, but some things are more important to be aware of, especially at the beginning when you are becoming keto adaptive - meaning your body is learning how to burn fat for fuel instead of carbs," says obesity expert Dr Wendy Scinta. "Hypoglycemia initially can lead to increased heart rate, dizziness, nausea, and poor athletic performance, but this improves as your body learns how to run on ketones instead of glucose," she says.
"Some people don't do well when they eat and work out, but it's important to have energy," says dietitian Bonnie Taub-Dix. "If you're on the 5:2 program, you're consuming only 25% of your kilojoule needs two days a week, so I would reserve exercise for the other days of the week when you're eating normally," she advises.
Women on the 5:2 diet limit their kilojoule intake to 2,000kJ (500cal) and men 2,500kJ (600cal). But this kilojoule limit is separated by a 12-hour fast, so you can consume 1,000kJ (250cal) in the morning and another (250cal) at night. Men can break up their kilojoule intake evenly between the fasting period too. If you want to work out during your limited-kJ days, then it might make sense to exercise right before or after your fast. This way, you're working out while you're fueled and have the option to eat once your fast is over. What's also great about the 5:2 diet is that you can decide which days you want to eat normally and which days you want to eat very little, making it easier to schedule your workouts accordingly.
It's best to work out at the beginning of a fast period when you're already properly fueled and at the end of a fast, so you can enjoy a pre- or post-workout snack.
Strength and conditioning rehab specialist Kulaa Bacheyie agrees that the 5:2 plan is more ideal than other intermittent fasting methods when you're new to IF and easing into a workout routine. "The 5:2 plan may be better than the 16:8 diet so you are fueled before your workout," he says. Bacheyie says it's best to work out at the beginning of a fast period when you're already properly fueled and at the end of a fast, so you can enjoy a pre- or post-workout snack.
Once your body has fully adjusted to an IF diet and is keto adaptive, making sure you're doing low-impact workouts over HIIT, running, and other high-impact exercises becomes less of a concern. "Initially, high-intensity exercises and resistance training will reduce blood sugar levels and glycogen stores, so avoid these in the beginning. But once you have been doing IF for a while, it is less of a problem," Bacheyie says.
Can you build muscle while following IF?
Research shows that combining an IF diet with a regular exercise routine can produce greater weight loss results than fasting alone. But the reality is IF isn't the most effective nutrition plan for building muscle mass, so if that's your goal, you want to consider following a different diet. "IF has a greater tendency to decrease your workload due to muscular fatigue. But you can build muscle if you train intensely enough and time your workouts properly, along with recovery days," Bacheyie says. "Loading your feeding time with protein will also help."
How to work out and fast the smart way
Other than timing your workouts so that they begin at the start or end of a fast, there are some other steps you can take to ensure that your workouts are effective while following IF.
1. Load up on protein, fat, and carbs during your eating periods. Taub-Dix says that combining protein, carbs, and fats in your meals will help you feel fuller during your fast and give you energy for your workouts. "It's important to replenish your glucose stores after a workout, so be sure to enjoy at least 15 grams of carbs. That's a half-cup of pasta or a slice of bread," she says. Go for lean sources of protein too, like grilled chicken, salmon, and grass-fed beef, and add some healthy fats, such as nuts and avocado.
Taub-Dix also stresses hydrating before and during your fast, as some people confuse thirst with hunger. "Drink a smoothie that has a good combination of protein and carbs, so it's easier to digest," she says.
2. Trick your brain into thinking you're actually fueling up. If you're new to IF and your body hasn't adapted to using fat as fuel yet, Bacheyie says swishing or gargling a carb-heavy drink in your mouth and then spitting it out can reduce your perception of fatigue and trick your brain into thinking that you're fueling it.
3. Save your more intense workouts for days you're not restricting kilojoules. If you're following the 5:2 plan, Taub-Dix says walking, doing yoga, Pilates, and other low-impact workouts are safer during the two days that you're limiting calories. "The calorie demand is greater when you're working out and 2,000kJ (500cal) isn't adequate anyway. If you're the kind of person that needs to work out every day, I would save the heavier workouts for later in the week," she says. If you're following another IF method that has longer fasting periods, like the 16:8 diet, then time your workouts at the beginning or the end of a fast.
4. Enjoy a healthy pre- or post-workout snack. When you time your workouts before or after a fast, you have the benefit of eating pre- or post-workout. There aren't hard or fast rules on whether it's better to eat before or after a workout (it depends on what works best for you), but the most important thing is that you're fueling up wisely.
Bacheyie says healthy high-glycemic carbs, like bananas, grapes and grape tomatoes, are best after a workout. "A recovery drink that has a 3:1 ratio of carbs to protein is best for replenishing glycogen stores and stimulating protein synthesis for muscle recovery," he adds. If you're working out before a fast, eat fruit, low-fat yoghurt, peanut butter, and other foods that are easy to digest. Your body is able to break down these foods quickly and use them as fuel. Taub-Dix says that Greek yoghurt with nuts, a smoothie, and whole-grain toast with peanut butter are some healthy pre-workout snack options.