How to Exercise Safely During Intermittent Fasting

If you’re trying IF or you’re fasting for other reasons and you still want to get your workouts in, there are some pros and cons to consider before you decide to work out in a fasted state.

Some research shows that exercising while fasting affects muscle biochemistry and metabolism that’s linked to insulin sensitivity and the steady management of blood sugar levels.

Research also supports eating and immediately exercising before digestion or absorption occurs. This is particularly important for anyone with type 2 diabetes or metabolic syndrome.

Chelsea Amengual, MS, RD, manager of Fitness Programming & Nutrition at Virtual Health Partners, says that an upside while fasting is that your stored carbohydrates — known as glycogen — are most likely depleted, so you’ll be burning more fat to fuel your workout.

Does the potential to burn more fat sound like a win? Before you jump on the fasted cardio trend, there’s a downside.

While exercising in a fasted state, it’s possible that your body will start breaking down muscle to use protein for fuel, says Amengual. “Plus, you’re more susceptible to hitting the wall, which means you’ll have less energy and not be able to work out as hard or perform as well,” she adds.

Priya Khorana, EdD, a nutrition educator at Columbia University, believes that intermittent fasting and exercising long term isn’t ideal. “Your body depletes itself of calories and energy, which could ultimately end up slowing your metabolism,” she adds.

If you’re set to try IF while continuing your exercise routine, there are some things you can do to make your workout effective.

1. Think through timing

Registered dietician Christopher Shuff says there are three considerations when making your workout more effective while fasting: whether you should exercise before, during, or after the fueling window.

One popular method of IF is the 16:8 protocol. The concept refers to consuming all food within an 8-hour fueling window and then fasting for 16 hours.

“Working out before the window is ideal for someone who performs well during exercise on an empty stomach, while during the window is better suited for someone who doesn’t like to exercise on an empty stomach and also wants to capitalize on post-workout nutrition,” he explains. For performance and recovery, Shuff says during is the best option.

“After the window is for people who like to exercise after fueling but don’t have the opportunity to do it during the eating window,” he adds.

2. Choose the type of workout based on your macros

Certified personal trainer and master pilates teacher, Lynda Lippin, says it’s important to pay attention to the macronutrients you take in the day before you exercise and when you eat after.

“For example, strength workouts generally require more carbohydrates the day of, while cardio/HIIT [high-intensity interval training] can be done on a lower carb day,” she explains.

3. Eat the right meals after your workout to build or maintain muscle

Dr. Niket Sonpal says the best solution for combining IF and exercise is to time your workouts during your eating periods so your nutrition levels are peaked.

“And if you do heavy lifting, it’s important for your body to have protein after the workout to aid with regeneration,” he adds.

Amengual says to follow up any strength training with carbohydrates and about 20 grams of protein within 30 minutes after your workout.

The success of any weight loss or exercise program depends on how safe it is to sustain over time. If your ultimate goal is to decrease body fat and maintain your fitness level while doing IF, you need to stay in the safe zone. Here are some expert tips to help you do just that.

Eat a meal close to your moderate- to high-intensity workout

This is where meal timing comes into play. Khorana says that timing a meal close to a moderate- or high-intensity workout is key. This way your body has some glycogen stores to tap into to fuel your workout.

Stay hydrated

Sonpal says to remember fasting doesn’t mean to remove water. In fact, he recommends that you drink more water while fasting.

Keep your electrolytes up

A good low calorie hydration source, says Sonpal, is coconut water. “It replenishes electrolytes, is low in calories, and tastes pretty good,” he says. Gatorade and sports drinks are high in sugar, so avoid drinking too much of them.

Keep the intensity and duration fairly low

If you push yourself too hard and begin to feel dizzy or light-headed, take a break. Listening to your body is important.

Consider the type of fast

If you’re doing a 24-hour intermittent fast, Lippin says you should stick to low-intensity workouts such as:

• walking
• restorative yoga
• gentle Pilates

But if you’re doing the 16:8 fast, much of the 16-hour fasting window is evening, sleep, and early in the day, so sticking to a certain type of exercise isn’t as critical.

The most important advice to heed when exercising during IF is to listen to your body.

“If you start to feel weak or dizzy, chances are you’re experiencing low blood sugar or are dehydrated,” explains Amengual. If that’s the case, she says to opt for a carbohydrate-electrolyte drink immediately and then follow up with a well-balanced meal.

While exercising and intermittent fasting may work for some people, others may not feel comfortable doing any form of exercise while fasting.

Check with your doctor or healthcare provider before starting any nutrition or exercise program.