Fasting With Exercise

By: Danielle Cook Kawash, MS, RD, NBC-HWC Owner First In Wellness

When it comes to nutrition, there is always a heated debate about the best way to fuel for particular situations or outcomes. One topic getting a lot of attention these days is whether to do a workout while you are fasting. Like every other nutrition topic, the answer is not cut and dried.

In general, I suggest eating prior to training – but that recommendation may vary depending on your goals, the type of exercise and intensity you are doing, and what and when you eat before or after your workout.

Let’s consider fasting. A practice that can vary widely, a fast can last between

12 and 24 hours, and in some cases for several days or weeks. Some people practice intermittent fasting (IF) or time-restricted eating (TRE) during which they fast and eat for set numbers of hours per day.

Fasting, IF, and TRE have become popular because of the apparent health benefits reported, which include weight and fat loss, improved blood sugar regulation, lowered inflammation and pain, and improved cognitive function. For example, you might fast for 16 hours between 6 pm and 10 am and eat only during the eight subsequent hours (10 a.m. to 6 p.m.). Some people fast a few days a week and eat regularly on the other days. Many people who fast also exercise – often first thing in the morning – something that should be evaluated in terms of goals and type of exercise.

The human body requires adequate calories for high level athletic performance, as well as adequate macronutrients such as fat, carbohydrate, protein, and water; and micronutrients including iron, B-vitamins, and potassium. Here’s what the research science has to say about exercising in the fasted state.

Fat Loss and Weight Loss

Fasting is often suggested to increase fat loss, which makes sense. When you begin exercising, you’re predominantly using glucose from stored glycogen. Once those stores are depleted, you use more body fat, an abundant source of stored energy, for fuel. Fasting depletes glycogen stores, so in a fasted state, you have less glycogen to burn through before you start tapping into stored fat.

Fasting also causes your insulin levels to drop. Given that one of the basic functions of insulin is to inhibit fat breakdown in the body (lipolysis), having less insulin means more fat is broken down into fatty acids and used to fuel your exercise. This all sounds good in theory, but like everything else with our bodies, it’s a bit more complicated. This theory seems to work only if the type of exercise you’re doing is low-intensity, aerobic exercise (e.g. walking or jogging super slowly with your out-of-shape friend).

Dial up the intensity of the exercise to something like running, lifting weights, or high intensity interval training (HIIT), and this theory falls apart. Although you’ll break down more fat, you won’t use the fatty acids quickly enough to prevent them from being repackaged and stored again after exercise.

If fat burning and weight loss are your goals, low intensity exercise isn’t the most effective choice. Assume you burn 500 calories at a higher intensity and only 150 calories at a low intensity for the same duration. Even though the percentage of the 500 calories from fat may be lower, the total amount of fat burned is higher at the higher intensity than the total amount burned at the lower level.

A second consideration is the number of calories you burn after exercise called “excess post-exercise

oxygen consumption” (EPOC). Imagine running up a flight of stairs. You’ll breathe hardest when you get to the top of the stairs, recovering from the exertion. When you exercise at a high intensity, your body increases oxygen consumption and metabolism to meet its demands for recovery and repair for several hours after exercise. Performing higher intensity or HIIT type exercise instead of lower intensity exercise can result in significantly more overall fatty acids and calories burned during exercise and elevated calorie burn after exercise.

It’s important to note that you can’t exercise your way out of a bad diet. If your goal is weight loss and/or achieving a lean, muscular physique, what passes your lips will need to match these goals. If you’re consuming too many calories or have a diet full of fluffernutter sandwiches, fasting during exercise or working out like a maniac won’t do much good.

Working out at a higher intensity, therefore, burns more total calories and total fat. If you’re fasting and hypoglycemic, your energy levels will be lower, and you won’t be able to work out as hard, which will mean that fewer overall calories and fatty acids will be burned. You’ll also miss out on an elevated metabolism post-exercise.

Studies on training in a fasted or fed state demonstrate that individuals can train more intensely if they’ve consumed a pre-exercise meal. Other studies have shown that consuming a small meal prior to exercise burns more calories in the post-exercise period than exercising in a fasted state. Eating a little food before exercise helps you work out harder and burn more calories and fat, both during and after your workout.

It’s important to note that you can’t exercise your way out of a bad diet

Getting Yolked

Trying to gain muscle? Exercising in a fasted state will most likely not support this goal. In general, training in the fasted state, at a higher intensity, increases muscle tissue breakdown.

This is especially true with cardio. Consuming some carbohydrates and/ or a high leucine food such as whey protein prior to performing moderate to intense cardio can slow muscle catabolism. With strength training we are more concerned with muscle growth after training. Eating a mixed carb/protein meal or a high leucine meal prior or within 60 minutes of strength training will enhance muscle growth.

Also, don’t forget doing heavy lifts and pushing harder will be difficult when you’re fasting, and this stimulus is needed to build muscle. If you’re working to increase performance and/or build a rock-hard physique, fueling before your workout will be beneficial and help you reach this goal.

Firefighter Specific Concerns

Your job as a firefighter is stressful: you’re sleep deprived, you’re juggling family and a social life with an unpredictable schedule, the tone could drop at any moment forcing your brain to be on constant alert, and you’re exposed to difficult situations on a regular basis. So, suffice it to say, your nervous system is most likely already out of balance and tipped towards a more stressed state. Is it a good idea to pile on more stress? Probably not. Exercise is a stressor and fasting is also a stressor. Both trigger a hormetic response, which is an adaptive response that strengthens the body. For example, when you lift weights, it causes small injuries to the muscle, which stimulates repair and adaptation to be able to lift a heavier load in the future. The key to keeping this a strengthening response (and not a depleting one) is the dose and the frequency. Exercise in excess, or not building in enough recovery time between training sessions (think overtraining), begins to break down

your body and weaken your defenses over time. Have you ever trained too hard and began getting random injuries or never feeling fully rested? The same goes for fasting.

If your body is already under too much stress, fasting can worsen your sleep, leave you cranky, deplete your energy, weaken psychological resilience, and cause weight gain. So as a firefighter, I’d caution you to listen to your body when it comes to both exercise and fasting, and especially if you decide to combine the two. Pay attention to your sleep, your moods, your energy, your strength, and your weight/body composition. If you notice a negative trend in any of these, consider backing off on potential stressors.

And since I’ve mentioned stress, starting a daily meditation practice will enhance mental and physical performance, support weight loss, improve sleep quality, and much more. I love the 12-minute program in the book Peak Mind by Amishi Jha.

Ok, that’s the science. Whether or not you train in a fasted or fed state is ultimately up to you.

It’s your body, and you know your body best.

HERE ARE A FEW GENERAL RECOMMENDATIONS BASED ON THE RESEARCH TO SUM IT UP:

TO DO FASTING:

Low-intensity exercise (30-minute walk or bike ride on a flat road, light yoga).

TO LOSE WEIGHT AND BODY FAT:

Best to work out at a moderate to high intensity or perform HIIT type training and eat a small meal such as a shake* within a few hours before training.

TO IMPROVE PERFORMANCE:

For all types of exercise (weight training, running, cycling, etc.) consume a small meal such as a shake* within a few hours before training. Depending on intensity and duration of exercise and temperature/humidity, replace electrolytes and fuel with carbohydrates.

TO PREVENT MUSCLE LOSS:

For cardio, consume a small meal such as a shake* within a few hours before training. For weight training, consume a small meal such as a shake* within a few hours before training or within 60 minutes after training.

TO GAIN MUSCLE (WITH WEIGHT TRAINING):

Consume a small meal such as a shake* within a few hours before training or within 60 minutes after training.

*mixed carb/protein or high leucine such as whey protein