When it comes to nutrition, there’s usually a heated debate about the best way to fuel for any particular situation or outcome. Fueling for exercise is no different. And like every other topic in nutrition, the answer is nuanced. How you eat around exercise depends on your goals, the type of movement you’re doing, your exercise intensity and duration, your blood sugar regulation (i.e., metabolic flexibility), and your regular diet.

Defining Different Types of Fasting

For the purposes of this discussion I’m considering the most common type of fasting, which is time restricted eating (TRE). I consider TRE an overnight fast of at least 13 hours, leaving an 11 hour window for eating. And most commonly TRE involves fasting for 14 to 16 hours overnight, allowing for a 10 to 8 hour eating window, respectively.

TRE is in included in the more general term intermittent fasting (IF), but IF can also include eating just one meal per day, or two nonconsecutive days per week of caloric restriction (i.e., a total of 500 calories for the day), and a few other variations. Intermittent fasting is nonspecific, so I try to use terms that are more clearly defined.

Why Fast Anyway?

I’m guessing that most people reading this already know that time restricted eating along with other types of more prolonged fasting, may offer benefits for cognitive health, cardiovascular health, cancer risk reduction, mitochondrial health, and improved metabolic flexibility. Because so many of us are focused on improving healthspan and preventing chronic diseases, TRE is popular amongst health conscious individuals.

Since many of us practice TRE for its myriad of health benefits, and because some of us fit exercise into our morning routines, we are often tempted to exercise while fasting. But this may not always be the best option depending on your goals.

What Are Your Exercise Goals?

Let’s consider a few common reasons we exercise:

1. Lose fat to improve body composition.

2. Increasing lean body mass (i.e., muscle mass and bone strength).

3. Improve metabolic flexibility.

4. Peak performance, such as with competitive athletics.

5. Because it feels good.

Now let’s consider each of these goals to see when you should be eating around exercise.

Will You Lose More Body Fat If You Work Out Fasting?

The short answer is no, but this depends on 3 main factors: Exercise type, intensity, and duration.

When you dial up the intensity of the exercise such as running, lifting weights, or high intensity interval training (HIIT), your main fuel is glucose. After a night of sleep, glycogen (a stored form of glucose) is 50% depleted. If you’re already on a low carb diet, you’re already starting at a pretty low amount. If you’re trying to do higher intensity exercise or lifting weights or a longer run, you won’t have the fuel to go as hard or as long.

Why A Greater Percentage Fat Burned Doesn’t Always Equal More Body Fat Lost

It’s true that you burn a greater PERCENTAGE of fat when working out at a lower intensity (think walking or jogging). The problem with this theory is that if fat loss (and weight loss) is your goal, low intensity exercise is not the most effective exercise.

There are 2 important pieces of the fat burning puzzle that were ignored:

● First, the effect of the percentage relative to the number of calories you burn during exercise. 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 those 500 calories from fat may be lower, the total amount of fat burned is higher than the total amount burned in the low intensity exercise.

● The second consideration is the number of calories you burn after exercise, or excess post-exercise oxygen consumption (EPOC). A good analogy to this is when you run up a flight of stairs. You’re going to 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 your body’s demands for recovery and repair for several hours after 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. Studies have also shown that when you consume a small meal prior to exercise you burn more calories in the post-exercise period than if you exercise in a fasted state. Bottom line, a little food before exercise helps you work out harder and burn more calories (and fat) during and after your workout. BUT, if your exercise intensity and/or duration is low, exercise while fasting is a good option. A quick note, about 80% of your cardiovascular training should be in Zone 2 (the intensity where you can still carry on a conversation with ease, but if you work any harder, you will start to become breathless) and 20% should be high intensity (think interval training). More on this topic in a future post.

I do also want to point out that you cannot exercise your way out of a bad diet. If your goal is weight loss and/or achieving a lean muscular physique, your diet has to reflect these goals.

If Your Goal Is Increased Muscle Mass, Fasting Workouts Are Not the Best Option

If you’re trying to gain muscle, exercising in the fasted state will most likely not support this goal. There is some evidence that fasted strength training increases muscle growth post-exercise. The caveat to this study is that participants consumed a protein/carb drink high in leucine (a branched chain amino acid) shortly after training. Leucine specifically promotes protein synthesis and prevents muscle break down. Exercise in itself stimulates muscle catabolism (muscle tissue breakdown). This is especially true with endurance cardiovascular exercise. Consuming some carbohydrates and/or a high leucine food, such as whey protein (whey has more leucine than collagen) prior to performing moderate to intense cardio can slow muscle catabolism.

With strength training we are more concerned with muscle growth after training. Everyone wakes in a catabolic state. Eating protein prior to exercise will not reduce muscle breakdown. However, eating adequate protein after a meal (it’s recommended to eat 1.6 g/kg, or about 0.7 grams per pound, body weight daily split between 3 meals with 40-60 grams of protein in your morning meal). Untrained individuals should eat within a few hours post exercise. For trained individuals the meal timing does not seem to be as critical, however, protein, and specifically the amino acid leucine, will stimulate muscle growth.

Eating some carbohydrates prior to training if you’re doing moderate to high intensity exercise (such as HIIT and heavy lifts) may improve the quality of your workout. I mentioned above, doing heavy lifts and pushing harder will be more difficult when your glucose stores are low, and this heavy stimulus is needed to build muscle and increase bone strength.

If You’re Exercising To Improve Metabolic Flexibility

Metabolic flexibility allows our cells to switch easily between using carbohydrates and fats as fuel. We can equate metabolic flexibility with stable blood glucose levels and insulin sensitivity (i.e., glucose moves easily into cells after eating carbohydrates).

Exercise offers a number of benefits towards improving metabolic flexibility:

1. Using your muscles will increase the uptake of glucose by those skeletal muscles.

2. After exercise, skeletal muscle takes up more glucose, and does so for several hours.

3. High intensity interval training (HIIT) effectively lowers fasting insulin levels (a marker of better insulin sensitivity).

4. Exercise, especially strength training, increases muscle mass, and since muscle is more metabolically active, this alone can improve metabolic flexibility.

So what does this mean in terms of eating before or after exercise?

All of this means that your body will handle carbohydrates best right before or after exercise. So let’s say you want to go out to dinner and enjoy some high carbohydrate foods but also keep some balance to your blood sugar. In this case you might enjoy a moderate to heavy strength training session one to three hours before your meal, to increase your muscle’s glucose uptake for several hours. Or you could enjoy a 20 minute walk after dinner.

Something as easy as a 15 minute walk after a meal can dramatically improve your blood glucose response to that meal.

Going For Gold

If you’re a competitive athlete or looking to set a personal record (PR) for your workout that day, then you’ll want to fuel up before your workout. Nutrition for peak performance is beyond the scope of this article, but know that if you’re going to push yourself, then your body benefits from the fuel to do this.

To Eat Or Not To Eat Before Exercise

So, should you eat before you exercise? Again, this depends on your goals, your blood sugar regulation, what your typical diet is like, and the type, duration, and intensity of your exercise. In general, as intensity goes up, you’ll rely more on glucose (and phosphocreatine) as fuel. At lower intensities you’ll use more fatty acids to fuel your exercise. If performance is important, and your activity requires moderate to high intensity, you will benefit from more carbohydrates in your diet in general, but also prior to activity (and depending on the duration of the activity, you may benefit from carbohydrates during exercise).

That’s the science, but 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.

Is It Good to Work Out While Fasting?