Principles of Exercise During Intermittent Fasting

fasting and exercises

If you’re just starting out with intermittent fasting, you probably have questions — questions such as, “Can I exercise while fasting?”

It’s a logical question; after all, your body needs fuel if you’re going to work out and fasting depletes your fuel stores, right? So working out might not be such a good idea… right?

Not necessarily. In fact, scheduling physical activity during fasting periods can have a profound positive effect on your body and your health. Here’s what you need to know.

Benefits of Physical Activity While Fasting

exercise while intermittent fasting

Increased fat burning is arguably the biggest benefit of scheduling your workouts while you’re fasting. A 2013 study from Northumbria University found, for instance, that simply exercising before breakfast rather than after allowed people to burn 20% more body fat.

More lean muscle is also a major benefit for a variety of reasons. More lean muscle = better performance. More lean muscle also means your body will burn more calories not just during your workout, but throughout the day since muscle needs more fuel than fat. Exercising while fasting can also improve VO2 max, which is essential for endurance workouts.

How Exercising While Fasting Affects Your Body

Engaging in physical activity while you’re doing IF has a profound physiological impact on your body. Not only does it help you burn more fat, it affects how your body responds to insulin, lowers your blood sugar, increases your body’s production of human growth hormone (we’ll explain why that’s important in a moment) and increases the amount of testosterone you produce. Yes, ladies, even for you — and that’s a good thing. It also revs up fat oxidation, boosting the potential for weight loss.

Let’s look at each of these points in more detail.

Fasted Exercise Improves Insulin Sensitivity

As anyone who is diabetic (or has a friend or relative who’s diabetic) knows, insulin is the hormone that controls how much glucose (sugar) is floating around in your blood. What you may not know, though, is that insulin also tells your body when to start accumulating fat, and that it also prompts your muscles to soak up glucose from your blood and store it for fuel. 

Here’s how it works and why insulin sensitivity is important even if you’re not diabetic.

When you eat, your blood sugar — in the form of glucose — rises. Once blood glucose hits a certain level, your body releases insulin. The insulin tells your muscle cells to pull glucose out of your blood, which the muscles then store as glycogen. When your muscles can’t absorb any more sugar, your body stores it as fat.

Poor insulin sensitivity, or insulin resistance — a problem which affects one in three Americans — means that not only is your body left with more excess glucose which it must store as body fat, but that your muscles aren’t getting the fuel they need in order to function at their best.

A growing body of research shows that exercising while fasting makes your body more sensitive to insulin — a big plus even for those with normal insulin sensitivity. This, in turn, means less weight gain/easier weight loss, better glucose control for those who are diabetic, pre-diabetic, or insulin-resistant and more balanced blood sugar levels overall.

Human Growth Hormone Production Increases — and That Means More Muscle

Human growth hormone (HGH or GH) is essential for repairing and regenerating tissue throughout the body. GH is involved in building muscle and in helping your body recover after exercise, and the production of this important hormone usually increases after a workout.

Insulin, however, restricts how much human growth hormone your body can make. In other words, the more insulin your body is dumping into your bloodstream, the less GH you’re producing. This not only slows down your recovery post-exercise but makes it harder to build lean muscle.

According to studies, exercising while in a fasted state can dramatically increase your GH levels. One study found that human growth hormone production increased by an astonishing 2,000% in men and 1,300% in women after a 24-hour fast. And while the shorter fasting period you practice in IF may not provoke quite such a profound response, the effect is still impressive.

 Testosterone Levels Rise

While we most commonly associate testosterone with men and male characteristics such as deep voices and facial hair, testosterone does more than make men look and sound like men; although they don’t produce nearly as much of the hormone as men, women’s bodies do also make testosterone.

So why is more testosterone a good thing in the context of exercise? Because we need testosterone in order to build lean muscle. Higher testosterone levels also mean higher energy levels — and less storage of body fat.

Fat Oxidization Is More Efficient So You Burn More Fat

Research shows that intermittent fasting practiced on its own helps you lose weight. Combining exercise with intermittent fasting can make it even more effective. Why? When you exercise, your body turns to the most accessible fuel source first. This means that it will first burn any glycogen stored in your muscles. Only after those glycogen stores — and the additional glycogen stored in your liver — are used up will it turn to burning fat for fuel.

When you work out after you’ve been fasting for some time (midway through your fasting period, for example, or right before your eating window), your body has already used up some — and sometimes most — of your stored glycogen. And once you have no glycogen left to burn, your body turns to fat. When you do this consistently, then over time your body adapts to the process and “learns” to tap into fat stores and burn fat more efficiently.

The takeaway: Getting your workout in while you’re fasting rather than feasting can get you better results. The physiological changes that take place can increase your body’s fat-burning capability while at the same time boosting your ability to build lean muscle.

Intermittent fasting and exercise

When Should You Eat?

There’s no one-size-fits-all rule for when you should eat in relation to when you work out; it depends on your individual body, your workout goals and what you do eat during your eating window. However, a good rule of thumb is to train before you eat if you’re doing cardio (thus maximizing its fat-burning potential), and to save your HIIT workout for 1–2 hours after you eat for best results.

How Your Body Reacts to IF Combined With Different Types of Exercise

In a nutshell: intermittent fasting and aerobic exercise both reduce your blood sugar, whether practiced separately or combined. HIIT (High-Intensity Interval Training) workouts, on the other hand, can raise blood sugar even when you’re working out while fasting.

The Final Word: Listen to Your Body

Get used to the routine of fasting before you add fasted exercise to the program, and don’t overdo it in the beginning. Jumping into a too-strenuous routine could lead to low blood sugar, so be alert for the signs of it (weakness, dizziness, “brain fog,” faintness, etc.) If you do find yourself feeling dizzy or faint, try a carbohydrate-electrolyte drink — and lighten up during your next workout.

Staying hydrated is also doubly important; it’s very easy to get dehydrated when you’re fasting, and adding physical activity to the equation makes it even easier. Drink more water than you think you need, and stay away from sports drinks unless your blood sugar drops uncomfortably low.

So, can you exercise while fasting? Yes. Should you exercise while fasting? Absolutely — but listen to your body.