If you’re a beginner to intermittent fasting, you may have some questions, and “Can I exercise while fasting?” might be at the top of your list. If so, you’re not alone.
It’s a logical question. You need fuel to work out and it seems like fasting will deplete your fuel stores. So you might think it’s best to skip your workout.
Not so fast. If you schedule physical activity during your fasting periods, it can have a profoundly positive effect on your health.
Exercise, Fasting, and You
When you participate in physical activity during IF, it can have a deep physiological impact. It will help you burn more fat, and it will affect the way your body responds to insulin. Exercise while fasting also lowers your blood sugar, increases your body’s production of human growth hormone, and increases the amount of testosterone you produce. If you’re female, you need testosterone too. When you exercise during a fast, it revs up fat oxidation, boosting the potential for weight loss.
Insulin is the hormone that controls how much glucose (sugar) floats around in your bloodstream. But did you know insulin also tells your body when to store fat, and prompts your muscles to soak up glucose from your blood and store it for fuel?
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.
Low insulin sensitivity, or insulin resistance is a problem which affects one in three Americans. It means your body is left with excess glucose which it must store as body fat, but your muscles don’t get the fuel they need to function optimally.
Research shows when you exercise during your fast, it increases your insulin sensitivity — a big plus if your insulin sensitivity is in normal range. If you’re diabetic, you may lose weight easier, and your blood sugar levels will be more balanced.
An Increase in Human Growth Hormone = More Muscle
Human growth hormone (HGH or GH) is essential for your body to repair and regenerate tissue. It also helps you build GH helps you build muscle and recover after exercise. Your production of this crucial hormone will typically increase after a workout.
There is an ideal ratio between insulin and human growth hormone that promotes optimal health. Exercise helps to improve this ratio by increasing the amount of GH in your system. Having high insulin and low GH leads to increased fat deposition in the body. When you have low GH, it can slow down your post-exercise recovery, and it can be more challenging for you to build lean muscle.
Studies show when you exercise in a fasted state, it can dramatically increase your HGH levels. If you practice shorter fasting periods you may not provoke a profound response, but the effects can still be impressive.
Testosterone Levels Rise
Testosterone isn’t exclusively a male hormone. Men typically have more testosterone which is what makes their deep voices and facial hair. Females produce testosterone as well, usually in a lesser quantity.
So why is more testosterone a good thing in the context of exercise? Because you need testosterone to build lean muscle. If you have a higher testosterone level, you’ll have more energy and store less body fat.
More Efficient Fat Oxidation
Research shows intermittent fasting alone can help you lose weight. But if you combine exercise with intermittent fasting, it can make it even more effective. Why? When you exercise, your body turns to the most accessible fuel source first, which is your muscles glycogen stores. Then you’ll use the additional glycogen stored in your liver. After you deplete all your glycogen, you’ll begin to burn fat for energy.
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 for energy.
The takeaway: If you exercise during your fast, you may see better results if your goal is fat loss. The physiological changes that take place can increase your fat-burning ability and help you build lean muscle.
Benefits of Physical Activity While Fasting
Increased fat burning is arguably the most significant benefit of scheduling your workouts during your fast. A 2013 study from Northumbria University found if you exercise before breakfast, you can burn up to 20% more body fat.
More lean muscle is also a primary benefit of physical activity during your fast. More lean muscle will give you better performance. And you’ll burn more calories 24 hours a day, not only while you workout. Muscle needs more fuel than fat.
How Your Body Reacts to IF Combined With Different Types of Exercise
In a nutshell: both intermittent fasting and aerobic exercise reduce your blood sugar, whether you practiced them separately or combined. But, some forms of exercise, such as HIIT (High-Intensity Interval Training), can cause an immediate rise in your blood sugar, because it prompts your liver to release stored glycogen. So, if you have diabetes, it’s vital you pay attention to your body regardless of what form of exercise you choose to use.
Why Working Out During IF May Not Work
While there may be some benefits to working out in a fasted state, there are some potential pitfalls.
Your blood sugar could fall dangerously low. Exercise promotes increased insulin sensitivity, allowing your muscles to absorb glucose more easily from your bloodstream. Exercising while fasting means there is even less glucose for your muscles to use, which could cause you to faint.
You could also experience a decrease in performance, especially if you are a trained athlete. Current data indicates either no change or a reduction in performance with fasting. A recent study recommends endurance athletes stay away from intensive training while fasting altogether.
Consider your fitness goals when deciding if fasting is right for you. If your main goal is to lose fat, fasting may work well for you. If you want to build muscle, be sure to eat enough during your eating window, or try a different eating plan.. Regardless, pay attention to any symptoms your body exhibits when you exercise, and don’t hesitate to ask your doctor if you have any concerns.
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 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.
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 initially. If you jump into a too-strenuous routine, it could lead to low blood sugar, so be alert for the signs of it (weakness, dizziness, “brain fog,” faintness, etc.) If you do feel dizzy or faint, try a carbohydrate-electrolyte drink – and lighten up during your next workout.
It’s crucial that you stay since it’s very easy to get dehydrated when you’re fasting. When you add physical activity to the equation, dehydration is even more likely. 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.