Intermittent Fasting and Exercise

Intermittent fasting is on the rise. The increasingly popular diet plan, which involves periodically switching between eating and abstaining from food entirely, for hours or even days at a time, is not only an effective way to manage weight, research suggests it can even lower high blood pressure, reduce oxidative stress and boost brain health.

But if food is what fuels our workouts, is it possible, or even healthy, to maintain a strenuous exercise program while intermittent fasting?

In short - yes, as long as you are careful, it can be very effective. Exercising on an empty stomach is not only safe, but it can also bring its own benefits.

When intermittent fasting, it’s not about what you eat, but when you eat. By restricting eating to specific hours of the day or days of the week, the body exhausts its sugar stores and starts burning fat. Beyond helping you to ‘lean up’, this biological process, known as ‘metabolic switching’, has been found to trigger numerous other beneficial reactions in the brain and body. Research demonstrates that combining the benefits of the metabolic switch with exercise, can actually ‘supercharge’ the effects.

When fasting is combined with HIIT training, it has been found to further increase Human Growth Hormone (HGH) production post-workout. Not only does HGH help build muscle, but it also increases sensitivity to insulin, which is associated with an increased life span and a leaner body composition.

For a number of years, elite athletes have used fasted training specifically to improve performance over time, by working out in a fasted state in the morning before breakfast. This has been standard practice among elite cyclists who will do long low-intensity rides, often for several hours, while in a fasted state. They do it to increase mitochondrial growth, which occurs during fasted training and leads to greater aerobic fitness. Scientists say that for the maximum benefit, the pre-workout fasting period should be 16-18 hours, but even after just abstaining overnight, you will feel the benefits.

It is a commonly held misconception that exercising on an empty stomach will cause your muscles to shrink as protein gets broken down. In fact, it is the far more abundant fat reserves that are burned when the fuel runs out. However, there are a few points of caution. When doing fasted aerobic exercise, it’s important to keep the intensity very low. It can be easy to creep into the anaerobic threshold on a long steady workout, but do that for too long in a fasted state, and you will quickly ‘bonk’ as you tear through the last of your glycogen stores. This can lead to dizziness, disorientation, and the inability to take another step until you eat some sugar. High-intensity workouts such as HIIT training or weight lifting in a fasted state are fine because high-intensity workouts are shorter and require energy only in bursts.

Ultimately, it’s all about timing. The beauty of intermittent fasting is that you can choose whether to exercise before, during, or after your eating window. Consider which period will suit different types of workouts. If you want to go for a brisk run or set a new 5k PB, it’s probably best to do it during or just after your eating hours to ensure you’re well-fueled. If you are doing a long slow jog or cycle, or a short HIIT workout, you will be better off waiting until the end of your fasting window. It's about finding what works for you. If you never get comfortable exercising on an empty stomach, just keep your workouts within fueled hours.

As with any healthy diet and exercise plan, listen to your body, and back it up with good sleep and a balanced diet.