We've all heard of 'carbing up' before a big race, but what about training on an empty stomach? Are there any benefits? We explore fasted cardio. 

What is a fasted workout?

Fasted cardio refers to doing a cardiovascular exercise like running or cycling on an empty stomach. Any workout can be done fasted, but fasted cardio is generally thought to have the most health benefits. It is usually done in the morning before breakfast as this is when your stomach is at its emptiest.

What are the benefits of doing fasted cardio?

Increased Insulin Sensitivity

When we eat, our body produces insulin to help us absorb nutrients from our food. Insulin takes the glucose (sugar) from our bloodstream and directs it to the liver, muscle and fat cells to be used as energy. When this energy is not used it remains stored as fat. Eating too much sugar without burning it off can make us more resistant to insulin’s effects and this is where health issues like Type II diabetes begin. Poor insulin sensitivity increases the risk of heart disease and cancer, as well as making it difficult to lose body fat. Fasting means the body produces insulin less often, so our sensitivity increases and these health risks reduce.

Increased Human Growth Hormone

Fasting can promote muscle gain and fat loss due to the increase in human growth hormone. Growth hormones help the body make new muscle tissue, burn fat, and improve bone quality, physical function, and longevity. A study by Intermountain Medical centre showed that 24 hours without food increased men’s human growth hormone by 2,000% and women’s by 1300%. This is a huge increase which could significantly improve your training.

Better Muscle Building

A combination of growth hormone and testosterone makes the perfect muscle building environment. Whilst fasting increases growth hormone, it doesn’t increase testosterone. This is where training comes in, in order to gain muscle, big compound lifts (deadlifts, squats) which activate multiple muscle groups in unison cause a dramatic increase in testosterone. Increased testosterone enhances muscle mass and reduces body fat while also improving energy levels, boosting libido, and even combating depression and heart problems—in both men and women.

Weight Loss

The theory behind training fasted for weight loss is that instead of burning food for fuel, the fasted state promotes fat to be burned as fuel during exercise. A recent study from the UK published in The British Journal of Nutrition found that when subjects had fasted before morning cardio they burned 20 percent more fat than when they had a meal beforehand. Fasted training also ensures the body absorbs the food eaten after more effectively, by ensuring macronutrients are delivered to the right places rather than being stored as fat.

Improved Endurance

There are also potential benefits for endurance athletes as fasted workouts can improve muscle glycogen storage efficiency. What this means is that the body learns to make better use of its energy stores. The occasional fasted training session can help improve your performance during regular fed training. When the body learns to exert itself without any food, it gets better at exercising when it does have fuel in the tank. Other studies also suggest that fasted training can increase your VO2 max, which is the maximal amount of oxygen our lungs can uptake at any given time, the better this is, typically, the higher the level of fitness.

Is fasted cardio good for weight loss?

Although there are many benefits for training fasted, it’s not without its setbacks. When glycogen is in short supply, your body also reverts to breaking down protein and eventually your muscles for fuel. So whilst you may burn more fat you may also burn muscle too, crucial for maintaining a higher BMR, greater bone density and strong, lean physique. Research also suggests that training fasted for long periods of time may slow your metabolism, making it more difficult to burn calories in the long run. This happens because your body adapts to the number of calories you give it in an effort to beat starvation.

A study by Pennington Biomedical Research Centre discovered that after 22 days of fasting every other day, both men and women encountered a fall in their BMR (base metabolic rate) of 5%, equivalent to 83 calories. Meaning in just over 3 weeks their bodies were already burning fewer calories.

In addition, motivation can be hard to find when your stomach is rumbling and you're low on energy and the strength you produce in your sessions may not be as high as when you are in the fed state.

Whilst fasting before training does have its advantages, it’s also fair to say it may not be the best training protocol for everyone. If you want to try it we suggest adding one or two fasted sessions into your routine per week and comparing your results to your fed sessions. Keep an eye on your weight and record levels of energy to see whether this is the workout protocol for you. Research has proven that you don’t have to eat before training, although fasting can be a difficult protocol to master as eating is our most basic human habit. If you find you get results, stick at it, but be careful of over using the principle and lowering our BMR.