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Exercising While Fasted: Dietitian's Opinion

Considering exercising while fasted? Read this blog to be sure exercising while fasted will help you meet your goals.

Runner exercising while fasted



Why are people interested in exercising fasted?

It's all to do with energy stores. Your body has 2 main energy sources. Carbohydrates  and fat. Your body stores around 300g of carbohydrate as glycogen. 300g of carbohydrate can only sustain 1 hour of exercise. Fat on the other hand is abundant in our bodies. Even the leanest individuals have kilos of fat! Fat has more calories per kilo than carbohydrates too, 1kg of fat is 9000kcal compared to 4000kcal in 1kg of carbohydrate. As a result if we can train our bodies to use fat instead of carbohydrates we can tap into big energy reserves, lose body fat and are not reliant on eating during endurance exercise to top up carbohydrate stores.

When you are fasted your carbohydrate stores are reduced and your body is forced to use more fat as fuel. In the graph below you can see individuals fat burning increased when they exercised in a fasted state. This could help you burn fat and could improve performance in endurance events.


What happens to your body when you exercise fasted?

Your body gradually becomes better and better at using fat as fuel. At the same time your body uses less carbohydrate as fuel. It does this by making more cellular machinery and enzymes needed to metabolise fat and stops maintaining the carbohydrate machinery.

You can literally think of this like a fat factory getting bigger and investing in more staff and technology, and a carb factory having no new investment and technology breaks and staff leave.

Fat needs more oxygen to metabolise than carbohydrates. It is also located away from the muscles and transport to the muscles takes time. This all makes fat a slow release energy source. As you become more adapted to burning fat as a fuel you make energy more slowly. This means you can lose top end speed and power.

On the flip side, you are capable of burning more fat at low intensity exercise. This can mean you require less nutrition during long events and are more capable of losing body fat.

When you are fasted your bodies carb stores are reduced. Your carb stores have a direct impact on your sensation of fatigue. Look at the diagram below. Glycogen stores and blood glucose both contribute to fatigue. This means exercising when fasted will likely feel harder and have a different training stimulus on your body.

exercising while fasted effect on fatigue
Image from: Newsholme, E. A., & Blomstrand, E. (2006). Branched-Chain Amino Acids and Central Fatigue. The Journal of Nutrition, 136(1), 274S-276S.


When is exercising fasted useful?

In elite sport it is debated whether improving the bodies ability to utilise fat is useful. Carbohydrates always provide more, faster release energy than fat. In events less than 3 hours it is possible to fuel before and eat during to provide enough carbohydrates to sustain high intensities. Beyond 3 hours and in multi-day events there is less evidence, but meeting carbohydrate demands becomes harder; making adaptations to fat more and more relevant.

There are anecdotes of ultra marathon runners using high fat diets to complete 8+ hour runs as they were unable to eat sufficient carbohydrates. There are also reports of polar explorers relying on high fat diets due to multi-day, high energy expenditures at a low intensity.

Becoming fat adapted through exercising while fasted can improve your ability to lose body fat. This is because you use more fat, but also you are more likely to be in a calorie deficit due to the fasting.

If you are burning more fat as fuel you will have more ketones in your blood. These are the form fat is burned as energy in our brains. Some people report increased concentration with higher ketones in their blood and may like fasted exercise for this reason.

Finally, exercising while fasted may be beneficial if there is no other way to fuel before your event. It's not ideal, but may be necessary.


Final Opinion

Exercising while fasted increases the bodies ability to use fat as a fuel during exercise. However, this comes at the cost of fast release energy and top end speed and power. Despite anecdotes, in my opinion, carbohydrates will always out perform fats. Training your body to use and digest carbohydrates during exercise will lead to enhanced performances. I have a gut training blog and nutrition plan to enable you to fuel events with carbohydrate for better performance. There are some unique events where meeting carbohydrate demands are impossible, for example rowing across the Atlantic. These may benefit from fasted training.

If your goal is not performance, but focus or fat loss, exercising while fasted may well help you achieve these goals.

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