Fasted workouts - why and how to maximize fat burning

You hear a lot about fasted training these days. Everybody who means something in the endurance sports embraces some form of fasted training and low-carb approach in their training methodology. Team Sky, team Jumbo-Visma, Norway triathlon etc. But what does it mean, and what does it do to you? How to train fasted correctly and avoid negative effects?

Firstly, let me frame the problem. I assume we are all mostly age-group and amateur endurance athletes who want to achieve their best possible self. The basic principles described below work for everyone, but for elite athletes, it is a bit more complicated and nuanced.

What is fasted training?

Imagine you have had your dinner, then went to bed, woke up in the morning, had only a glass of water and went for a 3 hour bike ride. The overnight calorie restriction and no-breakfast scenario create a basis for fasted workout. By doing so, you enter the workout with a limited amount of glycogen in your muscles. During the workout, you only consume water or low-carb electrolyte drinks (Nuun, SoS), depleting your glycogen stores even further.

This way, you force your body to use fat as fuel even at higher intensities, and therefore improving your fat oxidation capacity, becoming a fat burning machine in the process.

The advantage of being a good fat burner shows in long endurance events (such as Ironaman, ultra running, stage racing, audax), where you can achieve one of two things: either you are able to utilize both carbs and fat at high intensity, therefore being able to go faster for longer, or you can sustain aerobic effort for extended periods of time without losing power and with limited carbohydrate supplementation, which works for people who have problems with carb digestion during long races. Let's break down the two scenarios:

The Ironman Scenario

You want to be as fast as possible during your Ironman race. Usually, during an Ironman, you would burn somewhere around 700-800 kcal per hour, so for a 9 hour Ironman, that is in the vicinity of 7000kcal. 9 hour ironman is a relatively high intensity effort. Under normal circumstances, you can consume only about 80-100g of carbs per hour, which is roughly 400kcal. That is about half of what you burn every hour, so the longer you go, the more you fade. You are just not able to sustain the energy deficit for such a long time. In comes fat oxidation.

Body has basically unlimited amounts of fat calories, as each gram of fat is 9kcal of energy. The game comes down to your ability to burn them.

Ability of the body to burn fat as fuel is called fat oxidation, and is measured in grams per minute. A 0,5g/min of fat oxidation is a good base level, but you can get to 1g if you are really really good. 30g of fat oxidation per hour is 270 extra calories of energy for free, 500 if you are really a beast. You basically remove the deficit by utilizing fat well, feeling much better in the last 10k of the marathon, and maybe even enjoying the last hour of the race! (just kidding, it is still miserable AF).

The ultra-distance scenario

In ultra-distance races (UTMB, Audax races etc) you usually go at low intensity for 24 hours or even several days. It is impossible to eat enough carbs over this period without getting sick in the process. In order to be able to finish well in such races, you need to be a fat burning machine. If you train for these races and have the needed volume, you will already be very well fat adapted anyway, but by correct fasted training, you can squeeze much more from your body.

So how do you train fasted?

To achieve results, you need to be methodical and do this over a long period of time, as is usually the case, but every single fasted workout by itself will help you. Start by a simple rule: Do not have a breakfast or snack before a workout, if that workout is less than 60 minutes regardless of intensity, or 90 mins of lower intensity. Make sure you hydrate before and during, ideally with an electrolyte drink, because with low-carb approach, your body needs significantly more electrolytes than usual. This works for runs, bikes, swims or strength. Doing that on most weekdays, you will be able to get fat adapted very quickly.

On top of this basic rule, try to do a fasted long workout at least twice a month. Long workout will be a 1,5-3 hour run or a 3-5 hour bike on no or little calories. Get up, have a coffee, electrolytes and go out. During the workout, you can only drink water and electrolytes, or if you are afraid, you can take up to 25g per hour of carbs (I recommend CLIF bloks for this, as they can be easily dosed). Take it easy and do not do a 5 hour fasted workout on the first try. Also, this is NOT a workout which should have intensity in it. Zone 1 or 2 maximum, easy conversational riding. Once you get better, you can keep adding intensity, but stay away from it for the first few tries.

Now we have one more rule left, the most important one: fuel correctly after the workout, and make sure you catch up on your calories over the next 48 hours. Do not try to stay in a caloric deficit, as this has numerous negative effects on you, from muscle mass loss, low energy levels, immunity issues and hormonal imbalances. Again: make sure you eat enough calories! Carry extra CLIF bars in your pocket for the next two days, and fuel up on the go. The fasted workouts, especially in the beginning, take a higher energy toll and you need to make sure you correct it as soon as possible.

Over time, you will see progress. After about 2 months, I was able to do a 3hr bike on black coffee only, in Z2 with some Z3 on hills, without any problems at all. No hunger, no lack of energy. The clarity this brings will be addictive. Have fun trying it out!