The science of bioenergetics establishes that the energy derived from carbohydrates is quick and is the preferred substrate for power and speed. It can also be metabolised in the presence or absence of oxygen. In comparison, energy derived from fat requires the availability of oxygen and the power output is slow.

The downside, however, is that our bodies have a larger and almost unlimited store of fat, but a very limited amount of carbohydrate, which is stored in the body as glycogen. For instance, a 70kg man with say 20% fat has 14kg of fat. They will also have only 450g glycogen stored in the muscles, and about 80g glycogen stored in the liver.  

Carbohydrate (glycogen) stored in the liver is converted to glucose and is made available to the body for its cellular energy needs. The brain depends entirely on glucose for its functioning. When we sleep at night, the liver exhausts its supply of carbohydrates by about 60% to 80%. This is because it is used by the body cells all through the night. On waking up in the morning, we have a low supply of carbohydrates in the liver. If you begin your workout on an empty stomach, the muscles will utilize glycogen that is stored in the muscles, but soon the reserves in the liver dry up — ie, the liver then turns towards muscle glycogen to break it down and meet the energy needs of the brain and other cells.

Also read: Dietary Fats: How to Choose Right and Eat Smart

Thus, as you continue your exercise your glycogen stores start dipping and the brain sends out a distress signal, which will get you to reduce your effort. This is a stage where you end up feeling fatigued and you struggle with your run.

The option for the body is now to turn towards its fat stores to provide the necessary fuel for your run. However, as we have mentioned earlier, fat metabolism happens in the presence of adequate oxygen and this results in slowing you down. This is because fat metabolism requires that your effort level is about 65% to 70% of your heart rate max — and is called a conversational pace, as shown in the graph. 

In other words, you will not be able to execute fast runs or speed workouts when you run on an empty stomach. It has been suggested that training the body to burn fat will benefit you by sparing carbohydrates, thus making your runs last longer. However, this requires a long period of adaptation and cannot happen in a few runs. Also, remember that what is a fast run is relative. A marathoner with a three-hour time for a full marathon (FM) may put down a 5 min/km pace as easy. But a marathoner with a four-hour time of FM, will find the same pace too fast. So, what you need to consider is your effort and not the pace.

How to teach your body to burn fat

1. There are a few things to note in case you want to attempt running on an empty stomach to teach your body to burn fat.

2. Maintain an effort level, where you are able to talk during the run. If you would like to quantify this with a heart rate monitor, then it is suggested you run at about 60% to 70% of your heart rate max

3. Ensure you hydrate well before and during the run because dehydration along with low carbohydrate levels will compound your fatigue and cause extreme exhaustion

4. There should be a gap of at least about eight hours between your dinner and your run the next morning

5. Train your body to handle low carbohydrates in a progressive manner. Begin with a 30 to 40-minute run and then progress to an hour with a cap of 10% increase. Once you have adapted to this, you may attempt to do your long runs that last 90 minutes to two hours

4. Try to limit these runs to a maximum of twice a week. This will allow you to train more for speed on other days by taking in nutrition before and during your run

5. When carbohydrate stores are depleted, your blood sugar drops and the immune system is compromised because it has to repair muscle tissue that is stressed during the workout. For this, the immune system looks for glucose to feed the muscles and the non-availability of this substrate means that the restoration of equilibrium in the body would not be optimal. Thus, you should make sure you eat a quickly-absorbed carbohydrate meal as soon as you finish your runs on an empty stomach.

While you may choose to experiment and dabble with running on low carbohydrate stores, you should be aware that this will not have a very large impact on your performance. This is because in order to improve performance, your workouts have to be aggressive on speed and endurance, and both these are compromised when you run on an empty stomach. The only advantage with this approach is that you may not feel the distressing effects of low carbohydrate stores when running a marathon or an ultra-marathon.