Hybrid cars are the future. They run on petrol/diesel as well as electricity. When the vehicle is being driven comfortably and easily, it runs on electricity. The moment the accelerator is pushed, the car automatically switches to fuel.
Our bodies are a bit like these hybrid vehicles when it comes to performing endurance activities. It depends on two sources to fuel these activities: fats and carbs.
So does the body use both fats and carbohydrates simultaneously? Or does it use carbs and fats independently? Let’s find out.
Fat: The Slow & Steady Fuel
Fat is stored as triglycerides in your fat tissues and within your muscles. Even a lean, well-trained runner carries tens of thousands of calories stored in the form of fat. It is effectively an inexhaustible tank of energy. The problem lies not in its availability, but the speed at which it can be converted to energy and delivered to your working muscles. Fat requires more oxygen and more processing steps to be converted into usable energy. It is a time taking process.
This is why the fat oxidation system works well at easy paces- a slow jog, a recovery run, a gentle long run when your body has time to process it steadily.
At easy effort, fat handles roughly half your energy needs. That is why easy running builds your aerobic base. You are training your fat-burning system to work efficiently.
Carbs: The Fast Fuel That Falls Short
Carbohydrate is stored as glycogen in your muscles and liver. It converts into energy quite quickly, much faster than fat does. This quick delivery of energy is exactly what your body needs when you are running at marathon pace or faster.
The problem is that your glycogen stores are limited. Most runners carry enough glycogen to last roughly 75 to 90 minutes at race pace. After that, the tank begins to run low on reserves.
At moderate to high intensity exercise, carbohydrate becomes the predominant fuel because it can be converted into energy faster than fat, and at race pace it provides the vast majority of the energy your muscles need.
What Happens When Carbs Run Out
When glycogen runs low your body, you cannot simply swap over to fat and carry on at the same speed. Fat is available but it is too slow to power race pace effort. So your pace drops. Your legs go heavy. Your brain starts negotiating.
Runners call this the wall. It is not a mental weakness or a fitness failure. It is just your carb tank hitting empty while the fat tank cannot cover the shortfall quickly enough.
What This Means For You As A Runner
3 things change when you understand how these energy production systems work
- Your easy runs are largely fat-fuelled and that is exactly the point.
A 45-minute recovery jog at conversational pace does not significantly deplete your glycogen. You do not need a gel. Your fat system is handling the work. This is also why easy running builds your aerobic base. You are training the fat-burning system to become more efficient, which has a small but real effect on delaying when carbs become the dominant fuel as intensity rises.
- Your long runs and race efforts are carbohydrate-dependent.
The moment you push past easy pace and toward marathon effort, the equation flips. Carbohydrates are now doing the majority of the work. Glycogen is the limiting fuel. Highly trained endurance athletes metabolise fat more efficiently and rely less on carbohydrate during submaximal efforts but at high intensities, carbohydrate remains the predominant and obligatory fuel regardless of training status. No amount of fat adaptation changes this at race pace.
- Fueling early is the way to go.
By the time you feel your energy fading at kilometre 28 or 30, you are already well past the window to prevent glycogen depletion. The carbohydrate you take in now takes 15 to 20 minutes to reach your working muscles. You are fueling for where you will be later in the race, not where you are right now. This is why the first energy gel goes in at kilometre 7 to 8, not kilometre 20 when things start to feel hard.
To know more about how many gels or carbs you need for your marathon, head here.
