In the running community, the debate about fueling is probably some of the longest. After all, some follow nutrition based on what their coaches say since decades ago, while others try it on themselves, and share it with their friends. Over time, some became tried and tested, while others not exactly. So the big question is probably this:

Is there a correct answer?

Yes, and no. Everybody runs differently, and therefore fuels differently. As such, sports nutritionist like myself always plan fueling strategies based on individual data, and never in a cookie cutter fashion, causing the answer to something as simple to this to become more complicated. If you ask me, my answer is going to be “it depends”, and if you do a consultation with me, these are some of the questions that will determine my answer for you:

Long term goal (What you want to achieve through running)
Training session goal (Is this an easy run, intervals, or something else?)
The distance, duration and intensity of your training

Okay, cool. So Carbs or Fats?

To answer whether you should go for carbohydrate or fat based fueling strategies, we need to consider what the body needs. Therefore, the answer to this question really lies in physiology and biochemistry.

While some things can be left to debate, here are some undisputed facts known to science.

Fact #1: Your body uses both carbohydrates and fats at the same time

The difference is how your body use them, and when it happens.

between fat and carbohydrates, which fuel should i use for my runs? — **Figure 1:** How your body uses fuel at different training intensities (van Loon et al., 2001).

As you can see on Figure 1, the body uses both carbohydrates and fats throughout energy intensity. The main difference is how it prioritizes the percentage of fuel used.

When you further interpret the graph, the type of fuel you use for your runs really depends on your intensity. We typically use a higher to equal amount of fats and carbohydrates at lower intensities (depicted on 40-55 %Wmax), and as it increases, carbohydrates are used more than fats (depicted on 75 %Wmax).

What this means is simple: At lower intensities, your performance will not be affected by your choice of fats or carbohydrates, but as the intensity increases, your body will need to tap in for energy faster, and therefore will more likely use carbohydrates. If your body cannot tap into carbohydrate fuels at higher intensities, it will simply slow down due to the lack of fuel.

Fact #2: Between carbohydrates and fats, your body will generally choose carbohydrates as it’s preferred source of energy because science.

The advantage fat has over carbohydrates is the fact that it produces more energy per gram, and has a higher storage space in the body, giving you a theoretical unlimited source of energy for as long as you have fats in your body.

However, the body can process and use carbohydrates faster than fats at a higher quantity per second. For this reason, from a preference point of view, the body will prefer to tap into the carbohydrate source over fats. What this means in a mixed-diet scenario is simple: Your body will choose to utilize your carbohydrate sources before tapping into your fat source.

For this reason, individuals who are using running for weight loss goals would often consider consuming more fat and lesser carbohydrates to help the body become more fat adapted. However, this will only work well if you use the strategy in conjunction with lower intensity training. This does not mean that you must be fat adapted to lose weight. After all, the main principle in weight loss is to maintain a calorie deficit.

How do I use this information in a practical manner?

Understanding how the body works is the key to answering which fuel you should choose. Moving from there, you want to consider what your training and ultimate goals are. Some useful questions to start considering would be:

Are you trying to complete or compete in your race?
Is this mainly to help you lose weight, or are you trying to perform?
What kind of intensity are you training at during your runs?
Must you have one or the other fuel, or can you work with both?

As you run through the questions, reconsider your training scenario and performance data. If the intensity of your training needs to be kept high, you are probably going to use more carbohydrates while keeping fats to enough levels for health reasons.

However, if you are just trying to complete a run without hitting a personal best, or hoping to lose some weight, fats is a good option to aid adaptation. For preference and essential nutrients reasons though, you do still want to keep some carbohydrates available.

So, will you need more carbohydrates or more fats for your run?

Reference:

Van Loon, L., Greenhaff, P., Constantin-Teodosiu, D., Saris, W. and Wagenmakers, A., 2001. The effects of increasing exercise intensity on muscle fuel utilisation in humans. The Journal of Physiology, 536(1), pp.295-304.