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PART 1. METRICS ALL RUNNERS SHOULD KNOW: FAT & CARBOHYDRATE UTILIZATION

  • Optimize Performance and INSCYD partner to BRING YOU metabolic testing that you can do remotely! Here is an article that speaks about important metrics you’ll receive in your data*
Runner who wears multiple sport gps watches

BY LOEK VOSSEN @ Inscyd

It almost looks like a new running metric is introduced every day. This makes it easy to lose sight on the metrics that are important and fundamental. In this blog we don’t want to get lost in details like cadence, stride length and ground contact time. Instead, we are going to look at the big picture and tackle the basics: the fuel that enables us to move forward. Understanding fueling in terms of fat and carbohydrate demands, related training intensities and refueling will make you faster runner – period!

As a runner, you probably know your (estimated) 10k time. Maybe you even know at which pace or intensity your Anaerobic Threshold occurs. But do you also know where the energy is coming from during training & racing? This is important because you might be aware that carbohydrate is a precious fuel because stores as well as intake rates are limited. Your energy metabolism highly predicts the actual performance during short(er) and long(er) efforts!

Two runners can have the exact same 10k time or Anaerobic Threshold, but still perform very different from each other during a marathon or half marathon.

WHERE IS THE ENERGY COMING FROM?

Your muscles need energy to move forward. Energy is produced by energy systems that need fuel. Let’s discuss the two energy systems of interest and their fuel.

  • 1. The system that prepares the carbohydrates to enter the Aerobic energy system is called the (Anaerobic) Glycolytic energy system. This system uses glucose and produces lactate to fuel the Aerobic energy system. It also creates (anaerobic) energy itself. The amount of lactate produced is proportional to the amount of energy this Glycolytic system produces. Therefore, the maximal lactate production rate – or VLamax (V= flux, La= lactate, max= maximal) – tells us how well this system works within an athlete.
  • 2. The Aerobic energy system uses oxygen to burn carbohydrates and fat. The amount of oxygen uptake in the body is proportional to the amount of energy this Aerobic system produces. Therefore, the maximal oxygen uptake rate – better known as VO2max (V= flux, O2= oxygen, max= maximal) – tells us how well this system works within an athlete.
How the aerobic energy system burns fat and carbohydrates via the glycolytic energy system that uses glucose and produces lactate
Image 1. The Aerobic system burns Carbohydrates and Fat to produce energy. The Glycolytic system prepares the carbohydrates to fuel the Aerobic system and produces energy in doing so.

WHY DO VO2MAX AND VLAMAX MATTER?

The Aerobic energy system can only process a certain amount of fuel. When more carbohydrates enter the Aerobic energy system, less fat is combusted (burned). When you know your VLamax, you know how many carbohydrates you combust. When you know your VO2max, you know how much additional fat you can combust. If you want to run very fast, you need to be able to quickly burn carbohydrates. The downside however is, that you don’t have an unlimited amount of carbohydrates available in your body. So if you want to maintain a high speed, you need to be able to burn enough fat as well.

USING FAT AND CARB COMBUSTION

Did you ever wonder why many runners bonk at 30k during a marathon? They run out of carbohydrates! Running out of energy will be history when knowing your Fat and Carb combustion rates. Let’s look at an example of an athlete who uses INSCYD.

EXAMPLE: RUNNING A MARATHON USING INSCYD

Our athlete wants to run a marathon in less than 4 hours. Therefore, the aim is to run almost 3 m/s (5:30 min/km). By using the INSCYD test results, we can see that the carbohydrate combustion rate at this pace will be ~160 grams per hour.

INSCYD software showing fat combustion and carbohydrate combustion for runners
Image 2. The x axis shows the running pace/speed/intensity, the right y axis shows the combustion rate. The red line describes the carbohydrate combustion curve. It shows our athlete burns 159 grams carbohydrates per hour at a running pace of 3.01 m/s.

This means that our runner will combust 160 grams x 4 hours = 640 grams of carbohydrates during the marathon. Using the INSCYD software and body composition of our athlete, we know our runner has 420 grams of carbohydrates stored in the body. If our runner does not consume any carbohydrates during the marathon, he will (indeed) bonk after about 2,5 hours (420 grams divided by 160 grams per hour), which equals about 30 kilometers when running 3 m/s. This would however not happen if he knew he needed to consume 640 – 420 = 220 grams carbohydrates during the marathon. In other words, the race fueling strategy contains 55 grams of carbohydrates per hour! Click here to read more examples.

Carbohydrates stored + Carbohydrate intake = Maximal Carbohydrate combustion

You can now easily find out what the fueling strategy should be and whether your goals are realistic or not. If our desired pace would for instance result in a carbohydrate intake of 100 grams per hour, we would know for sure that this would be challenging, if not impossible.

Want to run faster tomorrow? Increase your carb intake! Already reached the upper limit of carb intake? Increase your fat combustion via training, so that you don’t need to burn as much carbohydrates! No time to increase fat combustion anymore? Decrease your pace up to the point where your carb combustion meets your carb availability! Want to know how professional long distance triathlete Jo Spindler uses carb combustion rates as a professional coach? Read it here!

IN SUMMARY

Yes, there are many metrics that could be interesting for runners. We however suggest you start with the most fundamental metrics that determine the vast majority of performance: VO2max and VLamax. As a result, you get to know your Fat and Carb combustion rates. This not only teaches you about fueling strategies, but also about a realistic pace and what to work on when you want to increase this pace. In the next blog we will tell you more about the benefits from knowing your VO2max and VLamax when creating the most effective interval training. Read part 2 of this blog here!

Loek Vossen

LOEK VOSSEN

Human Movement Scientist at INSCYD

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