Carbohydrate and fat combustion rates are among the most important features INSCYD can offer for runners, triathletes, and cyclists.

Here’s why: While fat combustion rates are a valid marker to monitor training progress and set training intensity zones, carbohydrate combustion is crucial not only in training – but even more in a race.

Carbohydrate storage is very limited in the human body, and so is the uptake rate an athlete can tolerate when training and racing. The ceiling of carbohydrate intake per hour is 60 to 90 grams/hour; but this also depends on the type of carbohydrates the athlete takes in, and how these types are mixed. Now, compare this stat to the actual carbohydrate combustion rate your athlete could face at the anaerobic threshold: a 70-kg age group triathlete easily combusts 250 g of carbohydrate per hour, and a professional cyclist (75-kg) can easily double that amount- up to 500g per hour, or even more.

It becomes clear: managing the effort in a race which lasts several hours – like a gran fondo, a marathon, a half Ironman, or a full Ironman race – is key for a great performance. On the given day, and given the specific training status, the perfect pacing strategy depends on three elements:

  1. How many grams of carbohydrates do your athletes actually combust?

  2. How big are the glycogen stores of your athlete?

  3. How many carbohydrates can your athlete take per hour?

With INSCYD, you hold all the tools you need to setup the perfect pacing and fueling strategies for your athletes.

1. Carbohydrate combustion rate

Three weeks before the targeted race, carry out a performance assessment with your athlete. A good time for this – often used by INSCYD coaches – is the end of the last training block, just before the beginning of the tapering period. In the INSCYD metabolic profile you will find the fat & carbohydrate combustion rate as a function of power (in cycling) or speed (in running).  The red graph shows you the carbohydrate combustion rate in kilocalories per hour (kcal/h), and in grams per hour (g/h). You can then use the cursor to move over the graph, in order to read the precise carbohydrate combustion rate at any given power or running speed. You will need this later to calculate the best power / running speed for the race.

2. Glycogen storage

There are a lot of scientific studies that took into account the amount of glycogen stored in the human bodies muscles. Here are some guidelines:
  • Untrained individuals; approximately 15 g per 1 kg muscle mass
  • Trained individuals; approximately 20 g per 1 kg muscle mass
  • Professional endurance athletes; approximately 25 g per 1 kg muscle mass.
All those guidelines relate to a full recovery status. This means at least 2 or 3 days with no (or very light) training, combined with a high carbohydrate diet. For a non-professional athlete, 20 g of carbs per kg of muscle mass has been proven to be a very accurate estimate. Even professionals, mainly because of hard training sessions (carried out one or two days before a race), hardly start a race with 25 g/kg muscle mass. How much muscle mass are your athletes actually using during the effort? Within INSCYD this is easy to calculate; open the “advanced body composition” menu when you run an analysis using INSCYD. Here you will find an estimate of the amount of muscle used during the effort. To calculate it specifically for your athlete, and help you to better understand the concept, we built a tool which INSCYD certified users can find in the user area

3. Carbohydrate intake during the race

The most common and general guideline for carbohydrate intake during a race is to stay close to 60 to 90 grams per hour. For long time, 60 g was seen as the maximum possible rate, but recently studies have found different types of carbohydrates (for example fructose and glucose) use different transport mechanisms. The 60 g/h was the ceiling of one specific transporter. However, if the body uses different carbohydrate transporters, from a mix of different kinds of carbohydrates, the upper limit rises- up to 90 g/h. To be clear- these rates do not mean the amount of carbohydrates your athlete can swallow in one hour! The bottleneck is the uptake from the gastrointestinal tract into the blood. So even when practicing to take in more than 90 g/h, not much more than this will actually get into the bloodstream and therefore become available to the muscles.

In all scenarios, you should have your athletes practice optimal carbohydrate intake during training. Consuming 60 g/h of carbs should be easy to achieve, while few athletes can ingest 90 g/h. It also depends on the type of exercise. Ingesting 70-80 g/h on the bike is doable for most athletes, contrasted with the run, where 60 g/h might be difficult.

Putting it all together: 5 Steps to calculate marathon pacing:

In order to calculate the best pace for a race, you now need to combine all three aspects. Step 1. Start with the estimated duration of the competition.
Step 2. Multiply the event duration (in hours) with the carbohydrate uptake rate (per hour).
For example: the goal is to run a marathon in 3h. We assume a carbohydrate intake rate of 50 g/h (so a relatively low intake rate). The total carbohydrate uptake rate is 3 (h) x 50 (g/h) = 150 (g) of total carbohydrate intake. Step 3. Estimate the glycogen storage. Let assume our athlete weights 75 kg (he’s a male and has approximately 12% of body fat). His total muscle mass is 40%, which equals to 30 kg (75kg x 40%). During the run, we assume he uses 70% of his total body muscle mass and therefore we get a total of 21 kg of muscle mass used. With a glycogen concentration of 20 g per 1 kg muscle mass we get: 20g x 21kg = 420 g of total glycogen available in the muscle involved during the marathon. Step 4. Add to this the 150 g of carbohydrates which is the total intake as calculated in the previous step. This gives us a total carbohydrate availability of 420 g (stored) + 150 g (feeding during race) = 570 g. Step 5. Divide this by the duration of the race.  In our case, this is a 3 hour marathon. This leaves us with a carbohydrate combustion rate of 570 g / 3h = 190 g/h. The only thing you need to do now is to read the speed at which this carbohydrate combustion rate happens. In the graph below, you can find out this combustion correspond to 3.98 m/s. At this speed the marathon would be completed in 2 hours 56 minutes and 42 seconds, so a bit below the originally assumed 3 hours. In a practical application, we recommend a similar approach, as it leaves a minimum room of tolerance (and safety) to achieve the desired goal. INSCYD certified coaches can download a little helper tool to plan this strategy here