Using ¹³C isotopic tracers, we measure exactly how much of the carbohydrates you consume your body actually absorbs and oxidises during exercise. We are one of the few laboratories in Europe that actually measures this.
You've probably heard it: "Take 60–90 g of carbs per hour." But that range is enormous — and the right answer is different for every individual.
Two athletes with similar body mass, similar power output, and similar training status can have completely different exogenous carbohydrate oxidation rates. One oxidises 47 g/hour of glucose, the other 78 g/hour. This means the optimal recommendation for one is around 80–85 g/hour, while for the other it's 140–145 g/hour. Without measurement, there is simply no way to know.
Similar athletes, completely different intake. The difference in recommended carbohydrate intake is nearly double — 80 g/hour versus 140 g/hour. Without measurement, there is no way to know.
"Take 60–90 g/hour"
"Your recommended intake: 140 g/hour (glucose + fructose in 1:0.8 ratio)"
We use carbohydrates labelled with the stable carbon isotope (¹³C). Simultaneously, we measure total substrate oxidation with a metabolic analyser. When ingested carbohydrates are oxidised, ¹³C appears in exhaled CO₂. Using mass spectrometry, we determine the ¹³C/¹²C ratio, enabling us to distinguish between exogenous (ingested carbohydrates) and endogenous (glycogen stores) oxidation. ¹³C is a stable (non-radioactive) isotope naturally present in food — its consumption is completely safe.
During the test, you drink a solution containing a precisely known proportion of ¹³C-labelled glucose at regular intervals.
Using a metabolic analyser (Cortex), we continuously measure total substrate oxidation (fats and carbohydrates) via gas exchange (VO₂, VCO₂).
Exhaled air samples are analysed with a mass spectrometer that precisely determines the ¹³C/¹²C ratio. Since the ingested glucose is labelled with ¹³C, we can accurately assess what proportion of oxidation comes from ingested carbohydrates and what proportion from glycogen stores.
By combining both measurements — total substrate oxidation and isotopic analysis — we determine your exogenous glucose oxidation rate (g/min) and create a personalised carbohydrate intake strategy, including the optimal glucose-to-fructose ratio.
Mixing chamber for collecting exhaled air enriched with ¹³C
The test consists of two parts — approximately 3.5 hours in total. In addition to your personalised carbohydrate utilisation data, you also get a complete picture of your endurance performance.
Progressive increase in intensity to exhaustion. We determine VT1 (first ventilatory threshold), VT2 (second ventilatory threshold) and VO₂max. These data points are essential for correctly setting the intensity in the second part of the test.
Time for recovery, preparation of the isotopically labelled drink, and equipment setup for part two.
5-minute warm-up followed by 2 hours 30 minutes at a constant intensity of 90–95% VT1. Throughout the test, you drink ¹³C-labelled carbohydrate solution at regular intervals. Exhaled VO₂ and CO₂ are measured at 30 and 15 minute intervals.
Exhaled air samples are sent for laboratory analysis. Results are ready in approximately 14 days.
This test doesn't just give you a single number — you receive a full set of data to optimise both your training and race-day nutrition.
From the incremental step test, you receive VO₂max values, VT1/VT2 thresholds, maximum heart rate, and individualised training zones.
Your personal exogenous carbohydrate oxidation rate — a data point that is impossible to obtain without isotopic testing.
Expert review of your results with our dietitian and physiologist. Together, you'll build a race-day nutrition strategy.
Exhaled air analysis results are ready in approximately 14 days after the test (isotope laboratory analysis time).
Complete test package: incremental step test + 2.5-hour steady-state CHO test with isotopic tracers + report + consultation.
After introductory period: 849 €/test
We currently offer the test for cyclists, runners, and triathletes who want to optimise their nutrition strategy for competition. It's especially valuable for athletes who experience GI distress during exercise or want a data-driven approach to race-day nutrition planning.
We'll send you detailed instructions before the test. In general: arrive well-rested (no intensive training 24–48 hours beforehand) and fasted. Avoid foods enriched with ¹³C (e.g. corn syrup) for at least 3 days before the test.
Bring your bike and gear, as if you were heading out for a training ride. The test is performed on your own bike.
A standard metabolic test (e.g. lactate profile or VO₂max) measures total fat and carbohydrate utilisation, but cannot distinguish between exogenous (ingested) and endogenous (glycogen stores) sources. The isotopic CHO test uses ¹³C-labelled glucose to precisely separate these two components and measure how much of your ingested carbohydrates your body actually uses — which is impossible to determine without isotopic tracers.
Incremental step test results (VO₂max, thresholds, training zones) are available within a few days. Carbohydrate oxidation analysis results are ready in approximately 14 days, as laboratory analysis of exhaled air samples is required to determine isotope ratios. Once results are prepared, we'll invite you for a personal consultation.
Yes! The test already includes an incremental step test, so you also receive a complete set of data about your aerobic performance. If you'd like additional coaching or nutrition guidance, we can arrange a package deal.