Athlete
Matthew Marquardt

Professional triathlete and Medical Student

Test

Exogenous carbohydrate oxidation

Terminology

Endogenous carbohydrates — fuel that comes from inside the body. Consumed before exercise commences and refers to fuel that is already stored in the muscles and liver as glycogen.

Exogenous carbohydrates — fuel that comes from outside the body. Refers to carbohydrates that are consumed and utilized by the body during exercise.

Purpose

It’s not about how much carbohydrate is consumed. It’s about analyzing how much of an athlete’s ingested carbohydrate is ultimately utilized as fuel for training and racing. Measured in grams of carbohydrates per hour (CHO g/hr).

It’s an analysis of efficiency. The test forms an understanding of how effectively an athlete can transform the consumed carbohydrates into an accessible fuel source to support the intended exercise load, while the whole system is stressed by the demands of exertion.

The implicit assumption is that all carbohydrate consumed during exercise becomes available as fuel. In reality, the route from mouth to muscle is a flawed pathway and not all carbohydrate will arrive at the intended destination to directly fuel a session.

The implicit assumption is that all carbohydrate consumed during exercise becomes available as fuel. In reality, the route from mouth to muscle is a flawed pathway and not all carbohydrate will arrive at the intended destination to directly fuel a session. The expectation is that Hydrogel Technology — by encapsulating the carbohydrates — enables athletes to metabolize and access more of the consumed fuel. However, reasons why efficiency of the system will never reach 100% include the following circumstances:

With blood being diverted away from the gastrointestinal (GI) tract to assist active musculature and transport oxygen, gastric emptying slows down and some carbohydrate may not leave the stomach in time for it to be used in that session.

Some athletes may exceed their own absorption capacity or tolerance. Once in the alkaline environment of the small intestine, the hydrogel component of Maurten fuels will dissolve, and the carbohydrate will be released to be absorbed — to cross the intestinal wall. This is done through protein transporters — SGLT1 for glucose or maltodextrin and GLUT5 for fructose. This absorption pathway has a limit — excess carbohydrate may saturate the transporter system. Carbohydrate that is blocked by the backlog in the transporter system may move through to the large intestine and will no longer contribute to the session as a rapidly absorbed fuel source.

Other possible diversions that may limit the throughflow of carbohydrates include:

- The lag of time — some carbohydrates arrive too late, replenishing depleted glycogen stores without directly assisting performance.

- The liver may retain some glycogen.

- Rapid expulsion via vomiting, due to intolerance or illness, will empty the stomach of its usable carbohydrate contents.

Physiological subtleties dictate that there is no template for accurately predicting endogenous carbohydrate oxidation capacity without testing. It’s personal and can change over time and training status.

Physiological subtleties dictate that there is no template for accurately predicting endogenous carbohydrate oxidation capacity without testing. It’s personal and can change over time and training status. Two athletes with similar VO₂ measurements may have differing tolerance and absorption capacities.

This is why we test. From the results, the intention is to establish specific and highly personalized fueling recommendations for Matthew’s Ironman triathlon racing and training.

Method

Matthew consumed Maurten Drink Mix labelled with Carbon-13 (¹³C) — a naturally occurring, rare, and non-radioactive isotope of carbon. Before starting any carbohydrate consumption, an initial breath bag is collected to create a carbon dioxide expiration reference that’s used in all calculations as the baseline measurement.

Most athletes will start utilizing more exogenous (consumed in-session) than endogenous carbohydrates from 90 and 120 minutes after starting exercise.

Most athletes will start utilizing more exogenous (consumed in-session) than endogenous carbohydrates from 90 and 120 minutes after starting exercise. For Matthew this is represented in Graph A that shows rising exogenous carbohydrate, reducing endogenous carbohydrate (as stores begin to deplete throughout the session), and a relatively constant total carbohydrate. The transition point to being reliant on consumed fuel to maintain performance happens around 110 minutes.

From 90 minutes onwards in the test, Matthew’s expired carbon dioxide (VCO₂) was captured every 20 minutes in breath bags. The ¹³C is then detected in this expired carbon dioxide enabling the calculation of the differentiation between how much of the exogenous versus endogenous carbohydrate is metabolized at one precise timepoint in the session — every 20-minute interval.

The test was completed twice — day 1 (160 gram of carbohydrates per hour) and day 3 (200 grams of carbohydrates per hour).

Periodically throughout the test (6 mins on/14 mins off), Matthew wore a mask that measured oxygen and CO2, from which it’s possible to obtain a respiratory exchange ratio (RER). This number informs the percentage of energy that’s coming from carbohydrate versus fat.

Day 1

Matthew completed 180 minutes on the bike. 290 Watts for the first hour followed by 298 Watts for the remaining two hours. He then ran for 1 hour on the treadmill at 15.5 kp/h.

Day 3
Matthew completed 180 minutes on the bike. 290 Watts for the first hour followed by 305 Watts for the remaining two hours. He then ran for 1 hour on the treadmill at 15.5 kp/h.

Protocol

BIKE
Power Output: Target ~300 W

Duration: 180 minutes

Mask: 14 minutes off / 6 minutes on

Lactate sampling every 30 minutes

RUN

Speed: 15.5 kp/h

Duration: 1 hour
Mask: 14 minutes off / 6 minutes on

Lactate sampling every 30 minutes

Test environment:
Indoor trainer session

Gradient: 0% (simulated)

Room temperature: 17.4°C

Humidity: 30%

Altitude: 47 meters above sea level

Fueling and hydration:

Carbohydrate intake: 160 g/h (day 1) and 200 g/hr (day 3)
Total fluid intake: 700 mL/hr

Water: Ad libitum

Findings

High carbohydrate oxidation rates were sustained throughout each test. Carbohydrate oxidation rates reached ~135 g/h when consuming 160 g/h and  ~170 g/h when consuming 200 g/h. Increasing the amount of carbohydrates consumed per hour did not have a negative impact on exogenous carbohydrate oxidation efficiency, which stayed at ~85%.

Matthew showed an increase in exogenous carbohydrate oxidation when consuming more carbohydrates, which is not always the case.

Graph B shows that utilizing more carbohydrate on day 3 (200 g/hr), Matthew was able to keep utilizing fuel and had not necessarily reached capacity. If the two lines were matched — following the same path — it would show that no further fuel was being utilized and there would not be a benefit to consuming more. It is possible that his exogenous carbohydrate oxidation ceiling might be even higher than what we tested, presenting an opportunity for future testing.

This now leads to an ability to deliver practical fueling recommendations that will ultimately support Matthew in Ironman training and racing.

As a top line recommendation, Matthew can increase carbohydrate intake during the bike and also go higher on the run, where he historically had reduced his fueling due to concerns about impaired absorption and oxidation.

Revised in-race fueling plan (g/hr):

BIKE

T1: Consume carbohydrates immediately

Hour 1: 160 to 200 grams
Hours 2 and 3: 180 to 200 grams

Hour 4: Slightly decrease on the 4th hour to prepare for the 
marathon

Last 15 minutes of the bike: No carbs — aim to clear the gut slightly

RUN

Target at least 120 g/h based on gut comfort. Increase if pace and comfort allow.

Result post-testing

1st Ironman South Africa, 2026. New course record.