Maximum Tolerable Carbohydrate Intake

We had previously discussed how GI distress can be more prevalent in the heat during racing and/or long duration events.

To help athletes navigate this approach to fueling in environmentally challenging conditions, we are introducing the conceptual framework of Maximum Tolerable Carbohydrate Intake. The purpose of this framework is not to calculate an exact carbohydrate target, but rather to illustrate how physiological and environmental stressors influence the amount of carbohydrate an athlete can comfortably tolerate. This will help coaches, practitioners, and athletes to understand and adjust fueling strategies to match the physiological limitations due to environmental factors.

MTCI = GC x PF x EF x DE

MTCI = maximum tolerable carbohydrate intake

GC = gut capacity

PF = physiological factors

EF = environmental factors

DE = Duration of exercise

Maximum Tolerable Carbohydrate Intake = Gut Capacity x physiological factors x environmental factors. This is a dynamic value that is variable within and between sessions.

Gut Capacity = the maximal carbohydrate intake that can be effectively processed, absorbed, and tolerated under ideal conditions (how many carbs per hour can the gut can inherently absorb).

This is a function of the transporters (SGLT1 and GLUT5) ability to move carbohydrates from your GI tract into your body.
It is also a function of the gastric (stomach) empty rate.
This is what athletes are trying to increase with gut training. As gut capacity increases, MTCI increases.

Physiological Factors = core temperature, hydration status, exercise intensity

Core temperature and exercise intensity will have a significant influence on the distribution of blood flood. The greater the core temperature and greater the exercise intensity will divert blood flow away from the gut reducing the rate at which carbohydrates can be absorbed. As core temperature and exercise intensity increases, MTCI will decrease.
Hydration status has a dramatic influence as it will also influence gastric emptying, GI tract blow flood, and osmotic balance. As hydration status decreases, MTCI will decrease.

Environmental Factors = humidity (impacts ability to evaporatively manage heat) and heat stress (ambient temperature, radiant, convective, and conductive heat). As heat and humidity increases, MTCI will decrease.

Duration of Exercise = exercise duration acts as a multiplier to the environment and physiological factors. For example, the longer the duration the greater the cumulative heat stress and the more total fluid is lost through sweat. Thus, the environmental and physiological impacts can have a greater impact on maximum tolerable carbohydrate intake. As the duration of exercise increases, MTCI will decrease.

Note that this is a dynamic conceptual framework that can change within a session and between sessions depending on the conditions; MTCI is not a static value, but it can change. Herein lies the misunderstanding that athletes may have, which results in GI distress deep into a race (late onset GI distress). An athlete may incorrectly assume that if they can tolerate a certain amount of carbohydrates per hour in ideal conditions then they can maintain this amount during hour 4, 5, 6, etc in environmentally challenging conditions. The magnitude of this decline varies between athletes, but the decline itself is a consistent physiological response.

Figure 1 describes the MTCI Capacity Curve figure used to illustrate the MTCI concept:

Figure 1. Maximum Tolerable Carbohydrate Intake (MTCI) is dynamic within a session. MTCI declines over time due to increases and accumulation of physiological and environmental stressors. When carbohydrate intake exceeds MTCI, GI distress risk increases. Note that values are illustrative and do not represent recommended values.

We suggest coaches, practitioners, and athletes who struggle with late onset GI distress to use this framework to build a better fueling and hydration strategy that goes beyond using gut capacity and gut training as the sole variable in determining a fueling strategy. We also provide some practical recommendation for fueling in the heat below:
Practical Recommendations

Match fluid intake with carbohydrate intake; the more carbohydrates that are ingested, the more water you should be consuming.
- This is why hydration is so important to being able to continue high carb fueling throughout a session. If you are not staying hydrated in challenging environmental conditions, then the absorption challenge will only get harder.
Protect the first half of the race. Athletes often experience GI symptoms late in an event, but the physiological strain that contributes to those symptoms may begin accumulating hours earlier. Falling behind early can have significant consequences during a long duration race with environmentally challenging conditions.
- Early in a session you may be able to get away with consuming little water with your fuel, however the longer the session, the more important it is to take water with your fuel.
- Even early in a session, consistent fluid intake with each carbohydrate dose helps prevent the dehydration that compounds GI distress later
If you experience GI distress deeper into a session, experiment with smaller carbohydrate amounts that you take at a time (e.g. instead of two 50 gram gels every 30 minutes, alternate one 50 gram gel and one 25 gram gel every 30 minutes.
- This way of drip feeding was discussed by Chris Lowe in a previous blog.
If possible, make efforts to keep yourself cool. At aid stations, pouring cool water over your body and head can help to reduce thermal load.
Perhaps the most powerful intervention to help protect against late onset GI distress is heat training. Heat training causes your body to make physiological adaptations to better handle the heat, thus lessening the impact of heat on MTCI.

We are here to support you with developing your fueling strategy. Please reach out to us for more detailed guidance on how to incorporate this framework into your fueling strategy.