Can You Train The Gut To Take On More Carbohydrate?

A significant proportion of my clients come to me because they’ve had gut issues. They want to take on more carbohydrate to support performance, but their gut seems to have its own number when it comes to how much feels comfortable.

I get it, for endurance athletes, carbohydrate availability can become a limiter to performance. Hence the interest in training the gut to potentially take on more… and more.

But what does “train the gut” actually mean?

Let me say this upfront, the data does suggest that repeatedly exposing the gut to carbohydrate and fluid during training can produce adaptations that allow an athlete to absorb, utilise and tolerate higher carbohydrate during exercise.

But it does not mean that everyone can, or should, work towards an arbitrary target such as 100 or 120g per hour.

The objective is to develop the highest practical intake that an individual athlete can sustain comfortably and consistently for their event, which might range from 6 hours to 6 days.

What happens when we consume carbohydrate?

Once carbohydrate has been broken into simple sugars, these sugars need to first get into the intestinal cell and second, they need to cross the intestinal wall to enter the bloodstream. 

This all takes place in the small intestine and it’s not a passive process.

There are three main transport mechanisms worth understanding.

SGLT1 – the glucose transporter

Glucose is transported into the intestinal cell primarily through SGLT1 (sodium-glucose cotransporter 1).

SGLT1 uses the sodium gradient to actively transport glucose into the cell. In simple terms, sodium helps drive glucose absorption, which is one reason sodium and carbohydrate transport remain closely linked in endurance nutrition.

GLUT5 – the fructose transporter

Fructose enters the intestinal cell using a different transport mechanism, primarily GLUT5.

This is why combining glucose with fructose can allow more carbohydrate to be absorbed overall – they use different transport pathways, increasing the total capacity for carbohydrate absorption.

GLUT2 – gets carb into circulation

Once inside the intestinal cell, glucose and fructose need to cross to the other side of the cell to enter the bloodstream. GLUT2 is an important transporter involved in this process.

The process is nuanced and can be influenced by multiple factors, including the concentration and combination of glucose and fructose present in the intestine, hydration and electrolyte status.

So, can we increase the function of these transport systems?

The evidence suggests yes.

There is evidence that repeated carbohydrate-rich feeding can influence both the activity and density of SGLT1 and GLUT5.

However, it’s important not to overstate what we know.

Increased SGLT1 and GLUT5 activity and density are plausible mechanisms behind the improvements seen with gut training, but direct evidence linking these changes to improved carbohydrate absorption in humans remains limited.

In other words:

We have good evidence that you can train the gut to improve the ability to absorb carbohydrate.

We have plausible physiological mechanisms that help explain why.

But we do not have proof that when you train the gut you automatically upregulate transporters in every athlete.

What else affects carbohydrate absorption?

Several things may contribute.

Gastric emptying rate

The stomach plays an important role.

Before carbohydrate can be absorbed, it must first leave the stomach and reach the small intestine.

The rate of gastric emptying therefore influences how quickly carbohydrate and fluid become available for absorption.

Some research suggests that repeated exposure to carbohydrate during exercise may influence gastric emptying. However, this isn’t consistent across studies, so it’s better to think of gastric adaptation as one possible component to train the gut rather than a guaranteed response.

The concentration of a carbohydrate solution can also influence gastric emptying, with more concentrated solutions generally emptying more slowly. Temperature may play a role too, in that cold fluids generally pass through the stomach more quickly.

When we’re talking about solid food, the amount and type of fibre, along with other components such as fat, protein and food structure, can also influence gastric emptying.

Practically, I find concentration, temperature and fibre content are major influencers as time and distance increase.

Learned tolerance of physical volume

There is also a perceptual and behavioural component to gut training.

Repeated exposure to eating and drinking during exercise can reduce the sensation of fullness and gut discomfort.

This matters because an athlete may have the physiological capacity to process a certain amount of carbohydrate but still struggle to feel comfortable while consuming it.

Gut training can therefore involve both physiological adaptation and learning to tolerate the experience of eating and drinking during exercise.

Being physiologically capable of processing something isn’t necessarily the same as being comfortable consuming it.

Does gut training work for everybody?

Not necessarily, and not to the same degree.

There is considerable individual variation in gut responses during exercise.

Two athletes can consume exactly the same nutrition at the same intensity and have completely different experiences.

Gut tolerance can be influenced by:

• carbohydrate type
• carbohydrate concentration
• total carbohydrate intake
• glucose-to-fructose combination
• gastric emptying rates
• fibre intake
• fluid intake
• exercise intensity
• exercise duration
• heat
• altitude
• previous dietary exposure
• individual gut physiology
• training status

There is no guarantee that an athlete who ‘trains their gut’ will eventually tolerate a particular carbohydrate target.

So, avoid comparison, and avoid chasing an amount simply because your training partner or an influencer says you should.

Gut training does not eliminate physiological limits

This is perhaps the most important misconception to avoid.

Gut training is not a competition to see how much carbohydrate an athlete can force through their mouth every hour. 

Remember, it has to leave the stomach, enter the intestinal cell and cross into the bloodstream before it becomes available to the body.

An athlete may successfully progress from:

40 g/h → 50 g/h → 60 g/h → 70 g/h

without ever needing to reach what has become the “holy grail” of 100–120 g/h.

And that may be exactly right for them.

The appropriate carbohydrate intake depends on the athlete’s output over time, the event, the intensity, the duration and overall fuelling strategy.

More is not automatically better. The right amount is the amount that supports performance and can be tolerated, absorbed and utilised effectively.

So, what should gut training actually look like?

The most useful approach is progressive.

Rather than suddenly introducing a very high carbohydrate intake on race day, expose the gut to increasing carbohydrate loads during training.

Start with what is currently tolerated

Establish the athlete’s current carbohydrate intake and identify any symptoms.

Increase gradually

If the athlete is tolerating the current intake comfortably, increase carbohydrate progressively in small increments rather than making a large jump.

Practise during relevant sessions

Gut training is most useful when the athlete practises consuming carbohydrate under conditions that resemble competition.

That might mean practising during:

• long rides
• high-intensity sessions
• hot-weather training
• altitude training
• back-to-back endurance sessions, exposing the gut to carbohydrate when fatigued

Experiment with carbohydrate sources

Different athletes respond differently to different products, foods, volumes and combinations, so experiment.

Find the individual sweet spot

The goal is not to reach the highest number on paper.

The goal is to find an intake that the athlete can absorb, utilise and tolerate repeatedly.

The “Sip & Drip” principle

For many endurance athletes, particularly those racing for very long periods, carbohydrate delivery does not need to mean large amounts consumed at once.

A more sustainable approach can be to provide carbohydrate consistently throughout the session.

Small, frequent amounts can allow the athlete to maintain a relatively continuous supply of carbohydrate while reducing the temptation to periodically overload the gut transport systems.

This is the thinking behind my “Sip & Drip” approach.

Small amounts, frequently and consistently, rather than large amounts consumed intermittently, provide a wonderful background carbohydrate operating system.

The exact carbohydrate target still needs to be individualised and aligned with the event, and that target should follow the individual’s physiology and the demands of the event, not the other way around.

The practical takeaway

The gut is flexible and adaptable.

Repeated exposure to carbohydrate during exercise can improve an athlete’s ability to tolerate and absorb carbohydrate, through a combination of intestinal, gastric and perceptual adaptations.

But gut training has limits, and those limits vary between individuals.

The objective is not to force the highest possible carbohydrate intake.

It’s to develop a fuelling strategy that is appropriate to the athlete, the event, the intensity, the environmental conditions and the duration of the event.

And, critically, comfortable enough to execute consistently throughout the event.

There’s no point in being comfortable for the first 6 hours of a 12-hour event and then suffering through gut distress to the finish.

Because the best fuelling strategy isn’t the one that looks most impressive on paper.

It’s the one that allows the athlete to keep taking in fuel, keep absorbing it and keep performing when it matters.

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Interested in understanding what fuelling strategy works for you, your performance, and your gut?

I work with ultra-cyclists, endurance runners and adventure racers.

Let’s chat!