Managing Gut Issues in Endurance Sport
Highland Fuel Nutrition's Practical Guide to Fueling Without Fear
Gut problems are one of the biggest performance limiters in endurance sport.
Cramps. Sloshing. Bloating. Nausea. Emergency stops.
For many runners, it’s not their legs that fail first, it’s their stomach. The good news? Most gut issues aren’t random. They’re predictable. And they’re trainable.
At Highland Fuel Nutrition, we believe fueling should feel controlled, steady and repeatable, not like a gamble on race day, so here’s how to manage it properly.
Understand Why Gut Issues Happen
During endurance exercise, blood flow shifts away from the digestive system and towards working muscles. The harder you go, the less your gut wants to cooperate.
Common causes of GI distress include:
Consuming more carbohydrate than your gut can absorb
Inconsistent fueling patterns (nothing… then too much)
Poor hydration balance
High fructose loads without glucose pairing
Trying new products on race day
Stress and intensity spikes
Most issues come down to one thing:
Mismatch between intake and tolerance.
Start With the Right Carb Target
You do not need 120g of carbs per hour just because someone on YouTube does. Carb intake should be based on body weight, effort and gut training status.
A practical starting point:
0.8–1.0g carbohydrate per kg bodyweight per hour
Examples:
60kg runner → 48–60g/hr
70kg runner → 56–70g/hr
80kg runner → 64–80g/hr
Effort-based guidelines:
Moderate endurance: 40–60g/hr
Long endurance/racing: 60–90g/hr
Elite / highly gut-trained: 90–120g/hr
Higher isn’t automatically better.
Higher is only needed if your gut can handle it.
Use Multiple Carbohydrate Sources
The gut absorbs glucose and fructose via different transporters.
That’s why we use a 2:1 glucose-to-fructose ratio — it allows higher absorption capacity while reducing the risk of overload from one pathway.
But even with a 2:1 ratio, total intake still matters.
Absorption efficiency does not override tolerance.
Fuel Little and Often
Large intakes are one of the biggest causes of distress.
Instead:
Start fueling early (within 30–45 minutes)
Take small amounts consistently
Avoid “catch-up fueling”
For example: If using Highland Fuel:
1 scoop ≈ 25g carbs
Easy/steady sessions → 1–2 scoops/hr
Long runs/ultras → 2–3 scoops/hr
Hard efforts (trained gut) → 3+ scoops/hr
Consistency is easier on the gut than spikes.
Use Multiple Sources If You Need To
You do not need to stick to one single product or one single brand. The goal is effective fueling, not brand loyalty mid-race.
In many cases, combining sources actually improves tolerance:
Carb drink mix as your base
Small gel top-ups when intensity rises
Occasional chews or real food in longer ultras
Alternating textures to avoid flavour fatigue
For example:
Use carb water steadily throughout the hour, then add a gel before a climb or pace increase. That layered approach is often easier on the stomach than trying to hit everything from one heavy bottle.
Different formats empty at different speeds. Changing texture can reduce nausea. Spreading intake reduces overload.
If something isn’t sitting well, adjust it. The body doesn’t care about logos. It cares about absorption.
Train Your Gut Like You Train Your Legs
The gut adapts.
If you normally take 40g/hr and suddenly try 90g/hr on race day, your stomach will protest.
Instead:
Increase carbs gradually over weeks
Practice race intake on long runs
Simulate race intensity during fueling practice
Replicate environmental stress (heat, hills, fatigue)
Gut training is progressive overload, Just like fitness.
Manage Hydration Properly
Carbohydrates and fluid work together.
Too little fluid → concentrated solution → delayed emptying
Too much fluid → dilution and discomfort
General guidance:
500–750ml per hour
Adjust for heat and sweat rate
Drink to thirst, not panic
Sodium supports fluid absorption and retention, but more is not always better.
Balance > excess.
Don’t Chase Numbers Over Comfort
There is a growing trend toward extreme carb targets.
For some athletes, that works, but for many, it creates unnecessary gut stress.
Ask yourself:
Are you actually running at an intensity that requires 100g+ per hour?
Is your race duration long enough to justify aggressive intake?
Have you trained at that level?
Often, 60–75g/hr done consistently beats 100g/hr done poorly.
Performance comes from execution.
Test Under Real Conditions
Easy long runs are not enough.
To truly test tolerance:
Practice fueling during tempo efforts
Use it during back-to-back long days
Test in heat and wind
Simulate race pacing
The gut behaves differently under stress so train it under stress.
Signs You’re Overdoing It
Watch for:
Sloshing stomach
Persistent bloating
Early nausea
Sudden aversion to sweetness
Sharp intestinal cramping
When these appear, it’s usually not random.
It’s either:
Too much
Too concentrated
Too late
Or too new
Race Day Do’s and Don’ts
Do:
Eat what you normally eat
Start fueling early
Stick to practiced volumes
Adjust for heat and hills
Stay consistent
Don’t:
Try new products
Double intake to “catch up”
Over-hydrate
Chase a carb number at the expense of comfort
Ignore early warning signs
Final Thought
Your fueling strategy should give you confidence and not anxiety. The goal isn’t maximum carbs it is maximum sustainable output.
That comes from:
Structured intake
Gradual gut training
Appropriate carb targets
Hydration balance
Rehearsed execution
Simple. Repeatable. Built for long days.
Fuel smart.
Train the gut.
And let performance follow.
And remember
Scoop. Sweat. Suffer. Succeed.