Trail Running Hydration: How Much to Drink and How to Get It Right
- vincent1405
- Mar 18
- 4 min read
Updated: May 6
Introduction
Most runners do not blow up because they forgot a gel. They blow up because they get hydration wrong. Too little and your heart rate drifts, your pace drops, and everything feels harder. Too much and your stomach starts sloshing, nausea kicks in, and you cannot eat.
Trail running hydration is not just about water. It is about fluid, sodium, and how both affect your ability to absorb carbs. This guide gives you clear numbers, and more importantly, how to turn them into something you can actually execute on a trail.
Table of Contents
The simple principle
Hydration is about replacing enough fluid and sodium to maintain performance without overloading your gut.
Here are the key numbers most runners should start from:
Fluid: 400 to 800 ml per hour
Sodium: 300 to 800 mg per hour
Carbs (linked to hydration): 60 to 90 g per hour
Why these ranges?
Sweat rates vary massively between runners, from around 0.5 L to over 1.5 L per hour
Sodium losses typically range from 200 to 1000 mg per litre of sweat
Carbohydrate absorption depends on fluid availability
The goal is not to replace 100% of what you lose. That usually leads to overdrinking. The goal is to stay within a manageable deficit without impairing performance.
What actually works in practice
Start simple, then adjust.
For most trail runners in moderate conditions:
500 to 700 ml fluid per hour
400 to 700 mg sodium per hour
Adjust based on:
Heat and humidity
Increase fluid towards 700 to 900 ml per hour
Increase sodium towards 600 to 1000 mg per hour
Cold conditions
Fluid often drops to 400 to 600 ml per hour
Sodium still matters, do not ignore it
Intensity and terrain
Climbing increases sweat rate
Technical descents reduce drinking opportunities
Signs you are underdoing it:
Dry mouth
Rising heart rate at same effort
Cramping late in the race
Signs you are overdoing it:
Bloated stomach
Sloshing feeling
Frequent clear urination
There is no perfect number. There is a range that works for you.
Turn this into a plan
Numbers mean nothing if you cannot execute them.
Let’s take a simple target:
600 ml fluid per hour
600 mg sodium per hour
70 g carbs per hour
Now translate that into real intake.
Example using common products:
500 ml soft flask with electrolyte mix:
30 g carbs
500 mg sodium
1 gel:
25 g carbs
50 mg sodium
A few chews or half a bar:
15 g carbs
negligible sodium
Example 3 hour trail run
Hour 1:
500 ml electrolyte drink
1 gel
Small handful of chewsTotal: ~600 ml, 70 g carbs, ~550 mg sodium
Hour 2:
500 ml electrolyte drink
1 gel
Half barTotal: ~600 ml, 70 g carbs, ~550 mg sodium
Hour 3:
500 ml electrolyte drink
1 gel
Top up water at aid station if neededTotal: ~600 to 700 ml, 70 g carbs, ~550 mg sodium
Reality check:
You need to carry 1 to 1.5 L depending on refill points
Gels must be accessible without stopping
Flasks should be easy to squeeze on technical terrain
If you cannot physically execute this, the plan is wrong.
Common mistakes
Drinking only water
Leads to diluted sodium levels and poor absorption
Waiting until thirsty
You are already behind
Trying to match 100% of sweat loss
Often causes gut issues
Ignoring sodium
Especially in long or hot races
Overcomplicating the plan
If you cannot remember it, you will not follow it
How to personalise
This is where most runners either improve massively or stay stuck.
Gut tolerance
Some runners handle 800 ml per hour, others struggle above 500 ml
Train your gut just like your legs
Training vs race
In training, test the upper limits
In races, stay slightly conservative
Conditions
Heat increases both fluid and sodium needs
Altitude can increase fluid requirements
Sweat rate testing
Weigh yourself before and after a run
1 kg loss roughly equals 1 L fluid
Example:
You lose 1 kg in 1 hour
You drank 500 ml
Actual sweat rate is ~1.5 L per hour
You are not going to replace all of that. But now you know you are a heavy sweater.
How to apply this with Atalen
This is exactly the type of problem Trail Fuel is built for.
You can:
Set targets per hour
for example 70 g carbs, 600 ml fluid, 600 mg sodium
Build your plan using real products
gels, drinks, bars you actually use
See totals by hour
not just daily totals
Test in training
adjust based on what your gut tolerates
Log feedback
what worked, what did not
Over time, you stop guessing. You start executing.
Conclusion
Trail running hydration is not about drinking more. It is about drinking the right amount, with enough sodium, in a way your gut can handle.
Start with:
500 to 700 ml fluid per hour
400 to 700 mg sodium per hour
Then test, adjust, and simplify until it works in real conditions.
If you can drink and eat consistently from hour 1 to hour 6, you are already ahead of most runners.
Sources
Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic PerformanceAmerican College of Sports Medicine https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6019055/
Exercise and Fluid ReplacementAmerican College of Sports Medicine https://pubmed.ncbi.nlm.nih.gov/17277604/
Consensus Statement of the International Society of Sports Nutrition: Nutrient TimingInternational Society of Sports Nutrition https://jissn.biomedcentral.com/articles/10.1186/s12970-017-0189-4
Sodium Intake During Exercise and Heat StressGatorade Sports Science Institute https://www.gssiweb.org/sports-science-exchange/article/sse-159-sodium-intake-during-exercise
Hydration for Endurance AthletesBritish Journal of Sports Medicine https://bjsm.bmj.com/content/46/Suppl_1/i8
Fueling the UltrarunnerAsker Jeukendrup https://www.mysportscience.com/post/fueling-the-ultrarunner
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