Marathon Fuelling: What Actually Matters (Carbs, Fluids, Sodium)

Mark Cullen
Apr 7
2 min read

How much should you drink? How many carbs per hour? Do you actually need sodium or is that just marketing?

Let’s simplify it and focus on what actually matters.

There Is No Perfect Formula

First thing to understand:

There is no one-size-fits-all hydration plan.

What works for one athlete can completely miss the mark for another.

Your needs depend on:

Body size
Pace
Temperature
Sweat rate
Fueling strategy
Sodium loss

This is why generic rules like “drink X per hour” or “8 glasses a day” don’t really work for endurance athletes.

What “Good Hydration” Actually Looks Like

You’re probably in a good spot if:

Your weight is stable day to day
Morning urine is light yellow
You’re not constantly thirsty
You’re peeing regularly

Nothing fancy. Just consistent signals your body is in balance.

The 2% Rule

You do NOT need to replace every drop of sweat during a marathon.

A good guideline: Don’t lose more than ~2% of your body weight

Example:

68 kg athlete
Can lose ~1.3–1.4 L before performance really drops

Trying to fully replace sweat = overdrinking risk

Not drinking enough = performance drop

The sweet spot is somewhere in between

What to Do Before the Race

Keep it simple:

2–3 hours before: ~750-1000 ml fluid w/ some sodium in it 500-1000mg
30 mins before: ~150–250 ml top-up

That’s it.

You want to show up:

Hydrated
Not bloated
Not chugging fluids last minute

During the Marathon (The Big 3)

Use this planner to help you plan The Fuel & Hydration Planner by Precision Fuel & Hydration

1. Fluids

Most athletes land somewhere around: 300–700 ml per hour

But again—this depends heavily on your sweat rate and conditions.

The key:

Know your sweat rate from training
Don’t guess on race day

2. Carbs

Aim for 50-75g carbs per hour

Over 60g per hour try to use a gel or carb sources that are a combo (2:1 or 1:0.8 of glucose/fructose)

3. Sodium (Most People Overthink This)

For most marathoners: You probably don’t need extra sodium

Why?

Sports drinks already contain it
Your daily diet covers the rest

Where it might matter:

Hot conditions
Very long races
You’re a heavy/salty sweater (salt stains, very salty sweat)

If you are worried about it even just getting in 250-500mg per hour during will likely be lots enough

After the Race

Don’t overcomplicate this either.

If you lost 1 kg during the race:👉 Aim to replace ~1 L over the next few hours

Key points:

Drink gradually
Pair with food
Don’t chug everything at once

Your body will handle the rest.

What About Shorter Races?

5K / 10K: No hydration, fluids or sodium needed during (could have some water if its really hot)
Half marathon: Less fluid than a full, but carbs still matter (40–60g/hr) - dont need additional sodium if you get some in the AM