Stop Guessing Your Race Nutrition: How to Build a Fueling Plan That's Actually Yours

The problem with every fueling guide you've ever read

You've done the research. You've read the articles. And they all say some version of the same thing:

• Drink 400–800ml of fluid per hour.
• Take in 60–90g of carbohydrates per hour.
• Aim for 300–1,000mg of sodium per hour.

Those ranges are technically correct. They're also completely useless.

A 58kg runner finishing a half marathon in 2:15 on a cool October morning does not have the same needs as a 90kg runner grinding through a marathon in 4:30 in July heat. But every generic guide treats them as interchangeable.

This is why you bonked at mile 22 despite "following a plan." The plan wasn't yours. It was an average, built for nobody in particular.

Here's how to stop guessing and find the numbers that actually work for your body, your pace, and your race.

Step 1: Find your sweat rate

Your sweat rate is the single most important number in your hydration plan. It tells you how much fluid you're losing per hour of exercise, which dictates how much you need to replace.

The good news: you can measure it at home with a scale, a water bottle, and a training run.

The protocol:

1. Weigh yourself in minimal clothing before a 60-minute run.
2. Track exactly how much you drink during the run (in ml).
3. Don't use the toilet during the run if possible.
4. Weigh yourself again immediately after, in the same clothing.
5. Calculate: (pre-run weight − post-run weight in grams) + fluid consumed in ml = sweat loss in ml.

That's your sweat rate for those conditions. A typical range is 500–2,000ml per hour, but conditions matter enormously. Heat, humidity, intensity, and fitness all move the number.

The critical detail most guides skip: you need to test this in conditions that resemble your race. A sweat rate measured on a cool morning in April will underestimate what happens on a warm race day in September. If your race is in the heat, test in the heat. If you'll be running faster than training pace, account for that too.

Run the test at least three times across different conditions. You're not looking for one number — you're building a range that you can apply contextually.

Step 2: Find your carb ceiling

The science says trained athletes can absorb up to 90–120g of carbohydrates per hour using dual-transport carbohydrate sources (glucose + fructose). That number dominates social media and podcast discussions right now.

But here's what the science also says: most recreational athletes can't tolerate anywhere near 90g per hour without GI distress. And the ones who can didn't get there overnight.

Your carb ceiling is the maximum amount of carbohydrate you can absorb per hour without your stomach turning on you. Finding it is a process, not a single test.

How to find yours:

Start at 40g of carbohydrates per hour on your next long run. That's roughly one gel plus some sports drink. Note how your stomach feels at every 30-minute mark.

If 40g sits well, move to 50g the following week. Then 60g. Keep pushing the dose by 10g per week until you hit symptoms — bloating, nausea, cramping, or the urgent need to find a toilet.

Drop back 10g from wherever the trouble started. That's your current ceiling.

Three rules for this process:

1. Always pair gels with water. Taking a gel without 150–200ml of plain water is the number one cause of gel-related nausea. The gel sits in your stomach as a concentrated syrup until water arrives to dilute it.

2. Use the same products you'll race with. Your gut adapts to specific formulations. If you train with Brand A and race with Brand B, your gut hasn't rehearsed that scenario.

3. Test at race-relevant effort. A gel that sits fine at easy long-run pace may cause problems at marathon race pace, because blood flow shifts away from your gut when intensity increases.

Most runners land somewhere between 50–80g per hour. If you're under 60g, that's fine — it doesn't mean you're doing it wrong. It means your gut has a lower current tolerance, and you can train it up over 4–8 weeks if your goal race demands more.

Step 3: Find your sodium needs

Sodium is the electrolyte that matters most during endurance exercise. You lose it in your sweat, and the rate varies wildly between individuals — from 200mg to over 2,000mg of sodium per litre of sweat.

This is why the standard advice of "take an electrolyte tablet every hour" is so hit-or-miss. If you're a heavy sodium sweater and you're only replacing 300mg per hour, you're falling behind. If you're a light sweater taking in 1,000mg per hour, you're wasting money and potentially upsetting your stomach.

How to estimate your sodium needs without a lab:

You won't get a precise number without a sweat composition test (offered by companies like Nix, Precision Hydration, and hDrop). But you can get a useful estimate from clues your body already gives you:

• White salt stains on your kit or skin after running = you're likely a saltier sweater (above average sodium concentration).
• Sweat that stings your eyes badly = higher sodium content.
• Craving salty food after long runs = your body is telling you something.
• History of muscle cramps in the second half of long events = may indicate sodium depletion (though the research on this is more nuanced than the old "cramps = low sodium" assumption).

If you show two or more of these signs, start with 500–700mg of sodium per hour and adjust. If none apply, 300–500mg per hour is a reasonable starting point.

Combine with your sweat rate: If you lose 1.5 litres of sweat per hour and your sweat sodium concentration is roughly average (about 900mg per litre), you're losing approximately 1,350mg of sodium per hour. Your sports drink and gels will replace some of that, but you may need supplemental sodium tabs to close the gap.

Putting it together: your personal fueling plan

Now you have three numbers:

1. Sweat rate — how much fluid you lose per hour.
2. Carb ceiling — the maximum carbohydrate your gut can handle per hour.
3. Sodium estimate — how much sodium you need to replace per hour.

Here's how to build the plan:

Fluid plan: Aim to replace 60–80% of your sweat losses. If you lose 1,200ml per hour, target 720–960ml per hour. You don't need to replace 100% — slight dehydration (up to 2–3% body mass loss) is normal and doesn't impair performance for most athletes.

Carb plan: Divide your hourly carb target into 15–20 minute intervals. If your ceiling is 60g per hour, that's 15g every 15 minutes — roughly half a gel. Start fueling early (within the first 30 minutes of exercise), not when you feel tired. By the time you feel the bonk, glycogen depletion is already advanced.

Sodium plan: Calculate the gap between what your drinks and gels provide and what you estimate you're losing. Fill the gap with salt tabs or high-sodium electrolyte mixes. Distribute evenly across the race rather than front-loading.

Map it to real products: This is where most plans fail — they give you numbers but not actions. Sit down before your next long run and write out exactly what you'll consume and when:

• 0:00 — Start running. First sip of sports drink.
• 0:20 — First gel + 200ml water.
• 0:40 — Sports drink top-up. Salt tab if needed.
• 0:60 — Second gel + 200ml water.

Write it on your arm, tape it to your water bottle, or use an app that prompts you. The best plan in the world is useless if you forget to execute it.

The training block: 4–6 weeks of dress rehearsals

Your fueling plan is a hypothesis until you've tested it. You need at least four long runs where you execute the full plan under conditions that approximate race day.

Week 1–2: Execute the plan at easy long-run pace. Focus on timing and tolerability. Does the schedule feel manageable? Does your stomach cooperate?

Week 3–4: Increase intensity on portions of the long run to simulate race effort. Include a race-pace segment in the middle or final third. Note whether your gut handles the fuel differently at higher intensity.

Week 5–6: Refine. Adjust quantities based on what you've learned. Try your backup options (different gel flavour, different electrolyte mix) in case your primary choice isn't available on race day.

Keep a simple log after each run: what you took, when you took it, how your stomach felt, and how your energy held up in the final 30 minutes. Patterns will emerge.

Why your watch data matters more than you think

Your Apple Watch or Garmin already tracks heart rate, training load, workout duration, and environmental conditions. That data tells a story about your fueling needs that most athletes never read.

Higher training load weeks mean higher caloric expenditure and greater glycogen depletion — which means your recovery nutrition and pre-run fueling need to scale up. A long run in 28°C heat will produce a meaningfully different sweat rate than the same run in 15°C, even at the same pace.

If you're tracking your runs on Strava, you have months of intensity and duration data sitting in an app, unconnected to your nutrition. The gap between "data you collect" and "actions you take" is where most athletes lose performance.

This is the problem Sweatr was built to solve. It pulls your training data from Apple Watch, Garmin, and Strava, combines it with your personal metrics, and calculates a fueling plan tailored to your next workout — not a generic range, but specific gram-by-gram, sip-by-sip guidance. Your plan updates as your fitness, training load, and conditions change, because a static plan can't keep up with a dynamic body.

The real cost of guessing

Every generic fueling guide is a gamble. Sometimes you get lucky and the averages work. Sometimes you cramp at mile 18, bonk at mile 22, or spend 10 minutes in a portaloo because your gut rejected a gel it wasn't prepared for.

The frustrating part is that the data to avoid all of this already exists — in your watch, in your training log, in the sweat stains on your shirt. The hard part has always been connecting those dots into a plan you can actually follow.

You can do it manually with a scale, a spreadsheet, and six weeks of disciplined testing. Or you can let Sweatr do the maths for you, automatically, using the wearable data you're already collecting.

Either way, stop guessing. Your next race is too important for averages.