The 3 Numbers That Make or Break Your Race-Day Nutrition Plan

You've done the long runs. You've tapered the mileage. You've laid out your race-day outfit three times already.

But ask yourself this: do you actually know how much fluid you lose per hour at race pace? Do you know the maximum grams of carbs your gut can absorb before it rebels? Do you know how many milligrams of sodium you need to replace every sixty minutes?

If you answered "roughly" or "I follow what my friend does," you're building your race-day nutrition plan on guesswork. And guesswork is how you end up cramping at mile 22, nauseous at the second aid station, or walking the last 10K wondering what went wrong.

Here's the good news: there are only three numbers you need to lock in. Get them right and your nutrition plan practically writes itself. Get them wrong — or ignore them entirely — and no amount of fitness will save you on race day.

Number 1: Your sweat rate

Your sweat rate is the volume of fluid you lose per hour of exercise. It's the foundation of every hydration decision you'll make on race day.

The problem is that sweat rate varies wildly between athletes. One runner might lose 500 ml per hour in moderate conditions. Another, at the same pace and temperature, might lose 1,800 ml. Generic advice to "drink 400–800 ml per hour" is the sports nutrition equivalent of telling everyone to wear a size 9 shoe.

Why it matters on race day

If you drink less than you lose, you dehydrate. Performance drops roughly 2–3% for every 1% of body weight lost to sweat. For a 70 kg runner, that's just 700 ml of unrecovered fluid before your pace starts slipping and your heart rate starts climbing.

If you drink more than you lose, you risk hyponatremia — dangerously low blood sodium from dilution. It's rarer than dehydration but far more dangerous, and it happens when well-meaning athletes follow the "drink as much as possible" advice that still floats around.

How to find yours

The simplest method is a weigh-in test. Weigh yourself (minimal clothing, empty bladder) immediately before a 60-minute run at your target race pace. Don't drink during the run. Weigh yourself immediately after. The difference in grams is your hourly sweat loss in millilitres.

Run this test at least three times in different conditions — cool morning, warm afternoon, humid day — because your sweat rate isn't a single number. It shifts with temperature, humidity, pace, and fitness. What you're building is a range, and your race-day plan should target the conditions you expect on race morning.

If you ran the test in 28°C summer heat but your October marathon forecast is 14°C, your race-day fluid needs could be 30–40% lower than what you measured. Planning for the wrong conditions is almost as bad as not planning at all.

Sweatr pulls temperature, humidity, pace, and heart rate data from your Apple Watch or Garmin and calculates your sweat rate automatically across multiple workouts — building the personalised range you need without the kitchen-scale routine.

Number 2: Your carb ceiling per hour

Your carb ceiling is the maximum grams of carbohydrate you can absorb and use per hour during exercise without triggering GI distress. It's the number that separates "I felt strong in the last 10K" from "I spent the last 10K looking for a portaloo."

The science is clear: carbohydrate fueling during endurance events longer than 60–90 minutes meaningfully improves performance. Your muscles burn glycogen at a rate that outpaces your stored supply, and gels, chews, or drink mixes bridge the gap. But there's a ceiling to how much your gut can process, and that ceiling is personal.

The 90g debate

You've probably heard that elite athletes now fuel at 90–120g of carbs per hour. This is real — TrainingPeaks' new Fueling Insights feature, developed with Dr. Iñigo San Millán, is built around supporting carb intakes up to 125g/hr for elite cyclists. Tadej Pogačar's fueling strategy runs at the high end of that range.

But here's what the headlines skip: those athletes have spent months training their guts to handle that load. They use carefully calibrated glucose-fructose blends that exploit dual transport pathways. And they have sports science teams monitoring every gram.

For most age-group runners and triathletes, a realistic starting ceiling is 40–60g per hour. Some athletes can work up to 80–90g with systematic gut training. Others hit their wall at 50g. The number depends on your gut physiology, your pace (slower runners generally tolerate more because blood flow to the gut is less compromised), and how much you've practised.

How to find yours

Start at 40g per hour on your long runs. That's roughly one standard gel every 30 minutes, or a consistent sipping strategy with a carb drink mix. If your stomach stays settled after three long runs at that intake, bump it up by 10g. Keep testing until you find the point where your gut pushes back — nausea, bloating, urgent bathroom stops.

Your ceiling is the intake one step below that threshold. Write it down. This is the number that goes into your race-day plan.

The critical mistake most athletes make is testing their fueling only once. Your gut adapts over weeks, and what felt impossible in June might be comfortable by September — but only if you practise consistently. A single successful long run doesn't confirm your ceiling. Three in a row does.

Sweatr tracks your training load, heart rate, and workout intensity to estimate your carb burn rate per hour, then recommends a fueling target calibrated to your tested ceiling — so your plan matches both what your body needs and what your gut can handle.

Number 3: Your sodium target

Sodium is the electrolyte that matters most during endurance exercise. You lose it in sweat, and when levels drop too far, you cramp, you fade, and in extreme cases you put yourself at medical risk. But "take some electrolytes" is about as useful as "eat some food." You need a number.

Why sodium and not just "electrolytes"

Your sweat is mostly water and sodium chloride. Potassium, magnesium, and calcium are present but in much smaller quantities. When endurance articles talk about "electrolyte replacement," they primarily mean sodium — and the amount you need to replace is directly tied to two things: how much you sweat and how salty your sweat is.

Sweat sodium concentration varies between 200 and 1,500 mg per litre depending on the athlete. That's a sevenfold range. An athlete with dilute sweat who sweats moderately might need 300 mg of sodium per hour. A salty sweater with a high sweat rate might need 1,200 mg. Both are following "electrolyte guidelines." Only one has the right number.

How to find yours

The gold standard is a lab-based sweat composition test, but those are expensive and not widely available. A more practical approach combines observation with testing:

Visual cues: After a hard workout, do you see white salt stains on your kit? If your shorts, hat, or sports bra routinely show crusty white residue, you're likely a salty sweater — north of 800 mg sodium per litre.

Trial and error: Start with 400–500 mg of sodium per hour (roughly one electrolyte tablet or a standard sports drink plus a gel). If you still cramp or fade in the back half of long runs, increase by 200 mg per session until the cramping resolves. If you feel bloated or puffy, you're probably taking too much.

The Nix biosensor offers real-time sweat sodium measurement, but at $130+ for the starter kit and single-use patches, it's a significant investment for a number you test occasionally.

Once you have your sodium target, the execution is straightforward: divide your hourly sodium need across your fluid intake and any separate salt tabs. If you need 800 mg/hr and your sports drink provides 300 mg per 500 ml, you know you need an additional 500 mg from capsules or salt in your bottles.

Sweatr combines your sweat rate data with environmental conditions to estimate your sodium needs per hour and maps that to specific products in its curated library — so you know exactly which electrolyte tabs, drink mixes, or salt capsules to pack.

How the 3 numbers work together

Here's where it gets interesting. These three numbers don't exist in isolation — they interact, and getting one wrong throws off the others.

If you underestimate your sweat rate and drink too little, your gut gets less blood flow. Less blood flow means lower carb absorption. Suddenly you can't hit your carb ceiling even though you tested it successfully in training — because you're dehydrated and your gut has shut down.

If you overshoot your carb ceiling, the unabsorbed sugar pulls water into your intestines (osmotic diarrhea, the nightmare scenario). Now you're losing fluid you can't afford to lose, your sodium balance is disrupted, and your hydration plan is meaningless.

If you miss your sodium target, water absorption in the gut slows. You can drink plenty and still dehydrate because the fluid isn't making it out of your stomach efficiently. And you're probably cramping, which wrecks your pace, which changes your fueling timing, which cascades into a race you'd rather forget.

The three numbers form a system. Change one and the others move. That's why a race-day plan built on "take a gel every 45 minutes and drink when you see an aid station" falls apart — it doesn't account for your specific physiology or the conditions you're racing in.

Building your plan from the numbers

Once you have your three numbers, the plan is mechanical:

Fluid per hour = your sweat rate for the expected race-day conditions, minus roughly 20% (you don't need to replace 100% of losses — a small deficit is fine and avoids GI overload).

Carbs per hour = your tested ceiling. Split it between gels, chews, drink mixes, or real food — whatever you've practised. Time your intake evenly across the hour; front-loading causes spikes and crashes.

Sodium per hour = your estimated target. Split it between your sports drink and any supplemental capsules. Don't rely on aid station cups of unknown sports drink concentration.

Map this plan to the actual aid station locations on your race course. Know where you'll take fluids, where you'll take gels, and what you're carrying versus what you're picking up. Practise the exact sequence on at least two long runs before taper.

Stop guessing, start measuring

Most athletes spend months building fitness and minutes building a nutrition plan. The irony is that nutrition is more likely to end your race than fitness. You can be in the best shape of your life and still bonk, cramp, or vomit your way to a time you'll never speak of again — because you didn't know your numbers.

The old approach was to weigh yourself, test on long runs, and hope your estimates were close. It worked, but it was slow, imprecise, and most athletes didn't do it consistently.

Sweatr calculates your sweat rate, carb needs, and sodium target directly from your Apple Watch, Garmin, or Strava data. It tracks how your numbers shift across temperatures and training loads, and builds a race-day plan you can follow gel by gel, sip by sip. No lab visits. No guesswork. Just three numbers, personalised to you.

Your fall race is weeks away. The fitness is done. The numbers are what's left.