Your fueling plan should change every month of marathon training

The plan you set in April won't work in September

You probably did the right thing. Somewhere around the start of your training block, you sat down, worked out a rough fueling plan, and committed to it. Maybe you Googled "how many gels for a marathon," read a few guides, and landed on a number. Maybe you tested it on a long run and it felt fine.

Then summer happened. Your training volume climbed. Your body adapted. The temperature jumped 15 degrees. And the plan you set four months ago is now quietly falling apart — you just haven't noticed yet.

Here's the thing: your body in week 1 of marathon training is physiologically different from your body in week 12. Your sweat rate changes. Your gut tolerance improves. Your carbohydrate oxidation rate shifts as your fitness builds. A static fueling plan ignores all of this, and the consequences show up where it hurts most — on race day.

This guide walks you through how to adjust your fueling plan month by month across a typical 16–20 week marathon block. It won't give you a single magic number because the right number keeps moving.

Month 1: Base building (weeks 1–4)

Your training load is low to moderate. Runs are shorter, intensity is controlled, and the weather (if you're starting a fall block in June or July) is just beginning to heat up. This is the month most runners skip entirely when it comes to nutrition planning. That's a mistake.

What's happening in your body

Your aerobic base is developing. Your muscles are efficient at burning fat at lower intensities, and your carbohydrate needs during easy runs are relatively modest. Your sweat rate reflects spring or early-summer conditions — not the heat you'll face later.

What to do

Establish your baseline sweat rate. Weigh yourself before and after a 60-minute run. Account for any fluid consumed. The difference, converted to millilitres, is your hourly sweat rate in those specific conditions. Write it down — you'll compare it later.

Start testing products. This is the lowest-risk time to try different gels, chews, and electrolyte mixes. Your runs are short enough that a bad reaction won't ruin a key session. Try one new product per week on your mid-week moderate run.

Set a conservative carb target. For runs under 75 minutes, you probably don't need in-run carbs at all. For longer efforts, start with 30–40g per hour. This is below what you'll target on race day, but right now the goal is to get your gut used to processing fuel while running — not to maximise intake.

Don't worry about perfection. The plan you set this month is a starting point. It will change. That's the point.

Month 2: Building volume (weeks 5–8)

Training volume increases significantly. Long runs get longer. You might introduce tempo work or marathon-pace sessions. If you're training through summer, the temperature has risen noticeably since month 1.

What's happening in your body

Your cardiovascular fitness is improving, which means your heart is pumping more blood per beat. Your body is beginning to adapt to heat — you start sweating earlier in a run and your sweat rate increases. Your total fluid and electrolyte losses per session are climbing.

Your gut is also adapting if you've been practising fueling on runs. Research shows that two weeks of consistent gut training produces a noticeable improvement in tolerance, and by six weeks, most runners can handle significantly higher carbohydrate loads.

What to do

Re-test your sweat rate. Run the same test you did in month 1, ideally in similar conditions. If you're training in summer, also test in the heat — your sweat rate in 30°C can be 50–100% higher than in 15°C. If your number has changed, your hydration plan needs to change with it.

Increase your carb target. If you tolerated 30–40g per hour comfortably in month 1, push toward 45–60g. Introduce carbs earlier in your long runs — within the first 30–45 minutes — rather than waiting until you feel tired. By the time you feel fatigued, your glycogen stores are already depleted and you're playing catch-up.

Lock in your electrolyte strategy. By now you should know whether you're a salty sweater (white residue on your kit, stinging eyes) or a lighter one. Salty sweaters typically need 500–1,000mg sodium per hour in hot conditions. Lighter sweaters can get away with 300–500mg.

Adjust your pre-run hydration. If your runs are starting earlier to beat the heat, make sure you're drinking 400–600ml of water with electrolytes in the 2–3 hours before you head out. Dehydration that starts before the run compounds faster than dehydration that develops during it.

Month 3: Peak training (weeks 9–14)

This is the hardest block. Weekly mileage peaks, you're running your longest long runs (30–35km), and workout intensity is highest. If you're racing in autumn, you're training through the hottest part of summer. Your fueling plan from month 1 is now almost certainly wrong.

What's happening in your body

Your fitness is near its peak. You're burning more total calories per session because you're running further and faster. Your fat oxidation has improved, but at marathon pace, you're still relying heavily on carbohydrates. Your gut — if you've been training it — can now handle higher carb loads than it could three months ago.

Your heat acclimatisation is well advanced. You start sweating earlier, produce more sweat per hour, and your sweat is slightly more dilute (lower sodium concentration per litre, but higher total sodium loss because of the increased volume). This means your total fluid and electrolyte needs are at their highest point in the training block.

What to do

Push your carb target to race-day levels. Current guidance recommends 60–90g of carbohydrates per hour for marathon racing. Your peak-block long runs are the place to practise this. If you've been gradually increasing since month 1, jumping to 60–75g should be manageable. If you're aiming for 90g, make sure you're using a dual-carb source (glucose plus fructose in roughly a 2:1 ratio) — your gut can absorb more total carbohydrate when it uses two transport pathways instead of one.

Do a full race-day rehearsal. Pick one of your longest long runs and execute your planned race-day nutrition from start to finish: same pre-run breakfast, same timing, same products, same quantities. Note everything — did you feel sick at any point? Did you need more fluid than expected? Did you carry enough? This is the most valuable session in your entire training block.

Re-test your sweat rate again. Yes, again. You're fitter and more heat-acclimatised than two months ago. Your sweat rate has almost certainly changed. One study found that heat-acclimatised runners increased their sweat rate by 10–15% over a four-week period. That means your hydration volumes need to go up to match.

Audit your electrolyte intake. Higher sweat volumes mean higher total sodium loss, even if your sweat concentration has dropped slightly. If you were taking 500mg sodium per hour in month 1 and your sweat rate has increased by 30%, you may now need 600–700mg per hour.

Month 4: Taper and race week (weeks 15–20)

Volume drops. Intensity stays. Your body is recovering and storing glycogen. This is where runners make two common mistakes: they either abandon their nutrition plan entirely ("I'm only running 40 minutes, I don't need to practise") or they panic-change everything in the final week.

What's happening in your body

Your muscles are repairing and supercompensating. Glycogen stores are filling up. Your sweat rate may drop slightly as training stress decreases, but your heat acclimatisation is maintained if you're still training in warm conditions. Your gut tolerance is at its highest — all that practise pays off now.

What to do

Don't change your products. Whatever gel, chew, or drink you've been using successfully for the last two months is your race-day product. This is not the week to try something new because someone at your running club recommended it.

Carb-load properly. In the final 2–3 days before the race, increase your carbohydrate intake to 8–12g per kilogram of body weight per day. For a 70kg runner, that's 560–840g of carbs per day. This is a lot — think pasta, rice, bread, cereal, juice, and sports drinks. You're not eating more food; you're shifting the macronutrient balance heavily toward carbohydrates.

Rehearse your race morning. Two weeks before race day, do a short easy run with your planned race-morning routine: wake up at the same time, eat the same breakfast at the same time, hydrate the same way. Confirm that everything works and nothing surprises you.

Finalise your hydration volumes. Take your most recent sweat rate measurement and use it to calculate how much fluid you need per hour on race day. Build in a buffer — race-day adrenaline and pacing often push sweat rates 10–15% higher than training. If your training sweat rate in warm conditions was 1.2 litres per hour, plan for 1.3–1.4 litres per hour on race day.

The signals your watch is sending (that most runners ignore)

Your Garmin, Apple Watch, or Strava feed is full of data that directly relates to your fueling needs — you're just not connecting the dots.

Training load is climbing? Your carbohydrate needs per week are climbing too. Higher training load means more glycogen depletion, which means more aggressive refuelling between sessions and higher in-run carb intake on your long days.

Heart rate at the same pace is dropping? Good — your fitness is improving. But it also means your metabolic efficiency at marathon pace has shifted. You may be burning proportionally more fat and less carbohydrate at easy paces, which changes your fueling needs during base runs (though not at race pace).

Your runs are starting in warmer conditions? Your sweat rate is higher. This isn't a guess — it's physiology. Every degree of temperature increase above your comfort zone adds measurably to your fluid loss. The hydration plan you tested in March training is not the hydration plan you need in August racing.

The problem is that no watch, app, or platform connects these data points to your fueling plan. They show you the inputs — training load, heart rate, temperature — but leave you to figure out the outputs: how much to drink, how many grams of carbs per hour, when to take your first gel.

This is exactly the gap Sweatr fills. It pulls your wearable data — training load from Garmin, activity data from Strava, heart rate from Apple Watch — and uses it to build a fueling plan that updates as your data changes. When your sweat rate goes up, your hydration volumes adjust. When your training load peaks, your carb targets rise to match. When the temperature spikes, your electrolyte recommendations shift.

You don't have to re-test, recalculate, or rebuild from scratch every month. The plan adapts because the data adapts.

Start now, not race week

If you're reading this in late June or July and your fall marathon is in October or November, you're in the perfect window. You have 16–20 weeks of training ahead of you and four natural checkpoints to adjust your fueling plan.

Here's the minimum you should do this week:

1. Establish your baseline sweat rate during your next easy run
2. Pick two gel or chew products to test over the next month
3. Set a conservative starting carb target (30–40g/hr for long runs over 75 minutes)
4. Put a reminder in your calendar to re-test your sweat rate at the start of each month

Or let Sweatr do it for you. Connect your Garmin, Apple Watch, or Strava account, and Sweatr builds your personalised fueling plan from day one — then adjusts it automatically as your training and conditions change, all the way through to race day.

Your body is going to change a lot between now and the starting line. Your fueling plan should change with it.