How Hills Change Your Marathon Fueling Plan

You have a fueling plan. You know your gels, you have your timing strategy, you tested it on your long runs. Then you look at the elevation profile of your fall marathon and realise: this is not the flat course your plan was built for.

Most fueling advice treats all marathons the same. Take a gel every 45 minutes. Drink 400–800ml per hour. Hit 60g of carbs. And on a flat, steady-paced course, that advice works well enough.

But marathons with serious elevation — NYC's five bridges and Central Park climbs, Marine Corps' Hains Point and the 14th Street climb, Twin Cities' rolling hills through St. Paul — change the equation in three ways that generic plans ignore.

Hills burn more carbs. A lot more.

Your body has two main fuel sources during a marathon: fat and carbohydrates. At lower intensities, you burn a higher proportion of fat. As effort increases, you shift toward burning more carbs.

Running uphill forces your heart rate higher even if you try to hold back. A climb that raises your heart rate from Zone 2 to Zone 3 can increase your carbohydrate burn rate by 20–30% during those minutes. Over a marathon with 300m of cumulative elevation gain, those extra carbs add up.

The maths matters. If a flat marathon at your pace burns roughly 60g of carbs per hour, the same pace on a hilly course might burn 70–80g per hour during climbing sections. If your fueling plan was built for 60g intake, you are running a deficit every time the road tilts up.

This is why runners who fueled perfectly in training on flat routes can still bonk at mile 20 on a hilly race. The plan was right for the terrain it was tested on — but the race terrain was different.

Your pacing changes, and your fueling should follow

On a flat course, most runners settle into a rhythm. Every kilometre feels similar. Your stomach has time to process fuel because your effort level is consistent.

Hilly courses break that rhythm. You push harder on the ups, recover on the downs, and your effort level oscillates through the race. This matters for fueling in two ways.

First, higher-effort segments redirect blood away from your digestive system. Your gut works best when you are running at a comfortable, aerobic pace. Push into threshold territory on a steep climb and your stomach's ability to absorb the gel you just took drops sharply. This is why runners often feel nauseous after taking a gel right before or during a big climb.

Second, pacing variation means your energy expenditure per hour is not constant. The first half of a hilly marathon might burn significantly more than the second half if the major climbs come early — or vice versa. A fueling plan that delivers the same amount of carbs every 45 minutes regardless of terrain is feeding a body whose needs are changing by the kilometre.

When to take gels on a hilly course

On a flat race, the standard advice is straightforward: take your first gel at 30–45 minutes, then every 30–45 minutes after that. Simple. But on a hilly course, you need to think about the terrain ahead, not just the clock.

Fuel before the climb, not during it. Take your gel 5–10 minutes before a major hill. Your stomach absorbs fuel best when your effort is moderate. By the time the gel hits your system (roughly 15–20 minutes for most gels), you will be on the climb and burning through those carbs. Taking a gel mid-climb, when your heart rate is high and your gut blood flow is reduced, is a recipe for nausea.

Use downhills as recovery and fueling windows. The back side of a hill is when your heart rate drops, your breathing settles, and your gut has the best chance to process fuel. If the course profile shows a descent after a major climb, plan to fuel at the top or early in the descent.

Map your gels to the course, not just the clock. Print the elevation profile. Mark the major climbs. Then plan your gel timing around those features. If a big hill comes at mile 8, take your gel at mile 7. If there is a long flat section from mile 10–14, that is your best window for steady fueling.

This is where a personalised plan makes the difference. Generic timing ignores terrain. A plan that accounts for YOUR pace on THIS course — including where you will slow down, where you will speed up, and where your heart rate will spike — can place your fueling in the windows where your body can actually use it.

Hydration changes on hills too

Sweating is driven by effort, not distance. You sweat more running up a hill at a higher heart rate than you do cruising a flat section at the same pace. A hilly course with significant climbing can increase your total fluid loss compared to a flat race of the same distance and temperature.

But here is the catch: you are less likely to drink on climbs. Grabbing a cup at an aid station while grinding up a hill is harder than on flat ground. Many runners skip water points on hills simply because it is too difficult to drink while climbing.

The result is a mismatch: you need more fluid on climbs, but you are drinking less. Over a full marathon, this creates a compounding deficit that catches up with you in the final 10K.

The fix: drink on the approach to climbs and during descents. If you carry a handheld or vest, take small sips on flats and descents, not on the steep sections. If you rely on aid stations, check the course map and note which stations come on flat or downhill sections — those are your priority stops.

The downhill problem nobody talks about

Downhill running is easier on your cardiovascular system. Your heart rate drops, you recover, you feel good. But it is brutal on your gut.

The repetitive impact of downhill running — the pounding as your quads absorb the grade — compresses your digestive system with every footstrike. Runners who experience mid-race GI distress often report it worsening on sustained downhills, not climbs. This is part of why NYC Marathon runners sometimes struggle through the long descent off the Queensboro Bridge, and why the rolling terrain of Marine Corps can cause stomach issues that a flat course would not.

If your race has significant downhills, consider using liquid carbs (drink mix or sports drink) instead of gels during those sections. Liquids pass through the stomach faster and are less likely to cause distress when your gut is being jostled.

How to adjust your fueling plan for elevation

Here is a practical framework for adapting your existing plan to a hilly course.

Step 1: Get the course elevation profile. Every major marathon publishes one. Study it. Note where the big climbs are, where the flats are, and where the descents come.

Step 2: Increase your total carb target. If you planned for 60g of carbs per hour on flat terrain, budget for 70–80g on a hilly course. You will burn more, so you need to take in more. If your gut has not been trained for higher intake, start practising in your remaining long runs.

Step 3: Retime your gels around terrain. Move each gel 5–10 minutes earlier if a climb follows. Use flat and downhill sections as your primary fueling windows. Do not force fuel during the hardest climbing sections.

Step 4: Front-load your hydration before hills. Drink more in the kilometres approaching a climb. Take smaller sips or skip aid stations on steep climbs. Make up the deficit on descents and flats.

Step 5: Practise on hills. If you have access to a hilly route, do at least two long runs on terrain that mimics your race. Test your gel timing around the climbs. Note how your stomach handles fuel at different effort levels. This data is more valuable than any generic calculator.

Your watch data already holds the answer

If you train with an Apple Watch, Garmin, or Strava, you already have the data you need to build a hill-adjusted fueling plan. Your watch records heart rate, elevation, pace, and effort level for every run. The question is whether you can turn that data into a fueling plan that accounts for how YOUR body responds to hills at YOUR fitness level.

A runner who averages 150bpm on flat terrain but spikes to 170bpm on climbs has a completely different carb burn profile on a hilly course than someone whose heart rate stays at 155bpm throughout. The first runner needs significantly more carbs per hour than a flat-course calculator would suggest.

Sweatr pulls your elevation data, heart rate patterns, and pace data to build a fueling plan that matches the actual course you are racing. It does not assume your marathon is flat. It does not assume your effort is constant. And it adjusts your gel timing and fluid recommendations to the terrain — not just the clock.

If your fall marathon has hills, your fueling plan needs hills too. The data is already on your wrist. The question is what you do with it.