How Much Fuel Do You Actually Need? A Body-Weight-Based Guide to Marathon Nutrition

Every marathon fueling guide on the internet tells you the same thing: aim for 60 to 90 grams of carbohydrate per hour. And then it leaves you there, staring at a pile of gels, wondering where in that range you actually fall.

That 30-gram gap isn't a rounding error. It's the difference between a comfortable final 10K and a walk-shuffle through the last three miles. It's the difference between three gels per hour and five. And the reason the range is so wide is that it was never designed for individuals. It's a population average, derived from lab studies on athletes of varying sizes, intensities, and conditions.

Your body is not a population average. Your fueling plan shouldn't be either.

Why Body Weight Changes Everything

The most overlooked variable in marathon fueling is the simplest one: how much you weigh.

A 55kg runner burning through a marathon at 5:00/km pace has a fundamentally different metabolic engine than a 90kg runner covering the same distance at 6:30/km. Their total energy expenditure differs by 40-60%. Their glycogen stores differ. Their sweat rates differ. And yet most fueling guides hand them the same 60-90g recommendation.

Here's what happens at the metabolic level. During sustained marathon-pace running, your body burns a mix of fat and carbohydrate. As intensity increases, carbohydrate becomes the dominant fuel source. The absolute amount of carbohydrate you burn per hour scales with your body weight and your running intensity — heavier runners and faster runners burn through glycogen faster.

Your body stores roughly 400-500g of glycogen in your muscles and liver when fully loaded. That's enough for about 90-120 minutes of marathon-pace running, depending on your size and speed. Everything after that depends on what you put in during the race.

The Body-Weight Formula

Sports nutrition research gives us a practical starting point. For marathon-pace running, most athletes oxidise carbohydrate at a rate of approximately 1.0 to 1.4 grams per kilogram of body weight per hour.

Here's what that looks like in practice:

If you weigh 55kg:

• Lower end: 55g/hour (about 2 standard gels per hour)
• Upper end: 77g/hour (about 3 gels per hour)

If you weigh 70kg:

• Lower end: 70g/hour (about 3 gels per hour)
• Upper end: 98g/hour (about 4 gels per hour)

If you weigh 85kg:

• Lower end: 85g/hour (about 3-4 gels per hour)
• Upper end: 119g/hour (about 5 gels per hour)

If you weigh 95kg:

• Lower end: 95g/hour (about 4 gels per hour)
• Upper end: 133g/hour (about 5-6 gels per hour)

Notice something? For a 55kg runner, the old "60-90g" guideline is roughly correct. But for runners above 75kg, it systematically underfuels. And for lighter runners, the top of that range might be more than their gut can handle.

This is why the same advice produces wildly different outcomes for different people.

Where You Fall in the Range

The 1.0-1.4g/kg/hour range isn't random. Where you land depends on three things:

1. Your pace relative to your threshold

The harder you run relative to your lactate threshold, the more carbohydrate you burn. A runner cruising at 70% of their max heart rate burns proportionally less glycogen than one grinding at 85%. If your marathon is a comfortable, well-paced effort, start at the lower end (1.0g/kg/hour). If you're racing aggressively — pushing closer to your half-marathon pace — you'll need the upper end.

2. Your finishing time

This is counterintuitive: slower marathoners often need more total fuel, not less. A 3:00 marathoner is on course for 3 hours. A 4:30 marathoner is out there for 50% longer. Even if the slower runner burns slightly fewer carbs per hour, they burn more total carbs because they're running for an extra 90 minutes. This is the single most common mistake in marathon fueling — assuming that slower means easier means less fuel.

3. Conditions on race day

Heat increases your carbohydrate oxidation rate. Running in 28°C versus 15°C can push your hourly carb burn up by 10-15%. If your fall marathon ends up warmer than expected, your body-weight calculation needs to shift toward the higher end of the range. Humidity compounds this effect by raising core temperature faster.

How to Put This Into Practice

Step 1: Calculate your baseline

Multiply your body weight in kilograms by 1.0. That's your floor — the minimum grams of carbohydrate per hour you should target.

Step 2: Adjust for intensity

If you're racing (not just finishing), add 0.2g/kg. If you're running in heat, add another 0.1-0.2g/kg. If both, you're looking at the upper end of 1.4g/kg/hour.

Step 3: Convert to gels (or whatever you use)

Most gels contain 20-25g of carbohydrate. Some newer options (like Maurten or SiS Beta Fuel) contain 40g. Divide your hourly target by the carbs per gel to get your gel count.

For a 75kg runner racing in mild conditions: 75 × 1.2 = 90g/hour. With 25g gels, that's roughly 3.5 gels per hour — or one gel every 17 minutes.

Step 4: Test in training

Your gut is a trainable organ. If you've never taken more than two gels per hour, jumping to four will end badly. Start at the lower end of your range in your next long run and build up over 4-6 weeks. Increase by one additional gel every two weeks until you reach your target.

Step 5: Account for other carb sources

Gels aren't your only option. Sports drinks at aid stations typically deliver 30-40g of carbohydrate per 500ml serving. If you're drinking a full cup at every aid station, subtract that from your gel count. The goal is total carbohydrate per hour from all sources — not total gels.

The Glucose-Fructose Factor

There's a ceiling on how fast your gut can absorb glucose: roughly 60g per hour through a single transporter. If your body-weight calculation puts you above 60g/hour (which it will for most runners over 60kg), you need a fuel source that uses multiple transporters.

Products with a 2:1 or 1:0.8 glucose-to-fructose ratio bypass this bottleneck by using two absorption pathways simultaneously. This is why dual-source gels and drink mixes exist — they can push absorption rates to 90-120g per hour in trained guts.

If your target is above 60g/hour, check the label. Single-source glucose gels won't cut it. Look for maltodextrin + fructose or glucose + fructose combinations.

What Most Runners Get Wrong

Mistake 1: Using the same plan regardless of body weight. A 60kg runner and a 90kg runner following identical gel schedules will have vastly different experiences. The lighter runner might feel fine; the heavier runner will bonk.

Mistake 2: Eating less because they're running slower. Slower finishing times mean more time on course, more total energy expenditure, and more fuel needed. A 5:00 marathoner needs more total carbohydrate than a 3:00 marathoner — just spread across more hours.

Mistake 3: Not accounting for pre-race carb loading. Your in-race fueling plan assumes you start with full glycogen stores. If you skipped carb loading or ate a light pre-race breakfast, you're starting in a deficit and need to make up the difference early.

Mistake 4: Waiting until they feel hungry. By the time you feel hungry during a marathon, you're already behind. Glycogen depletion signals arrive 20-30 minutes after the deficit begins. Start fueling at the 20-minute mark and stick to a schedule — don't wait for your body to ask.

From Manual Calculation to Automatic

The body-weight formula gives you a solid starting point. But it's still a calculation based on averages. Your actual carbohydrate oxidation rate depends on variables that change week to week — your training load, your fitness level, your heat acclimatisation, your sleep quality, and your pre-race nutrition.

Sweatr pulls your heart rate, pace, training load, and environmental data from your Apple Watch or Garmin and calculates a fueling plan calibrated to you — not to a formula. It adjusts as your fitness changes through a training block and recalculates when race-day conditions differ from your training environment.

If you've been guessing somewhere in the 60-90g range and hoping for the best, try building a plan from your actual data. Your body weight is the first step. Your wearable data is the rest.