Marathon Fueling: From the Old 60 g/h Ceiling to a Modern 90–120+ g/h Strategy

For decades, runners were told the gut tops out at 60 grams of carbs per hour during exercise. You will leave minutes on the course if you cap fueling at 60 g/h. That "limit" came from single-carb studies using glucose alone. When you feed multiple transportable carbs—specifically glucose plus fructose—the ceiling moves. A lot. Trained athletes routinely operate at 90–120+ g/h without blowing up their stomachs. The difference is the substrate and the training of the organ you race on: your gut.

The old 60 g/h rule isn't wrong so much as incomplete. It's what happens when you saturate a single transporter system. But your intestine has multiple doors. Use them all, and the throughput changes dramatically. With this modern understanding in mind, let's look at the gut's actual physiology and what it means for marathon fueling. This isn't fringe science or bro-science hype. It's settled physiology that elite endurance athletes have been exploiting for years while the rest of the field clings to outdated guidelines.

The Physiology You Actually Need

Here's what matters. Glucose and fructose take different pathways in your gut. Glucose uses one route. Fructose uses another. If you use only glucose, you overload one pathway and leave the other unused. Combine them in the right mix, and you use both at once.

Many runners thrive on a 2:1 glucose-to-fructose ratio. Others do better near 1:0.8. The ratio determines not just absorption but usable oxidation—the rate at which ingested carbohydrate actually gets burned by working muscles. Translation: your muscles can actually burn what your gut takes in, rather than having it sit in your stomach like a brick.

Research demonstrates that when athletes consume glucose-fructose blends at high rates (90–120 g/h), exogenous carbohydrate oxidation rates can reach 1.75 g/min compared to just 1.0–1.2 g/min with glucose alone. That's not a marginal gain. That's a 40–50% increase in fuel availability during the exact window when your glycogen stores are depleting.

The practical implication: products matter. A gel that's pure maltodextrin (glucose polymer) hits the SGLT1 ceiling. A gel or drink that blends glucose sources with fructose can break through it. Read labels. Check ratios. Don't assume all "90 calorie gels" deliver the same result.

How do I train my gut to handle higher intake?

Capacity isn't genetic destiny. It's trainable. Over two weeks, you can move the needle. Over eight to ten weeks, you can transform your ceiling from "I'm a sensitive stomach person" to "I tolerate real numbers." The process is progressive overload for your GI tract, not a one-shot dare.

Start where you are—even if that's 30–40 g/h—then step up methodically until you find the sweet spot between performance and comfort. Studies show that just 2 weeks of high-carbohydrate intake during training (90 g/h) significantly reduces GI symptoms and improves absorption compared with low-carbohydrate training groups. The adaptation is real, and it's fast.

Consistency is key. Random big doses won't help—they just cause discomfort. Regularly increasing amounts trains your gut to handle more fuel. Your gut becomes faster and more tolerant over time.

This isn't optional if you're serious about performance. Elite marathoners routinely fuel at 100–120 g/h. Age-groupers trying to break three hours often fuel at 40–60 g/h and wonder why they hit the wall. The difference isn't talent. It's trained capacity.

How much sodium per hour do runners need?

Sweat sodium concentration differs significantly between athletes, ranging from 200 mg/L to over 2,000 mg/L. Sweat rate also varies with factors such as body size, fitness, heat acclimation, and genetics, ranging from about 0.5 L/h to more than 1.5 L/h. Because these factors vary so much, standard advice, such as "aim for 500 mg/h," often does not fit individual needs. Adjust your sodium intake based on your sweat loss and sodium concentration to avoid both too little and too much.

In August, racing the Chicago Marathon, you may need several times your February dose at the Tokyo Marathon. Observe your clothes after hot runs. Look for salt residue—white streaks or crusty patches. That's visible evidence of high sodium loss. If your hat stiffens or your shirt has white tide lines, you're a salty sweater. If not, you're probably not.

Instead of relying on generic recommendations, use your measured data to determine sodium intake. For runners who lose a lot of sodium in sweat during hot races, intake can go above 1,500–2,000 mg/h. Those with lower sodium loss may only need 300–500 mg/h in cooler conditions. Match your sodium plan to your actual losses, not a standard amount.

How to Estimate Your Sodium Needs:

1. Weigh yourself before and after a 60-minute run in race-like conditions. The weight loss (in kg) multiplied by 1,000 gives you sweat loss in mL.
2. Check for salt residue. Heavy visible residue suggests concentrations toward the higher end of the range (1,000+ mg/L). Minimal or no residue suggests lower concentrations (400–700 mg/L).
3. Cross-reference with perceived cramping history. Frequent cramping during hot races, despite adequate hydration, often signals a sodium deficit.
4. For precision, consider a sweat sodium test. Several companies now offer at-home testing kits that directly measure sodium concentration.

What changes in heat and humidity?

Heat increases carbohydrate oxidation, which means you burn through fuel faster, even at the same pace. If you don't adjust, you under-fuel by 20–30 percent. Research shows that exercising in heat (35°C vs. 20°C) increases muscle glycogen utilization by approximately 25% at the same relative intensity. Your fuel tank drains faster, period.

The catch is classic: heat also reduces gut headroom. Blood flow shifts to the skin for cooling, leaving less available for the GI tract. Gastric emptying slows. Absorption efficiency drops. You need more fuel precisely when your tolerance for it decreases.

The fix is tactical. Shift toward liquids that empty faster than gels. Shrink the bolus size—take smaller, more frequent sips rather than large gulps. Increase frequency to hit intake targets across more touchpoints. And accept a slightly lower ceiling on very hot days. If you normally tolerate 110 g/h in cool conditions, plan for 90–100 g/h when it's 80°F and humid.

Marathon fueling targets by pace and weather

Pace band	Cool 50–60°F	Moderate 60–75°F	Hot 75°F+
Sub-3:00 (≤6:50/mi)	100–120 g/h, 400–600 mL/h, 600–900 mg Na/h	100–110 g/h, 500–700 mL/h, 800–1200 mg Na/h	90–100 g/h, 600–800 mL/h, 1000–1500 mg Na/h
3:00–3:30 (6:50–8:00/mi)	90–110 g/h, 300–500 mL/h, 500–800 mg Na/h	90–100 g/h, 400–600 mL/h, 700–1000 mg Na/h	80–95 g/h, 500–700 mL/h, 900–1300 mg Na/h
3:30–4:00 (8:00–9:10/mi)	80–100 g/h, 300–400 mL/h, 400–700 mg Na/h	80–95 g/h, 350–550 mL/h, 600–900 mg Na/h	75–90 g/h, 450–650 mL/h, 800–1200 mg Na/h
4:00+ (≥9:10/mi)	70–90 g/h, 250–350 mL/h, 300–600 mg Na/h	70–85 g/h, 300–450 mL/h, 500–800 mg Na/h	65–80 g/h, 400–600 mL/h, 700–1000 mg Na/h

How should I pace fluids without overdrinking?

Even pacing in the heat often leads to blow-ups late if you pace too bravely early on. The physiological cost of pace rises exponentially with temperature. What feels controlled at mile 8 becomes unsustainable by mile 18 when core temperature climbs and cardiovascular strain compounds.

Start conservatively. In hot conditions, that often means 10–15 seconds per mile slower than your cool-weather goal pace for the first half. Manage replacement percentage rather than chasing impossible volumes. Complete fluid replacement is neither necessary nor achievable for most marathoners. Research suggests that maintaining body weight loss under 2–3% of starting weight is sufficient to avoid performance decrements.

And here's a detail that matters: room-temperature fluids empty faster than cold ones, even if they feel less refreshing. Gastric emptying rate peaks around 10–15°C (50–60°F). Ice-cold fluids (0–5°C) slow gastric emptying by up to 50%. You're optimizing throughput, not vibes. If performance is the goal, skip the ice bath in your water bottle.

Does caffeine help marathon performance?

Caffeine works. The evidence is overwhelming. Moderate doses (3–6 mg/kg body weight) improve endurance performance by 2–4%, reduce perceived exertion, and delay fatigue. For a 70 kg runner, that's 210–420 mg total—roughly two to four cups of coffee, or two to four caffeine gels.

Dose and timing are individual, but the position stands from the International Society of Sports Nutrition backs its performance benefit across a wide range of endurance events. Build it into training so race day is just repetition. You're not trying new tricks on the last lap of a project.

Sample race-day fueling timeline

None of this matters unless you can execute a plan at your actual pace and in your actual weather. Before diving into race prep, make sure you've nailed your 48–72 hour carb-loading plan so your race-day fueling strategy starts with fully topped-off glycogen stores.

Troubleshooting and a quick reset protocol

When distress shows up, you need a reset protocol, not stubbornness. GI issues mid-race aren't a sign of weakness—they're feedback. Ignoring them doesn't make you tough. It makes you slow.

Back off briefly. Switch to smaller sips. Dilute concentration if you're drinking high-carb fluids. Pause solid intake for 10–15 minutes. Resume at a reduced rate once symptoms settle. That's how you save a day.

Frequently asked questions

How many gels do I need for a marathon?

A typical gel has about 25 grams of carbs. At 90–120 g/h for a 3–4 hour race, plan 11–16 gels or an equivalent mix of gels and sports drink. Use more liquids in hot weather.

What is the best glucose to fructose ratio?

A 2:1 glucose to fructose ratio is a reliable default and some athletes do well near 1:0.8. Test in training to find your tolerance.

Should I fuel differently in hot weather?

Yes. Shift 60–70% of your carbs to liquid form in hot conditions, reduce your total target by 10–15% if needed, and increase frequency of smaller doses rather than large boluses.

Can I carb load and fuel at high rates if I normally eat low-carb?

Yes, but the transition needs careful management. Start your gut training earlier (10–12 weeks out) and allow extra time for digestive adaptation. Your gut can adapt, but it needs progressive exposure.

The Bottom Line

Modern marathon fueling is not the 60 g/h ceiling your coach learned in 2005. It's a multi-transporter, gut-trained, climate-adjusted system that starts with your individual sweat profile and builds from there. The protocol is specific: 90–120+ g/h from glucose-fructose blends, sodium matched to your losses, fluids titrated to conditions, caffeine timed strategically, and contingency plans for when things go sideways.

Execute that, and you'll arrive at mile 23 with something left. Skip it, and you'll arrive at mile 23 wondering what happened. The physiology doesn't negotiate. Fuel the engine or watch it quit.

Training gets you fit. Gut training gets you fueled. Both are necessary. Neither is sufficient alone. Build the system the same way you build your mileage—progressively, consistently, and with respect for adaptation time—and race day becomes execution, not improvisation.

Use the Marathon Fueling Calculator to dial in your exact targets and test them during your next training block.