Fueling and Nutrition

You can train perfectly for eighteen weeks and still fall apart at mile 20. Not because your aerobic base failed, but because you ran out of fuel. Nutrition is not a garnish on top of a training plan - it is one of the inputs the plan depends on. This section covers what to eat to support training, what to take in during a race, and why the marathon has a "wall" while the 5K does not.

This is educational material, not medical or dietary advice. Individual needs vary widely. If you have a medical condition, a history of disordered eating, or specific dietary requirements, work with a physician or a registered dietitian.

The Energy Systems View

Your body runs on two main fuels: fat and carbohydrate. Fat is effectively unlimited. Carbohydrate is not.

Even a lean runner carries tens of thousands of calories of stored fat - enough to run for days. What you cannot store much of is carbohydrate, held as glycogen in muscle and liver. A trained runner stores roughly 1,500-2,000 kcal of glycogen, and running burns roughly 100 kcal per mile.

Do the arithmetic and the whole problem appears:

• A 5K burns ~300-350 kcal. You could run it after an overnight fast and never come close to the bottom of the tank.
• A marathon burns ~2,600-2,800 kcal. That is more than you can store.

Why Intensity Changes the Math

Fat and carbohydrate are not interchangeable. As intensity rises, the fuel mix shifts toward carbohydrate, because carbohydrate produces energy faster and needs less oxygen per unit of ATP.

• Easy running: a large share of energy from fat, glycogen spared
• Marathon pace: predominantly carbohydrate, but with a meaningful fat contribution
• Threshold and above: almost entirely carbohydrate

This is why stored glycogen lasts roughly 90-120 minutes of hard running. Run harder, and you empty the tank sooner.

The Wall, Explained

"Hitting the wall" is glycogen depletion. When muscle glycogen runs low, you cannot generate energy fast enough to hold pace, and blood glucose falls, which is why the experience is both muscular and cognitive - your legs feel hollow and your judgment goes with them. The pace you drop to is the pace fat alone can support, and it is much slower than the pace you trained for.

Key insight: The marathon is the only common race distance where nutrition can beat your fitness. A 5K runner does not bonk. A marathoner absolutely can, and it is the single most common way a well-trained marathon goes wrong.

There are only three levers against it, and you want all three:

1. Store more (train the aerobic system; carb load before the race)
2. Spend less (start at the right pace - see Race Day Execution)
3. Take some in (fuel during the race)

Daily Nutrition: Fueling the Training

Before you worry about gels, get the ordinary days right. Most runners under-fuel training and then try to fix it on race morning. That is backwards.

Carbohydrate: Scale It to Your Training Load

Daily carbohydrate needs are expressed in grams per kilogram of body weight per day (g/kg/day) and scale with how much you are actually running, not with how serious you feel.[1]

Most recreational marathoners in a build block land in the 6-8 g/kg/day range. Note how large those numbers are in real food: 500 g of carbohydrate is roughly 2,000 kcal from carbohydrate alone.

The practical version: carbohydrate intake should rise on hard days and long-run days and can fall on rest days. Fuel the work you are actually doing.

Signs you are chronically under-fueling carbohydrate:

• Workouts that used to feel routine now feel heavy
• Persistent fatigue that rest days do not fix
• Long runs that fall apart in the final third, every time
• Poor sleep, irritability, frequent illness
• Stalled progress despite consistent training

Protein: The Adaptation Nutrient

Carbohydrate fuels the run. Protein builds the adaptation the run was for. Training breaks muscle protein down; protein intake plus rest builds it back stronger, and that is what recovery and adaptation actually consist of.

• Target: 1.2-2.0 g/kg/day for endurance athletes, with higher end during heavy training or when in an energy deficit[1]
• Spread it out: roughly 0.3 g/kg per meal (about 20-30 g of quality protein), four to five times a day, beats one large hit at dinner
• After hard sessions: protein plus carbohydrate within a few hours supports both glycogen resynthesis and repair. The old "30-minute anabolic window" is oversold, but it matters most when you have another hard session within 24 hours

Fat, and the Bigger Picture

Do not run a very-low-fat diet. Fat carries fat-soluble vitamins and supports hormone production; keeping fat around 20-35% of total energy is a reasonable range.

The single biggest nutritional error in distance running is not eating enough total food. Chronically low energy availability - too few calories for the training load - degrades bone health, hormonal function, immunity, and performance in both men and women (this is the core of RED-S, Relative Energy Deficiency in Sport).[1] Running is not a sport where less is safer. If you are training hard, losing weight fast, and getting slower, the problem is almost certainly the total.

Training Nutrition vs Race Nutrition

These are different jobs and it is worth naming the difference explicitly.

A useful way to hold both: train mostly like a person who has to do this again tomorrow; race like a person who does not.

In-Race Carbohydrate: Targets by Duration

How much fuel you need during a race depends almost entirely on how long you will be out there.

Why 60 g/hr Is a Ceiling for a Single Sugar

Glucose crosses the intestinal wall through the SGLT1 transporter, and that transporter saturates at roughly 60 g/hr. Beyond that, extra glucose does not get absorbed - it sits in your gut, draws in water, and gives you the GI distress that ends races.

Fructose uses a different transporter, GLUT5. Because the two pathways are separate, combining glucose and fructose (commonly around a 2:1 or 1:0.8 ratio) lets you absorb and oxidize up to ~90 g/hr - substantially more total fuel, with less GI distress than trying to force the same amount through glucose alone.[2]

This is why product labels matter. A gel listing "maltodextrin and fructose" is engineered for this. A gel with glucose only is not, and stacking three of them per hour will not work.

What That Looks Like in Practice

A typical gel contains 20-25 g of carbohydrate. Most sports drinks run ~6-8 g per 100 ml (roughly 15-20 g per standard cup at an aid station, though you rarely drink the full cup).

Marathon example, 3:30 finish, targeting ~60 g/hr:

• Roughly 210 g of carbohydrate total
• About 8-9 gels across the race, or ~1 every 25 minutes
• Practically: take one gel every 4-5 miles starting around mile 4-5, with water (never with sports drink, which stacks concentration on concentration)

Do not wait until you feel low. By the time you feel it, absorption takes 15-20 minutes and you are already behind. Fuel on a schedule, from early, while your gut still works.

Half marathon: 1-3 gels for most runners, or a well-timed drink or two. Well-trained runners going under ~90 minutes may need very little.

10K and 5K: nothing. Drink if you want to. You will finish before fuel is ever the limiter.

Train the Gut

Your gut is trainable, and an untrained gut is the reason most in-race nutrition plans fail.

Absorbing 60-90 g/hr while running hard is a physiological skill. Intestinal transporter expression and gastric tolerance both improve with repeated exposure. A runner who has never taken a gel in training and swallows three on race day is running an experiment at the worst possible moment.

How to do it:

1. Use your long runs as the laboratory. They are the only sessions long enough to matter.
2. Practice at race pace where you can. A gel that sits fine on an easy jog can come straight back up at marathon effort. Fuel during your marathon-pace segments.
3. Practice the exact products, in the exact amounts, on the exact schedule you will use on race day.
4. Build up. Start at 30 g/hr, work toward your target over several weeks.
5. Rehearse race morning too. Eat your intended pre-race breakfast, at the intended interval before the run, before at least two or three long runs.

By race day, nothing about your fueling should be new information to your stomach.

Hydration

Hydration advice was wrong for a generation, and the correction matters: there are two failure modes, not one.

Drink to Thirst

For the large majority of runners, in the large majority of conditions, drinking to thirst is the right instruction. Thirst is a well-calibrated signal. The old advice to drink at fixed intervals, as much as tolerable, "before you feel thirsty," was not just unnecessary - it caused harm.

Both Directions Are Dangerous

Dehydration (losing more than roughly 2-3% of body mass) can degrade performance and, in heat, contributes to heat illness. This is real.

Hyponatremia (exercise-associated hyponatremia, or EAH) is the opposite problem: drinking so much fluid that blood sodium is diluted to dangerous levels. Symptoms range from nausea, bloating, headache and confusion to seizure and death. Crucially, it is caused by overdrinking, and it is the failure mode created by rigid drinking schedules.[3] It is more common in slower marathoners who take a cup at every aid station over four-plus hours.

The nasty part: early hyponatremia and dehydration can feel similar (headache, nausea, disorientation). If you have been drinking heavily, are gaining weight during the race, and feel bloated and nauseated, the answer is not more water.

Practical Guidance

• Drink to thirst. That is the headline.
• Do not force fluids on a schedule and do not aim to finish a marathon weighing the same as you started. Some weight loss over a marathon is normal and expected.
• Know your sweat rate if you want a sanity check: weigh yourself nude before and after a one-hour run in race-like conditions, without drinking. Each 1 kg lost is roughly 1 litre of sweat. This tells you the scale of your losses; it is not a prescription to replace all of it.
• Include sodium in longer events, via sports drink or salt supplementation, especially if you are a heavy or salty sweater and racing longer than about two hours. Sodium supports fluid balance and reduces hyponatremia risk.
• Heat changes everything. In hot conditions, fluid needs rise, pace expectations must fall (see Race Day Execution), and both risks increase.

Caffeine

Caffeine is one of the few legal supplements with genuinely strong evidence behind it for endurance performance.[4]

• Dose: 3-6 mg/kg body weight (roughly 200-400 mg for a 70 kg runner) taken ~45-60 minutes before the start. More is not better; higher doses mainly add side effects.
• Mechanism: primarily central - it blunts perception of effort and fatigue, so hard paces feel more tolerable.
• In-race: caffeinated gels are a reasonable way to top up in the back half of a marathon, when the psychological lift matters most.
• Side effects: jitters, elevated heart rate, sleep disruption, and - importantly for runners - accelerated GI motility. Some people cannot take it and race.
• Habituation: regular users still benefit, so you do not need to abstain for weeks beforehand. If you do want to cut back to sharpen the effect, taper caffeine gradually and never on race week alone - a caffeine-withdrawal headache on race morning is a bad trade.

And obviously: test it in training first.

Carbohydrate Loading

Carb loading is a real, evidence-based protocol. It is not "a big bowl of pasta the night before."

The goal is to supersaturate muscle and liver glycogen stores above normal levels, so you start the race with a bigger tank. One dinner cannot do this; glycogen storage is a multi-day process, and a single enormous meal mostly makes you feel heavy at the start line.

What It Actually Is

• Timing: the final 36-48 hours before the race
• Amount: roughly 10-12 g/kg of body weight per day (for a 70 kg runner: 700-840 g/day - a lot of carbohydrate)[1]
• Requires a taper: the modern protocol works precisely because your training volume is already low. Loading only works alongside reduced training. The old "depletion phase" (a miserable week of low-carb hard training first) is obsolete and unnecessary.
• It replaces fat and protein calories, not adds to them. You are shifting the composition of your intake, not simply eating more of everything.

Practical Notes

• Prefer low-fibre, easy-to-digest carbohydrate in the last 24-36 hours: white rice, white bread, pasta, potatoes without skins, sports drinks, bananas, low-fibre cereal. This is the one time to avoid a huge salad and a bean-heavy meal. Fibre in, fibre out, usually around mile 8.
• Expect to gain 1-2 kg. Every gram of stored glycogen binds roughly 3 grams of water. This weight is not fat, it is fuel and the water that comes with it. Do not panic on the scale and do not try to correct it.
• You will probably feel bloated, heavy, and sluggish. That is what a full tank feels like. It is also, unhelpfully, exactly what the taper feels like. Both resolve on race morning.
• Applies to the marathon; it is optional for the half and pointless below that. A 5K does not need extra glycogen.

Race Week and Race Morning

Race week: Do not experiment. Eat familiar food, keep hydration normal, and drop fibre in the last day or two. Alcohol impairs glycogen storage and sleep quality; skip it.

The night before: A normal-sized, familiar, carbohydrate-rich, low-fibre, low-fat dinner. Eaten reasonably early. Not a novelty restaurant, not a huge portion. The loading happened over the previous two days, not at this meal.

Race morning:

• Eat 2-4 hours before the start, allowing time to digest and use the bathroom
• 1-4 g/kg of carbohydrate depending on how much time you have and what your gut tolerates - for many runners, a bagel or toast with jam, a banana, and coffee is the whole answer
• Sip fluid normally; do not chug in the final hour
• Whatever you eat, you should have eaten it before several long runs already

The Golden Rule: Nothing New on Race Day

Not the gel. Not the drink. Not the breakfast. Not the caffeine. Not the shoes. Not the shorts. Not the sunscreen.

Race day is the day you execute a plan you have already rehearsed, not the day you try the interesting product in the goodie bag. Every element of your fueling should be something your gut has done before, repeatedly, at effort.

The expo will sell you a new supplement. The aid station will hand you a brand you have never used. A stranger in the corral will tell you what works for them. The answer is no. You are not missing an edge; you are being offered a way to lose one.

Common Mistakes

1. Under-fueling training, then over-fueling the race. Backwards. The daily plate matters more than the gel.
2. Waiting until you feel low to take a gel. By then you are 20 minutes behind absorption.
3. Taking three glucose-only gels in an hour and wondering why your stomach revolted. You cannot absorb it. Use glucose + fructose products above 60 g/hr.
4. Taking a gel with sports drink. Too concentrated. Take gels with water.
5. Drinking at every aid station on a schedule. This is how hyponatremia happens. Drink to thirst.
6. Treating "carb loading" as one enormous pasta dinner. That is a heavy stomach, not a full tank.
7. Never practicing race fueling in long runs. Your gut is trainable and untrained.
8. Trying anything new on race day. The most preventable mistake in the sport.

Key Takeaways

1. Fat is unlimited; glycogen is not. Stored carbohydrate covers roughly 90-120 minutes of hard running - which is exactly why the marathon has a wall and the 5K does not.

2. Daily carbohydrate should scale to training load - roughly 5-7 g/kg/day at moderate volumes, 6-10 g/kg/day in a real endurance build.

3. Protein (1.2-2.0 g/kg/day) is what turns training stress into adaptation. Spread it across the day.

4. In-race fuel is a function of duration: nothing under ~75 minutes, 30-60 g/hr from 1-2.5 hours, 60-90 g/hr beyond that using glucose + fructose mixes that exploit separate intestinal transporters.

5. Train the gut. Practice the exact products, amounts, and timing in your long runs, at race pace.

6. Drink to thirst. Both dehydration and hyponatremia are real, and rigid drinking schedules cause the second one.

7. Carb loading is 36-48 hours at ~10-12 g/kg/day during the taper, not a pasta dinner. Expect to gain water weight and feel heavy. That is the point.

8. Nothing new on race day. Ever.

Remember: Nutrition cannot make an undertrained runner fast. But bad nutrition can make a well-trained runner slow, and in the marathon it is the most common way that happens.

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References

[1] Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). "Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance." Journal of the Academy of Nutrition and Dietetics, 116(3), 501-528. DOI: 10.1016/j.jand.2015.12.006

[2] Jeukendrup, A. (2014). "A step towards personalized sports nutrition: carbohydrate intake during exercise." Sports Medicine, 44(Suppl 1), S25-S33. DOI: 10.1007/s40279-014-0148-z

[3] Hew-Butler, T., et al. (2015). "Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015." Clinical Journal of Sport Medicine, 25(4), 303-320. DOI: 10.1097/JSM.0000000000000221

[4] Guest, N. S., et al. (2021). "International society of sports nutrition position stand: caffeine and exercise performance." Journal of the International Society of Sports Nutrition, 18(1), 1. DOI: 10.1186/s12970-020-00383-4

[5] Burke, L. M., Hawley, J. A., Wong, S. H. S., & Jeukendrup, A. E. (2011). "Carbohydrates for training and competition." Journal of Sports Sciences, 29(Suppl 1), S17-S27. DOI: 10.1080/02640414.2011.585473

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Next: Race Day Execution