Running Race Day: Carb Loading, Fueling, Hydration, Pacing and Heat

This article contains affiliate links: if you buy through them we may earn a commission, at no extra cost to you. Learn more.

Corral C, 6:55 a.m. Thousands of runners bouncing on their toes, a last sip of water, a gel tucked into a waistband, a watch searching for satellites. Somewhere ahead the elites are already off. Whether it’s a turkey trot 5K, a high school cross-country championship, a half marathon with friends or a big-city marathon, race day comes down to a few things you control: what you eat in the days before, what you take in during the race, how you pace it, and how you handle the weather. This page covers all of it, by distance, with the numbers sports dietitians use.

General guidance for healthy adult runners. If you have diabetes, a heart or kidney condition, high blood pressure or special diet needs, follow your doctor’s or a registered dietitian’s plan. Back to the running hub.

The days before: carb loading, when it helps

The joint position statement on nutrition and athletic performance from the American College of Sports Medicine (ACSM), the Academy of Nutrition and Dietetics and Dietitians of Canada is the reference most sports dietitians use. Its guidance, by event length:

• Races under 90 minutes (5K, 10K, most half marathons for faster runners): no special loading. “Fuelling up” with 7–12 g of carbohydrate per kg of body weight over the day before is enough.
• Races over 90 minutes (the marathon, slower half marathons, ultras): carb loading means 10–12 g/kg per day for the 36–48 hours before. For a 70 kg (154 lb) runner that’s 700–840 g of carbs a day, far more than most people expect. Bread, rice, pasta, potatoes, cereal, juice and sports drinks make it possible; cutting back on fat and fiber makes room.
• The pre-race meal: 1–4 g/kg of carbs, 1–4 hours before, going easy on fat, protein and fiber to avoid stomach trouble. A bagel, banana and sports drink is a classic.

The golden rule from the same statement: practice it. Nothing new on race day, not the breakfast, not the gel flavor, not the shoes. Our first marathon guide covers rehearsing fuel on long runs.

Fueling during the race

Time on course	Carbohydrate (ACSM 2016)	What that looks like
Under 45 min (most 5Ks and 10Ks)	Not needed	Water if it’s hot
45–75 min	Small amounts, even a mouth rinse	A few sips of sports drink
1–2.5 hours (half marathon, faster marathons)	30–60 g per hour	1–2 gels an hour, or sports drink plus a gel
Over 2.5–3 hours (most marathons, ultras)	Up to 90 g per hour, using glucose + fructose blends	About 3 gels an hour, if your gut is trained for it

Gels make the math easy. A Maurten Gel 100 has 25 g of carbs (glucose and fructose at 1:0.8), a GU Energy Gel 21–23 g, and an SiS GO Isotonic 22 g. Many runners take one every 20–30 minutes from early in the race rather than waiting until they’re fading. The higher intakes need gut training on long runs; jumping to 90 g/h on race day is a recipe for a stomach upset. Caffeinated gels exist (Maurten CAF 100 has 100 mg per gel); caffeine isn’t for everyone, especially with heart conditions, pregnancy or caffeine sensitivity, so check with your doctor and try it in training first. Full comparisons in our energy gels guide.

Hydration: enough, not too much

• Before: ACSM suggests 5–10 mL per kg of body weight in the 2–4 hours before the start (350–700 mL for a 70 kg runner), aiming for pale yellow urine.
• During: sweat rates range widely, from 0.3 to 2.4 L an hour. ACSM’s general range for drinking is 0.4–0.8 L per hour, adjusted to your sweat rate and the weather. The goal is to lose no more than about 2% of body weight.
• Don’t overdrink. The National Athletic Trainers’ Association (NATA) says athletes should not gain weight during exercise, and that drinking to thirst is a safe approach when you don’t know your sweat rate.
• Sodium: heavy, salty sweaters may benefit from extra sodium (NATA). Drink mixes like Skratch Labs or tablets like Nuun Sport (300 mg sodium per tablet) add it. Follow the label, and check with your doctor if you have high blood pressure, heart or kidney problems or are on a low-sodium diet. See our electrolyte guide.

Why not just drink at every station? At the 2002 Boston Marathon, researchers tested 488 finishers: 13% had hyponatremia (low blood sodium), and 0.6% a critical level. The biggest risk factors were gaining weight during the race, finishing slower than 4 hours, and drinking more than 3 liters or at every mile. Symptoms such as headache, nausea, confusion and swelling can look like dehydration, and it can be life-threatening, so a runner who is confused after a race needs medical help, not just more water.

If you race long or on trails, a soft-flask vest like the Nathan VaporAir Lite 4L lets you carry your own fluid and fuel; compare options in our running hydration vest guide.

Pacing: start slower than you want to

The excitement of the start is the enemy of a good marathon. A 2026 analysis of split times from 78,912 Boston Marathon finishers (2015–2017) found that even pacing was linked to the fastest finishes at every age and level, with average finish times far lower than in runners who slowed sharply in the second half (217 vs. 276 minutes). 82% of sub-3-hour runners paced evenly, against about 20% of casual participants, and runners with erratic pacing were most likely to “hit the wall” (a pace drop of more than 20% late in the race).

• Pick a goal pace you’ve proven in training (long runs, tune-up races), not one you hope for.
• Run the first mile or two a little slower than goal pace; crowds and adrenaline will try to pull you faster.
• On hills, run by effort, not pace. GPS pace jumps around in tall-building city centers, so lap the mile markers if your watch drifts.
• In 5Ks and 10Ks, the same idea holds over a shorter span: controlled first half, then push.

Racing in the heat

Heat slows everyone, and slower runners most. A study of seven North American marathons over decades, including Boston, New York and Twin Cities, found that as the wet-bulb globe temperature (WBGT, a heat index that includes humidity and sun) rose from 5–10 °C to 20–25 °C, top men finished about 1.7% versus 4.5% off the course record, and runners further back slowed even more. Some days are simply too hot: on October 1, 2023, the Twin Cities Marathon was canceled before the start under “black flag” conditions, with the temperature forecast in the 90s °F.

The 2023 ACSM consensus statement on heat illness includes a table for when to modify or cancel exercise. For continuous events like races, it rates risk for all runners as increased from a WBGT of about 19–22 °C, high for unfit or unacclimatized runners from about 22–26 °C, and recommends canceling from about 25–28 °C, depending on the region’s climate (cooler northern regions get lower thresholds). If race day is hot:

• Adjust your goal. Slow your pace from the start; this isn’t a day for a personal best.
• Cool on the outside: light clothing, a cap or visor, water over the head and neck, ice in the hat at aid stations.
• Respect the flags. Many races use color flags for heat risk; when the course is black-flagged or closed, stop.
• Know the danger signs. Confusion, stumbling or collapse in a hot race can be exertional heat stroke, a medical emergency. Medical teams cool first and transport second, ideally with ice-water immersion. More on our running injuries page.
• Acclimatize ahead: NATA recommends 7–14 days of gradual heat exposure.

Race-day checklist

• Bib pinned and timing chip on, shoes and kit you’ve already run long in.
• Anti-chafe on feet, thighs and underarms; sunscreen.
• Gels counted for the distance, plus one spare; check what the course hands out.
• Watch charged; know whether you’re pacing by watch or mile markers.
• A throwaway layer for a cold start; a warm-up jog of a few minutes for shorter races.
• Emergency info on the back of your bib or in your phone; a plan to meet friends after.

Race-day fuel many runners use

After the finish line, the work switches to recovery. Everyday fueling is on running nutrition. For a team-sport version of game day, see soccer game day.

Race day: FAQs

Do I need to carb load for a half marathon?

ACSM reserves full carb loading (10–12 g/kg a day for 36–48 hours) for events longer than 90 minutes. If your half takes longer than that, it can help; if you’ll finish faster, eating a carb-rich diet the day before (7–12 g/kg) is enough.

How many gels do I need for a marathon?

It depends on your finish time and gut. At 30–60 g of carbs an hour, a four-hour marathoner needs roughly 5–10 gels of about 25 g; at up to 90 g/h, more. Count sports drink from the course toward the total, and practice your plan on long runs.

Should I drink at every water station?

Not automatically. Drinking at every mile was one of the risk factors for hyponatremia at the Boston Marathon. Drink to thirst or to a plan based on your sweat rate, and aim not to gain weight during the race.

What is the best pacing strategy for a marathon?

Even pacing. In a study of nearly 79,000 Boston finishers, runners who held an even pace finished fastest across ages and levels, and those who slowed sharply finished about an hour slower on average.

Can a marathon be canceled because of heat?

Yes. The Twin Cities Marathon was canceled in 2023 for record heat, and the Chicago Marathon was stopped mid-race in 2007. ACSM’s heat table recommends canceling continuous events at high WBGT readings; follow the race’s flag system and officials.