Fueling & Hydration for Runners

Every day, eat enough

The base is everyday eating. Eat enough for the training you do, and raise your carbohydrate as your training rises. Shorenstein, a sports dietitian for recreational and competitive endurance athletes, scales it at about 3 to 4 g per kilogram of body mass on days of under an hour of easy running and 5 to 7 g/kg for one to two hours, raised a slice of toast or a piece of fruit at a time. David Roche has the athletes he coaches eat 100 grams or more of protein a day, leaning each meal and snack a bit toward it. Do not cut calories and train hard at the same time. The guide on daily nutrition has the bands, what under-fueling looks like and what to eat around a run.

Weeks out, train your gut

Your stomach adapts to fuel the way your legs adapt to mileage. In a 2023 review, two trials that used two weeks of carbohydrate feeding during running or cycling cut gut discomfort by 47% on average¹. Start at least 12 weeks before a race you want to fuel hard in, and give yourself three to four months if you have frequent stomach trouble.

Build volume first and the carbohydrate rate second. Spend two to three weeks taking more fluid and food at once than usual on easy runs, then step the rate up on long runs, about 60 grams an hour for a couple of weeks and then 80 to 90. Rehearse near your race rate for four to six weeks, with the products and the clock you will race with. Harder sessions come last, because the gut copes least with fuel at high effort.

The adaptation fades. After two to three weeks without practice the top rates, 90 to 120 grams an hour, get hard again, so do the hard work close to the race. Keep a log of grams an hour, the form, how your stomach felt and how you felt the next day. The guide on training your gut matches the work to your symptoms.

The last day or two, carb load

Carb loading is for races longer than about 90 minutes: a marathon, an ultra, and a half marathon that takes you past 90 minutes. For a shorter race, eat normally. Coaches differ on how much: Shorenstein, a sports dietitian, scales carbohydrate to about 10 to 12 g/kg a day for a load, while David Roche, who coaches road, trail and ultra runners, does no grams-per-kilogram load, because the taper fills the muscle. In a 2026 trial of 11 endurance-trained adults, 10 g/kg produced clearly higher muscle glycogen than 6 or 8 g/kg did². Shift toward easy carbohydrate such as rice, bread, pasta, potatoes, oats and juice, keep fiber and fat low, and eat familiar food. Expect to feel full. The guide on carb loading has the detail.

Race morning

Eat the breakfast you ate before your long runs, at the same time before the start. Howe, who coaches trail and ultra runners at CTS, has athletes rehearse the race breakfast and its timing in training, because the gut needs to be ready for it. Carbohydrate eaten hours before the start raises liver and muscle glycogen, a 2004 review found³. Keep it low in fiber and light on fat. Take a gel or a few sips of sports drink in the last half hour only if that has gone well in training.

During the race, fuel by the clock

Under 90 minutes you need nothing in the race, so eat well beforehand and carry nothing. Past 90 minutes, carbohydrate you take in spares your own stores and keeps blood glucose up⁴. Take the first gel before you feel empty, at 60 to 75 minutes, and one about every 30 minutes after that, or every 35 to 40 if your stomach is touchy. Take it by the clock, not by feel. Shorenstein, a sports dietitian for recreational and competitive runners, sets the rate by how long you will be out:

• 90 minutes to about 2.5 hours: 30 to 60 grams an hour, starting nearer 30 to 40 if you have a history of stomach trouble.
• Longer than about 2.5 hours, which includes most marathons: 60 to 90 grams an hour, built up with practice.
• Ultras: Johnson, a sports dietitian for Western States runners, sees most of his athletes race at 70 to 90 grams an hour, holds them lower in heat or at altitude, and prefers a steady rate to a high start.

Which sugars and how to take them

Up to about 60 grams an hour, one sugar, glucose or maltodextrin, is enough. Above that, use a blend of glucose (or maltodextrin) and fructose, because fructose uses a different absorption route. Split the hour into equal feeds every 20 to 30 minutes, plan the amount per hour and not per bottle, and keep drinks dilute as runs get longer. A larger volume, a more concentrated solution and larger doses taken less often all raise the risk of stomach symptoms⁵.

Expect to miss a feed. Take the next one on schedule at the usual amount, because a double dose is more carbohydrate at once than your stomach has practiced. The guides on carbohydrate during exercise and race-day fueling have the amounts and the plan.

Fluid and sodium

Under an hour, drink when you are thirsty and plan nothing more. Past an hour, start from about 500 mL (17 oz) an hour in cool weather and up to 1 L an hour in heat, and let thirst steer. More than 1 L an hour is almost always too much. In a 2025 review of six studies in ultra runners, drinking to thirst was safe and effective for many experienced athletes, with no rise in hyponatremia and no measurable loss of performance⁶.

Overdrinking dilutes the sodium in your blood. In a 2005 study of the 2002 Boston Marathon, 13% of the 488 runners with a finish-line blood sample had low blood sodium, and it went with gaining weight during the race⁷. Treat thirst as the guide, and weight gained across a race as the warning sign.

For long runs and hot days, a drink with about 500 to 1,000 mg of sodium per liter suits most runners, toward the top for salty sweaters and in summer. A 2022 review calls the cause of cramps controversial and points to several factors acting together, and tired muscles are one of them⁸. If you cramp late, check both whether the run went past what you trained for and whether your drink carries enough sodium for your sweat. For heat, get used to it, slow down for it and cool during the race. The guide on hydration and electrolytes has the sweat test, the heat plan and the cramp advice.

Write the plan down

Nothing is new on race day, and that includes the rate: 500 mL an hour in training and a liter in the race is new. Write the plan down, with the grams an hour, the products, the time or mile marker of each feed, the drink plan and the breakfast, and write up what happened when you finish. A rate that felt fine in one build can turn on you in the next, and the last one on paper is how you know.