Fueling Long Training Sessions: Carbohydrate and Fluid Intake During Exercise

When During-Training Fueling Matters

Your pre-workout meal and your recovery meal bracket a session, but they do not fully cover what happens in the middle of a long one. Once training stretches past roughly an hour, the fuel you take in while you are moving starts to influence how well you hold pace, power, and concentration.

Sports nutrition guidance from the American College of Sports Medicine, summarized in a review of carbohydrate intake during exercise, recommends roughly 30 to 60 grams of carbohydrate per hour during endurance exercise lasting more than about one hour, or around 0.7 grams per kilogram of body weight per hour. For sessions of about two to three hours, roughly 60 grams per hour is the usual target. Ultra-endurance events push that range higher, to about 90 grams per hour, and only when the carbohydrate is supplied in a form the gut can absorb quickly.

A separate physiological limit explains why the type of carbohydrate matters. A single carbohydrate source — glucose or sucrose, for example — can be oxidized at rates up to approximately 60 grams per hour. Beyond that, the absorptive pathway that handles it becomes saturated and additional carbohydrate sits in the intestine, which is where nausea and cramping often begin.

If you simply want a quick rule for planning: sessions under about an hour generally need no in-session carbohydrate, sessions of one to three hours sit in the 30 to 60 grams per hour band, and longer or more demanding efforts can justify up to about 90 grams per hour with a mixed carbohydrate source. A carbohydrate mouth rinse can produce performance benefits during high-intensity efforts of about one hour even without swallowing anything, according to the same review.

• Under about 60 minutes: in-session carbohydrate is usually unnecessary; a mouth rinse may help high-intensity work.
• One to three hours: aim for roughly 30 to 60 grams of carbohydrate per hour, with about 60 grams per hour common for two- to three-hour efforts.
• Beyond three hours or very high demands: up to about 90 grams per hour, using multiple transportable carbohydrates.
• Carbohydrate recommendations during exercise are set by the type and duration of activity, not by body weight or training status.

Sources: PubMed Central (National Library of Medicine), The Journal of Nutrition (Elsevier, on behalf of American Society for Nutrition), Practical Gastroenterology

Hourly Carbohydrate Targets by Session Length

The table below turns the guidance above into a planning reference. Treat the numbers as ranges to test in training, not as prescriptions, and remember that the ceiling for a single carbohydrate source is lower than the ceiling for a blend.

One nuance that surprises many athletes: the recommended intake is independent of body weight and training status. A lighter athlete and a heavier athlete training for the same duration at the same intensity are working from the same hourly range. What changes is how much you personally tolerate and how much you have practiced.

The 90 grams per hour figure reflects a genuine absorptive limit, not a preference. To go above roughly 60 grams per hour, the carbohydrate has to use more than one intestinal transporter, which is why products designed for high intake combine glucose and fructose rather than relying on glucose alone.

Sources: PubMed Central (National Library of Medicine), Practical Gastroenterology, Gatorade Performance Partner

Choosing Carbohydrate Sources That Tolerate Well

Carbohydrate during exercise can come as a liquid, a semi-solid, or a solid. Oxidation rates and performance outcomes do not appear to differ between these forms, so the deciding factor is usually comfort and convenience rather than physiology. Solid foods and highly concentrated carbohydrate solutions, however, have been shown to slow fluid absorption, which matters when you are also trying to drink enough.

When intake exceeds 60 grams per hour, a 2:1 ratio of glucose to fructose is the common recommendation. The two sugars use separate transport pathways in the intestine, so pairing them allows higher oxidation rates than glucose alone and reduces the amount of carbohydrate that accumulates in the gut. One brand education resource describes an increase in oxidation of roughly 50 percent with a 2:1 blend compared with glucose alone; treat that figure as directional rather than precise, since it comes from a single secondary source.

Your capacity to absorb carbohydrate during exercise can also fall when you restrict carbohydrate for long periods, including low-carbohydrate, high-fat or ketogenic diets. If you have been training with very little carbohydrate and then attempt a high intake on event day, expect tolerance to be worse than it would otherwise be.

• Liquid, gel, chew, and solid forms can all work; there is no clear performance advantage to one form over another.
• Above 60 grams per hour, look for a glucose-to-fructose blend, commonly around 2:1.
• Highly concentrated solutions and solid foods can slow fluid absorption.
• Heavy carbohydrate restriction in daily training may reduce your ability to absorb carbohydrate during exercise.

Sources: PubMed Central (National Library of Medicine), Practical Gastroenterology, Gatorade Performance Partner, Cadence

Fluid Intake During Long Sessions

Fluid needs during long sessions vary enormously between individuals. Sweat rates can range from roughly 0.5 liters per hour to more than 3 liters per hour, so a single universal recommendation would be misleading. The practical approach is to estimate your own rate and then replace a sensible fraction of it.

A recommended method is to measure body weight before and after a training session and convert the change into fluid loss. This works best for sessions of about 45 to 75 minutes, and you can drink during the test if you want the estimate to reflect real conditions.

When you translate that estimate into a drinking plan, replace roughly 70 percent of your sweat losses, and no more than 100 percent. Going beyond full replacement risks over-hydration. The stomach empties larger volumes of fluid faster than smaller volumes, so sipping steadily tends to work better than large, infrequent drinks.

Carbohydrate concentration in your drink also affects how quickly fluid leaves the stomach. Sports drinks at up to about 6 percent carbohydrate — 6 grams per 100 milliliters — empty at rates similar to water, while beverages with an osmolality above roughly 500 mOsm/kg H2O can delay gastric emptying and are best avoided during exercise. Concentrated fruit juice and regular soft drinks sit in that range.

• Sweat rates commonly fall between about 0.5 and over 3 liters per hour.
• Estimate your own rate by weighing yourself before and after a 45- to 75-minute session.
• Replace about 70 percent of sweat losses, and never more than 100 percent.
• Keep drinks at or below roughly 6 percent carbohydrate to support gastric emptying.
• Avoid beverages above roughly 500 mOsm/kg H2O during exercise.

Sources: Practical Gastroenterology

Gastrointestinal Comfort and Practicing Your Plan

Gastrointestinal distress is the most common reason a well-designed fueling plan falls apart on event day. The gut is adaptable, though. Repeated exposure to carbohydrate and fluid during training — sometimes called gut training — can improve gastric emptying and intestinal absorption and reduce both how often and how severely symptoms occur.

Because of that, the plan itself needs rehearsal. Reviews of carbohydrate intake during exercise recommend practicing your competition nutrition strategy in training rather than debuting it on race day, and clinical guidance suggests allowing at least eight weeks to refine a hydration plan. That window gives you time to adjust concentrations, swap products, and find the timing that keeps your stomach settled.

Practicing is not a formality. A plan that works on paper can fail simply because the intake rate was never tested at race intensity, in race weather, with race nerves. Build toward your target intake gradually across weeks rather than jumping straight to your goal hourly rate.

1. Pick a target hourly carbohydrate intake and a fluid volume based on your session length and sweat-rate estimate.
2. Start at roughly half your target intake in training and increase over several sessions.
3. Use the same products, concentrations, and timing you intend to use in competition.
4. Note any GI symptoms, their timing, and what you changed — adjust one variable at a time.
5. Allow at least several weeks, and ideally around eight weeks for hydration, before your event.

Sources: PubMed Central (National Library of Medicine), Practical Gastroenterology, Gatorade Performance Partner

Adjusting Fuel for Heat and Intensity

Heat and intensity both raise the demand on your fueling plan, but they affect it in different ways. Higher intensity increases carbohydrate use, which pushes you toward the upper end of your hourly range for the session duration. Heat increases sweat rate, which pushes fluid and electrolyte losses upward.

The two pressures interact. If you are drinking more to keep up with sweat losses, you are also taking in more fluid volume, and if that fluid contains carbohydrate, your intake rate shifts. Keeping carbohydrate concentration moderate helps you meet fluid needs without overshooting on carbohydrate or slowing gastric emptying.

There is no single formula that converts temperature and intensity into a revised intake rate. The workable approach is to start from your tested plan, expect to drink more in heat, and treat any change to your carbohydrate rate as something to rehearse in training before you rely on it.

One area where athletes often look for more precision than the guidance provides is sodium. Estimating sweat rate is well supported, and the 70 to 100 percent replacement principle is well supported, but exact sodium amounts for a given session are not something to guess at from general guidance. If you have a history of cramping or a medical condition that affects fluid balance, that is a conversation to have with a qualified clinician rather than something to resolve from a chart.

• Higher intensity tends to raise carbohydrate needs toward the top of your hourly range.
• Heat raises sweat rate, so expect to drink more and to need more electrolytes.
• If your drink supplies carbohydrate, increasing fluid volume also increases carbohydrate intake — check both numbers together.
• Rehearse any heat- or intensity-driven change in training before using it in competition.

Sources: PubMed Central (National Library of Medicine), Practical Gastroenterology