Gastric emptying

Gastric emptying is the process by which liquids and food leave the stomach and enter the small intestine. In sports nutrition, it determines how quickly water and carbohydrates ingested during exercise become available for absorption and energy.

Factors that influence gastric emptying

Gastric emptying is an important step in hydration because fluid and carbohydrate must leave the stomach before they can be absorbed. In general, the stomach can empty fluid at a maximal rate of roughly 15–20 mL per minute, which equates to about one litre per hour, when gastric volume is maintained using water or a dilute carbohydrate drink. Several factors alter this rate. Dehydration above roughly 3 % body weight and heat stress reduce gastric emptying, and exercising at intensities greater than about 70–75 % of maximal oxygen uptake also slows emptying and intestinal water absorption. Carbohydrate concentration and osmolality are important because water follows solutes in the small intestine. Solutions containing 1–3 % carbohydrate stimulate sodium–glucose cotransport and improve fluid delivery compared with carbohydrate‑free drinks. As concentration rises, energy density and osmolality increase, causing a slower rate of gastric emptying and fluid delivery. Drinks with eight percent carbohydrate or higher have been shown to reduce emptying and increase gastrointestinal discomfort. Early studies found that mono‑ and disaccharides slow emptying in proportion to concentration; glucose slows emptying more than galactose, while fructose is relatively ineffective. Even very low amounts of simple carbohydrate (<2 g per 100 mL) reduce the emptying rate below that of water, whereas polymer (maltodextrin) solutions can empty at rates similar to water up to about five percent carbohydrate. Combining multiple transportable carbohydrates such as glucose and fructose may increase emptying and absorption at higher concentrations.

Balancing carbohydrate concentration and speed

For endurance training sessions, most sports drinks provide 6–8 % carbohydrate to balance fluid delivery and energy supply. A hypotonic solution containing about 3 % carbohydrate empties from the stomach quickly and encourages water absorption, whereas a concentrated drink containing 10 % carbohydrate slows emptying and may cause stomach fullness. Drinks that use maltodextrin in place of simple sugars can have lower osmolality at the same carbohydrate content, which supports faster emptying. Combining glucose and fructose in a 2:1 ratio uses different intestinal transporters to improve carbohydrate availability and reduces the likelihood of stomach upset. In recovery periods, thicker drinks such as milk or protein‑carbohydrate blends deliberately slow gastric emptying; the coagulation of casein in milk delays emptying compared with whey‑based drinks.

Understanding gastric emptying helps athletes select appropriate drinks for exercise and recovery. Lower‑osmolality carbohydrate solutions leave the stomach quickly and deliver both water and energy, while more concentrated or protein‑rich beverages slow emptying and are better suited for times when rapid fluid uptake is less important.

Related Terms: Intestinal absorption, Glucose:fructose ratio, Carbohydrate per hour, Carbohydrate periodization, Palatability