Intestinal absorption

Intestinal absorption is the uptake of fluids, electrolytes and nutrients across the walls of the small intestine into the bloodstream. In the context of sports nutrition it determines how quickly water and ingested carbohydrates move from the gut into the circulation.

How intestinal absorption works

After fluid leaves the stomach it passes into the proximal small intestine (duodenum and jejunum), where most water and solute absorption occur. Solutes such as glucose and sodium are taken up by specific transport mechanisms in the brush‑border membrane of intestinal epithelial cells, while water absorption is passive and depends on an osmotic gradient created by solute uptake. During exercise, dehydration and heat stress can impair gastric emptying and reduce intestinal water absorption. A small amount of carbohydrate stimulates sodium‑glucose cotransport; the World Health Organization’s oral rehydration solution contains about 1.35 % glucose for this reason. Studies with healthy individuals show that sports drinks containing 1–3 % carbohydrate improve fluid delivery compared with carbohydrate‑free solutions. High energy density or osmolality has the opposite effect; beverages with ≥ 8 % carbohydrate slow gastric emptying and fluid delivery and increase gastrointestinal discomfort.

The type of carbohydrate influences absorption because different sugars use distinct transporters. Glucose and galactose enter enterocytes via the sodium–glucose cotransporter 1 (SGLT1) and sodium facilitates water absorption, whereas fructose is absorbed independently through the GLUT‑5 transporter. Combining glucose or maltodextrin with fructose can improve water uptake by preventing saturation of SGLT1; however, large amounts of fructose or a fructose‑to‑glucose ratio greater than one can raise osmolality and reduce absorption. Drinks that supply only glucose are absorbed faster than fructose‑only drinks, but mixing multiple transportable carbohydrates increases solute and water absorption. Sodium is essential for co‑transport but the small intestine secretes sodium, so adding more sodium to a beverage plays only a minor role in intestinal fluid absorption. One study found that absorption was faster when sodium was paired with chloride rather than bicarbonate or sulfate. Protein and amino acids increase energy density and casein can clot in the stomach; protein‑rich beverages slow intestinal absorption but may be useful when slower fluid delivery is desired.

Applying intestinal absorption to sports drinks

A hypotonic sports drink with around 6 % carbohydrate and a small amount of sodium chloride is designed to promote rapid water uptake. Using maltodextrin in place of some simple sugars lowers osmolality at the same carbohydrate content, while including fructose alongside glucose uses different transporters to raise carbohydrate availability without saturating SGLT1. Highly concentrated drinks (≥ 10 % carbohydrate) or beverages sweetened primarily with fructose increase lumen osmolality and can slow absorption and cause gastric distress. In contrast, recovery shakes that combine protein and carbohydrate intentionally slow gastric emptying and absorption to support fluid retention and glycogen resynthesis.

Efficient intestinal absorption ensures that the fluids and carbohydrates consumed during exercise reach the bloodstream quickly. Athletes should choose drinks with appropriate carbohydrate concentration and mixed carbohydrate sources to optimise hydration and avoid gastrointestinal upset.

Related Terms: Gastric emptying, Glucose:fructose ratio, Electrolyte, Sodium, Palatability