Absorption is the step in digestion where already-broken-down nutrients cross the lining of the small intestine and enter the bloodstream. It follows mechanical and enzymatic digestion but precedes the metabolic use of nutrients: until a molecule has been absorbed, the body cannot act on it.
In horses, the small intestine is the primary site of absorption for simple sugars, amino acids, and fatty acids. Water and electrolytes are absorbed in the large intestine — the hindgut fermentation chamber that also extracts volatile fatty acids from fiber. This division matters clinically: a horse with colic affecting the large colon disrupts electrolyte and water absorption, which compounds dehydration risk. A horse on a high-starch diet that overwhelms the small intestine pushes undigested carbohydrate into the hindgut, where rapid fermentation produces lactic acid and raises the risk of laminitis — a failure of absorption geography, not just quantity.
Absorption rate is affected by feed composition, particle size, intestinal motility, and the health of the gut mucosa. Horses with parasitic damage, inflammatory bowel conditions, or prolonged antibiotic use may show reduced absorptive capacity even when diet is adequate, explaining body condition loss that does not respond to increased feed. Understanding absorption helps interpret a horse’s body condition score — a low score in a horse eating well points toward a problem upstream of use, often in absorption itself. See also how parasite load reduces absorption for practical implications.
Further Reading
- Pharmacological absorption — Wikipedia overview of how drugs and nutrients cross biological membranes.
- Intestinal nutrient transport in horses — peer-reviewed research on equine small-intestinal absorption mechanisms (PubMed Central).