Carbohydrate digestion and absorption studies in the horse

The ability of the horse to digest and absorb soluble carbohydrates was assessed using a series of oral disaccharide tolerance tests followed in the same animals by tolerance tests with the constituent monosaccharides. In horses older than three years, lactose did not produce an increase in the plasma glucose levels but induced the passing of soft faeces, indicating that adult horses are lactose intolerant. Horses of all ages could absorb the glucose: galactose mixture without any change in the faeces. The tolerance is due to a failure to hydrolyse lactose and does not involve the monosaccharide transport systems. These findings correspond to the known development pattern of brush border lactase activity in the equine small intestine. Both sucrose and maltose were rapidly hydrolysed, the resulting tolerance curves closely approximating to those for the constituent monosaccharides. Galactose was absorbed at a similar rate to glucose, although a dose of 1g/kg was necessary to produce galactosaemia. An oral lactose tolerance test (1 g/kg as a 20 per cent solution) could be of clinical value to determine small intestinal mucosal damage in diarrhoeic foals when the continued ingestion of lactose might be detrimental.