The hypothesis for this study was that a higher dietary proportion of soluble fibre would result in stable and constant plasma metabolite and regulatory hormone concentrations. The study was a 4×4 Latin Square design with a sequence of 17 days adaptation to the ration followed by 8 sampling days. The feed rations consisted of only timothy hay (H), hay plus molassed sugar beet pulp combined with either whole oats (OB) or barley (BB) and hay plus a loose chaff-based concentrate (M). Four horses were fitted with permanent caecal cannulas and liquid caecal content was withdrawn manually and blood was drawn from the jugular vein at 0, 3 and 9 h postprandial. The horses were exercised daily at medium level for about 1 h. Samples were analysed for short-chain fatty acids (SCFA) and metabolic traits. Caecal SCFA and propionic acid concentrations increased with increased dietary starch and soluble fibre. The diet highest in soluble fibre (M) resulted in the highest plasma glucose and insulin concentrations in the morning, which then remained stable and constant throughout the day. A strong interaction (P<0.01) between time and diet was measured for plasma urea, glucose, insulin and leptin. The greatest variations in plasma glycaemic and insulinaemic responses were associated with the cereal grain diets (OB and BB). There were indications of a negative energy balance, which was reflected in a significantly higher plasma β-hydroxybutyrate concentration and a numerically higher non-esterified fatty acid concentration. In conclusion, this study found that inclusion of soluble fibre resulted in increased total caecal SCFA and propionic acid concentrations. This consequently resulted in stable and constant plasma glycaemic and insulinaemic responses. Diets with a high content of soluble fibre provided enough energy for horses at medium work level.