Glycaemic index of cereal products explained by their content of rapidly and slowly available glucose

Elucidating the role of carbohydrate quality in human nutrition requires a greater understanding of how the physico-chemical characteristics of foods relate to their physiological properties. It was hypothesised that rapidly available glucose (RAG) and slowly available glucose (SAG), in vitro measures describing the rate of glucose release from foods, are the main determinants of glycaemic index (GI) and insulinaemic index (II) for cereal products. Twenty-three products (five breakfast cereals, six bakery products and crackers, and twelve biscuits) had their GI and II values determined, and were characterised by their fat, protein, starch and sugar contents, with the carbohydrate fraction further divided into total fructose, RAG, SAG and resistant starch. Relationships between these characteristics and GI and II values were investigated by regression analysis. The cereal products had a range of GI (28-93) and II (61-115) values, which were positively correlated (r(2) 0.22, P<0.001). The biscuit group, which had the highest SAG content (8.6 (sd 3.7) g per portion) due to the presence of ungelatinised starch, was found to have the lowest GI value (51 (sd 14)). There was no significant association between GI and either starch or sugar, while RAG was positively (r(2) 0.54, P<0.001) and SAG was negatively (r(2) 0.63, P<0.001) correlated with GI. Fat was correlated with GI (r(2) 0.52, P<0.001), and combined SAG and fat accounted for 73.1 % of the variance in GI, with SAG as the dominant variable. RAG and protein together contributed equally in accounting for 45.0 % of the variance in II. In conclusion, the GI and II values of the cereal products investigated can be explained by the RAG and SAG contents. A high SAG content identifies low-GI foods that are rich in slowly released carbohydrates for which health benefits have been proposed.