Both the amount and source of dietary carbohydrates have important physiological roles in human health and disease. However, characterisations based solely on chemical attributes do not adequately describe the physiologic fate of carbohydrates. The glycemic index (GI) has been proposed as a measure of carbohydrate quality, but does not necessarily distinguish between different types of low GI foods that have different nutritional properties. There is a need to establish a better description of the physiological attributes of carbohydrates, which is the main focus of this thesis. This thesis describes a physicochemical characterisation of carbohydrates based on their chemical composition and likely gastrointestinal fate. Modifications were made to existing methodology for the measurement of rapidly (RAG) and slowly (SAG) available glucose, and resistant starch. Using this method, with improved precision and quality control, a database of physicochemical carbohydrate fraction profiles for 189 foods was established. The content of the SAG fraction, a marker of slow release carbohydrates, ranged from <5% in potatoes, bakery products and breakfast cereals to >35% in some legumes, pasta and whole kernel cereals. The superiority of the RAG and SAG measures over conventional carbohydrate classification schemes in explaining glycemic responses was demonstrated in two studies. In one study (n = 23 cereal products) the RAGS:SAG model explained 69% of the variation in GI values for a selection of cereal products, which was considerably greater than other models (e.g. 34% for the starch and sugar model). The other study, incorporating two carbohydrate portion sizes, found that the RAG fraction explained 70% of the within subject variation in the glycemic responses to test meals, significantly greater than the 42% explained by the total carbohydrate intake. This study also demonstrated that the magnitude of the glycemic response was largely determined by RAG intake, rather than SAG intake which had a minimal effect. It appears that SAG is a marker of slow release carbohydrates that do not elevate the glycemic potential of foods. An epidemiological study (n=684) was used to establish the intake of the physicochemical carbohydrate fractions: RAG, 62%; SAG, 4%; RS, 2%; fructose, 21%; other (mainly lactose), 11%. Although the average intake of SAG was small, it had a 13 fold intake range (1.5-19.5g/day) indicating that it could be utilised in identifying dietary patterns. Indeed, it was shown that the physiochemical carbohydrate fractions were significant determinants of dietary GI and glycemic load values. The initial investigation of relationships with metabolic markers for disease risk found that correlations were generally poor, both for the physicochemical carbohydrate fractions and for other nutrients with previously established associations. An extended study adjusting for confounding factors is warranted. In conclusion, the RAG and SAG measures provide a new perspective on the physiological role of carbohydrates, which can be applied to metabolic and epidemiological studies investigating the complex role of carbohydrate quality in nutrition.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598852 |
| Date | January 2002 |
| Creators | Englyst, K. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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