We show the significance of in vitro measurements of RAG in relation to glycemic response in human studies. The simple in vitro measurement of RAG and SAG is of physiologic relevance and could serve as a tool for investigating the importance of the amount, type, and form of dietary carbohydrates for health.

The digestibility of the starch in plant foods is highly variable, and is dependent on a number of factors, including the physical structure of both the starch and the food matrix. An in vifro technique has been developed to categorize starch in plant foods according to its likely rate and extent of digestion in the human small intestine. The in vitro method provides values for rapidly digestible starch, slowly digestible starch and resistant starch (RS). In the present study values for the RS content of foods, as measured by the analytical technique, were compared with the recovery of starch from these foods when fed to healthy ileostomates. Nine ileostomy subjects were given a plysaccharide-free diet with a breakfast supplement, on each of 2 d (two subjects) or 3 d (seven subjects), of biscuits made from wheat, potato or banana flours or from moist-heat-processed wheat or maize flours. RS intakes measured in vifro ranged from 8.5 to 15.0 g/d for the test biscuits, and mean starch recoveries in ileostomy effluent were 1004 (n 5, range 91-106) % of those values, but there was substantial variation between individuals. It is proposed that RS is defined as 'the sum of starch and starch-degradation products that, on average, reach the human large intestine'. The analytical method for the measurement of RS in vitro based on this definition is shown to provide an accurate prediction of the average amount of starch that is likely to escape complete digestion and absorption in the human small intestine.Resistant starch: lleostomy : Human small intestine In 1982 we reported that a fraction of starch in cooled, cooked foods was highly resistant to digestion by pancreatic amylase (EC 3.2.1 . 1) in vitro, and used the term resistant starch (RS) to describe this fraction (Englyst et al. 1982). Following a series of studies in healthy ileostomy subjects, who were used as a model for digestion in the small intestine (Englyst & Cummings, 1985, 1986, 1987, it became clear that this type of starch represented only a proportion of the starch that can resist digestion in the human small intestine. Starch that is trapped within whole plant cells or within the food matrix, and some starch granules that have not been fully gelatinized, are hydrolysed only very slowly by a-amylase and therefore may escape complete digestion in the small intestine. We therefore extended the term and modified the analytical procedure to include these sources, i.e. the definition of RS and its measurement in vitro were enlarged to include starch and starch-degradation products (Englyst & Cummings, 1987;Englyst & Kingman, 1990; Englyst et al. 1992a). The material included in the definition of RS reaches the human large intestine and thus becomes a substrate for microbial fermentation. The end-products are H, and CO,, CH, in about half the population, and short-chain fatty acids, which are absorbed and utilized. However, the energy yield to the body from this source is less than that provided if starch is digested and absorbed in the small intestine. The h...

Methods for the measurement of dietary fibre as non-starch polysaccharides (NSP) are described. A common enzymic removal of starch and acid hydrolysis of the NSP to their constituent sugars are followed by one of three alternative techniques, gas-liquid chromatography, high-performance liquid chromatography or spectrophotometry, for measurement of the released sugars. The results obtained by the three methods are in good agreement for a wide range of raw and processed foods. NSP compose approximately 90% of the plant cell-wall material and are therefore a good index of this material. Values for NSP therefore provide a good marker for a diet rich in fruit, vegetables and high-extraction cereal products associated with health and recommended in dietary guidelines. Values for total, soluble and insoluble NSP may be obtained with any of the end-point techniques, and the detailed information obtained from the chromatographic methods is useful in studies of the relationship between the intakes of various types of NSP and health. The causes of some potential interferences in the spectrophotometric assay, especially from processed foods, have been identified and eliminated. The rapid spectrophotometric version is suitable for food labelling purposes and for quality control, and the changes described have made it more robust.

The glycaemic index (GI) is an in vivo measurement based on the glycaemic response to carbohydratecontaining foods, and allows foods to be ranked on the basis of the rate of digestion and absorption of the carbohydrates that they contain. GI values are normalized to a reference amount of available carbohydrate and do not reflect the amounts of carbohydrate normally present in foods; for example, a food with a low content of carbohydrates will have a high GI value if that carbohydrate is digested and absorbed rapidly in the human small intestine. This is potentially confusing for a person wishing to control his or her blood glucose levels by the choice of foods. The rate and extent of starch digestion in vitro has been measured using a technique that classifies starch into three major fractions: rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS). In addition, this technique gives a value for rapidly available glucose (RAG), which includes RDS, free glucose and the glucose moiety of sucrose. When the values for thirty-nine foods were expressed on the basis of the available carbohydrate content of these foods, highly significant (P < 0001) positive correlations were observed between GI and both RDS and RAG. The measurement of RAG in vitro provides values for direct calculation of the amount of glucose likely to be rapidly absorbed in the human small intestine and, thus, to influence blood glucose and insulin levels. These values can be used to compare foods, as eaten, on an equal-weight basis. Food-table RAG values would allow simple calculation of the total amount of RAG provided by single foods, by whole meals and by whole diets. Studies are planned in which RAG and the glycaemic response in man will be measured for identical food products.

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