Geoffrey J. Hudson

Determination of dietary fibre as non-starch polysaccharides with gas–liquid chromatographic, high-performance liquid chromatographic or spectrophotometric measurement of constituent sugars

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.

Measurement of rapidly available glucose (RAG) in plant foods: a potential in vitro predictor of the glycaemic response.

The glycaemic index (GI) is an in vivo measurement based on the glycaemic response to carbohydrate-containing 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 no 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 < 0.001) 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.

The classification and measurement of dietary carbohydrates

Abstract Major advances in our understanding of the digestive physiology of dietary carbohydrates and their potential benefit to health require new and more informative techniques to replace the traditional ‘by difference’ measurement. The human diet contains a range of chemically distinct carbohydrates and research, as well as labelling for dietary carbohydrates including dietary fibre, should be based on the classification and measurement of chemically identified components. Such values do not become obsolete and can be used in different combinations for different purposes. We present a new scheme for carbohydrate classification, including a new class of short-chain carbohydrates (SC). The classification and measurement of nutritionally important types of starch includes its division into rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS). In addition, a new category of rapidly available glucose (RAG) is described, which is the amount of glucose from free sugar and starch that is rapidly available for absorption. Values for dietary fibre based on the measurement of plant cell-wall NSP aid the consumer in choosing the type of high-fibre diet recommended in the dietary guidelines and are, therefore, appropriate for food labelling. The AOAC Prosky procedure, in contrast, is not specific for plant cell-wall material but includes substances that are formed by food processing and by treatment of analytical samples. Such values are not suitable for food labelling because they do not aid the consumer in choosing the recommended diet.

Determination of dietary fibre as non-starch polysaccharides by gas–liquid chromatography

An improved method is described for the measurement of total, soluble and insoluble dietary fibre as non-starch polysaccharides (NSP). An established procedure is modified to allow more rapid removal of starch and hydrolysis of NSP. In its present form the procedure is simpler and more robust than those previously published. In the modified method starch is removed enzymically within 50 min and NSP is precipitated with ethanol and then hydrolysed by treatment with sulfuric acid for 2 h. The constituent sugars can in turn be measured by gas-liquid chromatography, high-performance liquid chromatography or more rapidly by colorimetry. The improved procedure described here for the removal of starch and hydrolysis of NSP applies to all three techniques, but only the method for measurement of sugars by gas-liquid chromatography is described here in full.

The calcium and phosphorus intakes of rural Gambian women during pregnancy and lactation

The Ca and P intakes of 148 pregnant and lactating women in a rural village in The Gambia, West Africa, have been estimated by direct weighing of food on a total of 4188 d. The Ca and P contents of local foods were determined by analysis of raw ingredients, snack foods and prepared dishes. Information about the contribution of mineral-rich seasonings was obtained. Efforts were made to discover unusual sources of Ca that might not be perceived as food by subject or observer. The main contributors to daily Ca intake were shown to be leaves, fish, cereals, groundnuts and local salt. Cows milk accounted for only 5% of Ca intake. Unusual sources of Ca were discovered, namely baobab (Adansonia digitata) fruit and selected earths, but these were consumed infrequently and their contributions to Ca intakes were small. Cereals and groundnuts were the main sources of P. Ca and P intakes (mg/d) were shown to average 404 (SD 110) and 887 (SD 219) respectively. Seasonal changes in the availability of leaves, cereals and groundnuts resulted in variations in Ca and P intakes. The rainy season was associated with increased Ca intakes (by 16%) but decreased P consumption (by 15%). No difference was observed in Ca intake between pregnant and lactating women but P intake in lactation was 11% higher than that in pregnancy during the post-harvest season. The implications of these low Ca intakes require investigation.

The gel nature and structure of the carbohydrate of ispaghula husk ex Plantago ovata Forsk

Abstract Mucilage from the seed husk of Plantago ovata Forsk is an arabinosyl(galactosyluronic acid)rhamnosylxylan. A wide range of molecular sizes was revealed by chromatography of the methylated mucilage on Sephadex LH-60, and the resulting 12 fractions were homologous. Partial hydrolysis with acid and enzymic hydrolysis variously of the mucilage and carboxyl-reduced mucilage led to identification of the β- d -xylosyl residues forming the xylan backbone of the polymer. Similarly, the arabinose, rhamnose, and galacturonic acid residues were shown to be α- l , α- l , and α- d , respectively. A structure for the polysaccharide is proposed and related to gel formation. Electron microscopy of the gel revealed a co-linear association. This association is reversible, and therefore not the result of covalent bonding.

Determination of resistant short-chain carbohydrates (non-digestible oligosaccharides) using gas–liquid chromatography

Abstract We have proposed the term short-chain carbohydrates (SCC) for those species, other than the free sugars, that are soluble in 80% ethanol under well-defined conditions. We describe a technique for the measurement of resistant SCC (RSCC), which are not susceptible to pancreatic amylase or the brush border enzymes and therefore sometimes termed non-digestible oligosaccharides. In the procedure, alpha-glucans (starch and maltodextrins) are hydrolysed enzymatically to glucose and the non-starch polysaccharides (NSP) are precipitated in ethanol. Fructans are hydrolysed enzymatically and the monosaccharide constituents are reduced to acid-stable alditol derivatives before the remaining RSCC are hydrolysed with sulphuric acid. All the constituent sugars are measured as alditol acetate derivatives by gas–liquid chromatography. The protocol allows both the measurement of total RSCC and a separate, specific measurement of fructans.

Variation in the composition of Gambian foods: the importance of water in relation to energy and protein content.

Protein and energy contents of numerous samples of Gambian foods have been determined in order to examine variations in composition. There was a highly significant negative correlation between the energy and water content of the foods, which enables the energy content to be predicted accurately by measurements of the water content alone. Protein content of foods, as estimated from measurement of total nitrogen, was more variable than the energy; nevertheless, for most foods there was a significant negative correlation between protein and water content. Consideration of cereal and groundnut foods separately enabled the protein content to be predicted from the water content with almost as great an accuracy as for the energy. The accuracy of large‐scale studies designed to estimate the protein and energy intake of individuals would be increased considerably by adopting the approach described. It would be necessary, of course, to determine in each case the regression equations relevant to the range of foods i...

An automated enzymic micromethod for the measurement of fat in human milk

Most (98%) of the fat in human milk is present as triglycerides. This paper describes the use of a clarification procedure that enables the level of human milk fat to be determined by measurement of glycerol released by enzymic hydrolysis of triglycerides. The method requires only 10-50 microliters milk, thus presenting a possible technique for work with small mammals, and is suitable for use with autoanalysers, permitting rapid sample throughput.

The fibre/starch overlap: a new look at food carbohydrates : Dietary fibre

The standard classification of carbohydrates is unhelpful because it gives no indication of digestibility and hence of nutritional value. A new classification of starch is described, based on its variable digestibility in the human small intestine and the fact that starch escaping into the large intestine has some actions similar to those of some non-starch polysaccharides (dietary fibre). Only separate values for chemically defined components will allow full understanding of the physiological effects of dietary carbohydrates