The importance of dietary carbohydrates

Forty years ago carbohydrates (CHO) were regarded as a simple energy source whereas they are now recognized as important food components. The human diet contains a wide range of CHO, the vast majority of which are of plant origin. Modern techniques based on chemical classification of dietary CHO replaced the traditional by difference measurement. They provide a logical basis for grouping into categories of specific nutritional importance. The physiological effects of dietary CHO are highly dependent on the rate and extent of digestion and absorption in the small intestine and fermentation in the large intestine, interactions which promote human health. Current knowledge of the fate of dietary CHO means that the potentially undesirable properties of many modern foods could be altered by using processing techniques that yield foods with more intact plant cell wall structures. Such products would more closely resemble the foods in the pre-agriculture diet with respect to the rate of digestion and absorption of CHO in the small intestine. The potentially detrimental physiological consequences of eating sugars and starch that are rapidly digested and absorbed in the small intestine suggest that, as fibre, the form, as well as the amount of starch should be considered. Increasing consumer awareness of the relationship between diet and health has led to demands for more widespread nutrition labelling. The entry carbohydrate is required in most countries, and the value is usually obtained by difference and used in the calculation of energy content. However, the value provides no nutritional information per se. Food labels should provide values that aid consumers in selecting a healthy diet.

Rapidly available glucose in foods: an in vitro measurement that reflects the glycemic response.

BACKGROUND: A chemically based classification of dietary carbohydrates that takes into account the likely site, rate, and extent of digestion is presented. The classification divides dietary carbohydrates into sugars, starch fractions, and nonstarch polysaccharides, and groups them into rapidly available glucose (RAG) and slowly available glucose (SAG) as to the amounts of glucose (from sugar and starch, including maltodextrins) likely to be available for rapid and slow absorption, respectively, in the human small intestine. OBJECTIVE: We hypothesize that RAG is an important food-related determinant of the glycemic response. DESIGN: The measurement of RAG, SAG, and starch fractions by an in vitro technique is described, based on the measurement by HPLC of the glucose released from a test food during timed incubation with digestive enzymes under standardized conditions. Eight healthy adult subjects consumed 8 separate test meals ranging in RAG content from 11 to 49 g. RESULTS: The correlation between glycemic response and RAG was highly significant (P < 0.0001) and a given percentage increase in RAG was associated with the same percentage increase in glycemic response. After subject variation was accounted for, RAG explained 70% of the remaining variance in glycemic response. CONCLUSIONS: 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.

Measurement of resistant starch in vitro and in vivo.

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 vitro 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 polysaccharide-free diet with a breakfast supplement, on eac 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 vitro ranged from 8.5 to 15.0 g/d for the test biscuits, and mean starch recoveries in ileostomy effluent were 100.4 (n5, 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.

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.

Dietary intakes of starch and non-starch polysaccharides in a west African village.

Dietary intakes of starch and non-starch polysaccharides (NSP) have been estimated for a rural West African community. These people eat directly from shared bowls of cooked food, and so measurement of any individual's food intake is not possible. Recently developed methodology for estimating food intake under these circumstances and the analysis of samples for dietary polysaccharides are combined to yield estimates of intakes of about 375 g starch and 25 g NSP/d for adult males, with lower intakes for women and children, related to their smaller body weights. These intakes are not direct measurements but are extremely plausible when compared with values obtained for other societies where the complication of the shared food bowl does not impinge upon the measurements.

Food intake in a west African village. Estimation of food intake from a shared bowl.

Novel methodology is described for the estimation of food intake in the particularly difficult circumstance where groups of people eat directly from a shared bowl of cooked food. Detailed observation and measurement of meal preparation is combined with food table values for composition to calculate the nutrient content of each meal. The distribution of food between individuals is estimated by a suitable algorithm. The ability of the algorithm to identify seasonal changes in energy intakes is demonstrated by comparison of the calculated energy intakes with values for the total energy expenditure of free-living adult male subjects, as measured by the stable isotope, doubly-labelled water technique. This comparison suggests that the energy intake calculated from detailed observation of two cooked meals per day is equivalent to approximately 80% of the total energy expenditure and, by inference, total dietary energy intake. The remaining energy intake may well be derived from uncooked 'snack foods', such as raw fruit and vegetables, or from cooked food obtained, by purchase or as a gift, away from the home. This is the first description of a successful method for the estimation of food intake when people eat directly from shared bowls of food.

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.

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. Cow's 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.

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.

Aflatoxin M1 in human breast milk from The Gambia, west Africa, quantified by combined monoclonal antibody immunoaffinity chromatography and HPLC.

Maternal to child exposure of aflatoxin M1 in breast milk is an underevaluated risk factor from dietary exposure to aflatoxin B1. A molecular dosimetry study in The Gambia, West Africa, was initiated to explore the relationships between dietary intake of aflatoxins during a 1 week period and a number of aflatoxin biomarkers including aflatoxin metabolite excretion into breast milk. For the breast milk study, five lactating women were identified and milk samples were collected by hand expression once a day during days 3-7 for three women and during days 3-6 for the two other women. Aflatoxin M1 (AFM1) in human milk was measured in all five subjects by a preparative monoclonal antibody immunoaffinity column/HPLC method. In three of the five women, aflatoxin G1 was found. Estimates of the percentage of aflatoxin in the diet excreted as AFM1 in milk ranged from 0.09 to 0.43%. Thus, these data indicate that a rapid methodology exists to assess the levels of AFM1 excretion in human milk and to use this approach as a biomarker for exposure of children to this carcinogen.

Molecular dosimetry of aflatoxin-N7-guanine in human urine obtained in The Gambia, West Africa.

Hepatocellular carcinoma is one of the major human cancers, causing at least 250,000 deaths each year. Two of the major risk factors for this disease are aflatoxin exposure and hepatitis B virus. This study was undertaken to explore the relationship between dietary exposure to aflatoxins and the excretion of the major aflatoxin-DNA adduct and other metabolites into the urine of chronically exposed people who were either hepatitis B virus surface antigen-positive or -negative. The diets of 20 individuals, 10 males and 10 females, with ages ranging from 15 to 56 years, were monitored for 1 week, and aflatoxin intake levels were determined for each day. Starting on the fourth day, total 24-h urines were consecutively obtained for 4 days. The subjects were generally paired for hepatitis B virus status. Preparative monoclonal antibody affinity chromatography/high-performance liquid chromatography and competitive enzyme-linked immunosorbent assays were carried out on each of the urine samples, and the relationship between aflatoxin intake values and the excretion of (a) total aflatoxin metabolites and (b) aflatoxin-N7-guanine (AFB-N7-guanine) was determined. The average intake of total aflatoxins was 12.0 micrograms for the entire study group during the 1-week collection period. However, there was considerable day-to-day variation in exposures, from a low of zero to a high of 29.6 micrograms total aflatoxins/day. Initial efforts to characterize total aflatoxin metabolites in the urine samples were made by competitive enzyme-linked immunosorbent assay. The correlation coefficient for the analysis was 0.65, with P < 0.001.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary intake of aflatoxins and the level of albumin-bound aflatoxin in peripheral blood in The Gambia, West Africa.

Aflatoxin is implicated as a risk factor for hepatocellular carcinoma in areas of the world with a high incidence of this tumor. The present study was designed to validate the use of aflatoxin-albumin adducts in peripheral blood as a measure of individual exposure to this carcinogen. Dietary intake of aflatoxin was measured at the individual level in 20 residents of Keneba, West Kiang, The Gambia, over a 7-day period and correlated with the level of aflatoxin bound to peripheral blood albumin at the beginning and end of the study. Complementary enzyme-linked immunosorbent assay and high-performance liquid chromatography-fluorescence techniques were used to assay the aflatoxin adducts. All subjects were exposed to aflatoxin originating from several food types, with an average daily intake of 1.4 micrograms/day. A significant correlation (r = 0.55; P = < 0.05) was observed between the dietary intake and the level of albumin-bound aflatoxin at the end of the study. In addition, a good agreement was obtained with the two analytical techniques. A comparison of matched chronic hepatitis B surface antigen carriers with noncarriers did not reveal any difference in adduct formation for a given dietary intake of aflatoxin. These studies demonstrate the validity of aflatoxin-albumin adducts as a marker of human exposure to this carcinogen.

Aflatoxins isolated by immunoaffinity chromatography from foods consumed in The Gambia, West Africa.

An aflatoxin-specific, monoclonal antibody-based immunoaffinity chromatography method has been developed for the rapid isolation of aflatoxins from human foods. Aflatoxins were isolated by immunoaffinity chromatography from a variety of cooked foods, including maize, rice, millets, groundnut sauces, and leaf sauces, collected in The Gambia, West Africa. The aflatoxins were measured by direct fluorescence or high-pressure liquid chromatography. The highest levels were found for groundnut sauces, mean 162 ppb (range 18 to 943 ppb) for 18 positive samples, but aflatoxins were found in other foods; e.g., maize, mean 9.7 ppb (range 2 to 35 ppb) for nine positive samples. The food analysis results were used with records of the amounts of cooked food to estimate a mean daily intake for an individual of the order of 3.5 micrograms of aflatoxins per day. This approach for exposure assessment is considered in relation to other biomarkers of aflatoxin exposure using biological fluids.

Early sodium intake and later blood pressure in preterm infants.

The relation between early salt intake and later hypertension is debated. As part of a larger feeding study, 347 preterm infants were randomly assigned to receive early diets differing grossly in sodium content. Feeding a high sodium preterm formula in the neonatal period did not influence arterial blood pressure at 18 months (corrected) age.

The guinea pig as a model for the study of the effects of milk on growth and development.

Study of the effects of enteral nutrition on neonatal growth and development has been hindered by the lack of a suitable model. A model has been developed to measure the effects of an artificial milk formula on the growth of the newborn guinea pig. The composition of milk expressed from lactating guinea pigs 2 to 13 days postpartum was analysed for its protein, lactose, fat and metabolizable energy contents. Changes in osmolality, water and electrolyte contents were determined. An isotope (3H2O) dilution method was used to measure daily milk intakes of naturally suckled animals. An artificial formula, based on cow's milk (CMF), isocaloric with natural guinea pig milk was designed and synthesized. Neonatal guinea pigs were reared either naturally or on CMF from birth to 8 days. Comparable growth and milk intakes were achieved by the two feeding groups. No adverse effects of CMF were noted. The study represents the development of an effective means of raising a laboratory animal during the suckling period on an artificial milk formula. The model may be used to measure the impact of enteral nutrition on the ontogeny of the gut, and on neonatal growth and development, in a precocial species which more closely resembles man than the altricial species, such as rat and mouse, generally used for this purpose.

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.

Preterm milk as a source of protein for low birthweight infants.

A total of 588 samples of 24 hour collections of preterm milk obtained during the first month of lactation from 58 mothers of low birthweight infants have been analysed for total nitrogen content. In addition to the expected decline in milk protein content (total nitrogen X 6 X 38) seen with postnatal age, a strong negative correlation between milk protein and milk volume output has been shown. Thus, the greater the volume produced, the smaller the chance that preterm milk contains a sufficiently high protein concentration to meet the calculated requirements of low birthweight infants (given the constraints on the infant's volume intake). Postnatal age and the infant's body weight are identified as additional factors which influence the likelihood that theoretical protein needs will be met. It is speculated that the high protein content seen in preterm milk may not necessarily reflect a unique secretory ability of the mammary gland in mothers delivering preterm but may relate more to the low volume of milk produced by many donors.

Response of the reproductive system of male rats to protein and zinc deficiency during puberty.

Male weanling rats were fed four diets providing high or low levels of protein and/or zinc. One group on each diet was sacrificed every 2 weeks to measure the weights of the reproductive organs, plasma levels of follicle-stimulating hormone and luteinizing hormone, and the plasma and testicular concentrations of testosterone, dihydrotestosterone and zinc. The results demonstrate that hypogonadal states are produced in response to protein deficiency and zinc deficiency but the mechanisms involved are different. The specific effects of zinc deficiency are not observed in protein-deficient animals.