Intestinal degradation of dietary fibre in green beans--effects of microwave treatments

Intestinal degradation of dietary fibre in blanched and microwaved green beans was studied by using a rat experimental model. Content and composition of dietary fibre as well as molecular weight distribution of water-soluble polysaccharides (WSP) were analysed. There was a solubilization and a shift towards lower Mw of mainly uronic acid-containing polysaccharides with repeated microwave treatment in the raw material. Thus, the apparent Mw of water-soluble polysaccharides decreased from approximately 1,550,000 to approximately 300,000. After the beans had been digested the Mw of the WSP was significantly reduced, to approximately 100,000, and the differences in Mw seen between various processed raw materials had been evened out. After fermentation the Mw of the WSP decreased further approximately 10 times. Fibre fermentability was high (approximately 90%) and similar for the various processed beans.     

Changes in carbohydrate fraction during dehydration process of common legumes

The effect of thermal dehydration on carbohydrate fraction of legumes (chickpea and two cultivars of beans) was evaluated. The legumes showed important contents of total dietary fibre (TDF), with insoluble dietary fibre (IDF) as the main fraction (75–96%). The level of dietary fibre (DF) in legumes was affected by processing; dehydration was the process that exhibited a significant increase in soluble dietary fibre (SDF). The extent of DF changes depended on the type of legume and the process. Starch was also affected by processing which improved its digestibility, although this depends on the type of legume and treatment (soaking, cooking and dehydration). Legumes exhibited important levels of raffinose family oligosaccharides (RFOs), but the profile differed according to the seed. The dehydration process produced significant reductions of these soluble compounds: 76% for white bean, 57% for chickpea and 41% for pink-mottled cream bean. Therefore, dehydration was an efficient process to reduce flatulence compounds, and legume flours can be proposed as functional ingredients for their beneficial health effects.     

Determination of the dietary fiber content of some foods using a biological method

The evaluation of a biological method for dietary fibre determination in food was the main purpose of this study. Three-week old Wistar rats were used in this experiments. The rats were administered during two weeks a diet containing 1% of crude fibre from bread, grits, green beans, white cabbage and carrot. It was found that the biological method gave results similar to those obtained by the enzymatic one.     

New legume sources as therapeutic agents

This review evaluates the potential health benefits of three legume sources that rarely appear in Western diets and are often overlooked as functional foods. Fenugreek (Trigonella foenum graecum) and isolated fenugreek fractions have been shown to act as hypoglycaemic and hypocholesterolaemic agents in both animal and human studies. The unique dietary fibre composition and high saponin content in fenugreek appears to be responsible for these therapeutic properties. Faba beans (Vicia faba) have lipid-lowering effects and may also be a good source of antioxidants and chemopreventive factors. Mung beans (Phaseolus aureus, Vigna radiatus) are thought to be beneficial as an antidiabetic, low glycaemic index food, rich in antioxidants. Evidence suggests that these three novel sources of legumes may provide health benefits when included in the daily diet.     

Importance of processing for physico-chemical and physiological properties of dietary fibre

There is considerable loss of DM during wet heat treatment of vegetables, leading to an increase in dietary fibre. Correction for the loss of DM indicates that the effects on total dietary fibre are minor. There is, however, depolymerization of the dietary fibre polysaccharides. The degradation is related to the severity of the heat treatment. Souring, freezing and mild microwave treatment have no effects. The viscosity is in general related to the extent of polymerisation. Microwave treatment has different effects on various cultivars of green beans, and the addition of salt (NaCl and CaCl2) to the boiling water changes the physico-chemical properties of soluble fibre in carrots, depending on the cation. The higher viscosity of the soluble fibre in raw carrots may partly explain the lower glucose and hormonal responses observed in healthy subjects when compared with blanched and microwave-cooked carrots. In studies on rats the amount of butyric acid in the distal colon has been shown to be higher with dietary components containing high amounts of resistant starch. Further, the fermentability is lower and the butyric acid concentration higher with composite foods than with the corresponding purified fibre fractions. In human studies the faecal concentration of butyric acid has been shown to increase in patients with ulcerative colitis when [beta-glucan-enriched oat bran (20 g fibre) is added to the diet for 12 weeks. Also, an improvement of symptoms was reported.     

Effects of dietary fibre on intestinal absorption

Changes in the functions of the small intestine due to alterations in dietary fibre, and the mechanisms and importance of these changes, are reviewed.

Increase in dietary fibre causes an increase in fat content of the faeces but this is modest compared with the alterations found in disease of the small intestine or pancreas. There is presumptive evidence of effects of dietary fibre on carbohydrate absorption. Dietary fibre may well produce an alteration in transit time through the small intestine but information available is not conclusive and its significance cannot, at the moment, be assessed. It is likely that there are other effects of dietary fibre on the function of the small intestine.

Dietary fibre affects the absorption of nutrients; iron absorption has been particularly studied. There is evidence that the absorption of iron is reduced by wheat bran. Prolonged and detailed studies m·e required before the significance of this finding can be assessed . Calcium may well be affected in a similar manner to iron; perhaps zinc and magnesium too. It seems likely that there arc also effects on other nutrients; there is a certain amount of information available about folic acid but other nutrients remain to be investigated.

Absorption of drugs from the small intestine may be influenced by alterations in dietary fibre. Alterations may be in the direction of more efficient or less efficient absorption. There are many possible mechanisms by which this may be brought about.

It is clear that different forms of dietary fibre have different effects on the small intestine; a single type of dietary fibre may have varied effects on different functions. There arc also complex interactions between the different functions of the small intestine and between the absorptions of different nutrients which remain to be elucidated.     

Dietary fibre in infancy and childhood

There is very little evidence for the effects of dietary fibre in young children and current dietary guidelines are based on assumptions and data extrapolated from studies in adults. The first years of life may be critical for the establishment of a healthy colonic microflora, as well as good eating habits. The lack of clear and well-founded guidelines for the intake of dietary fibre in childhood may hinder both factors. The fears that a high-fibre diet in children < 5 years of age will lead to growth faltering and mineral imbalance are not well supported in the literature, especially for children in the developed world. Indeed, with the rising levels of obesity, fibre intake may be of benefit in reducing energy intake. A low-fibre diet may also be implicated in the aetiology of childhood constipation and appendicitis. The latest proposals for the definition of dietary fibre include oligosaccharides, which may act as prebiotics. There are potential health benefits of including oligosaccharides in the diets of infants and children, but more research is needed to consider the long-term effects. The immature intestine of the infant may also result in a greater amount of starch entering the colon during weaning, and this starch would now be considered dietary fibre under the new definitions. Much new research is needed to allow adequate recommendations for the intake of dietary fibre in childhood based on data collected in appropriate age-groups.     

The definition,analysis and properties of dietary fibre

It is now possible to propose a definition for dietary fibre (Trowell et al., 1976) which, even if it has not received complete acceptance, at least provides a basis for study.

Studies of analytical methods have reached the stage where methods based on different principles give similar values for foods for which they arc applicable. In other foods the relation between results obtained by different methods is understood. However, methods have not reached perfection; much remains to be done before we can be satisfied that we have an ideal or complete characterisation of dietary fibre.

By drawing on the work of the carbohydrate chemists , it is possible to identify properties of the components of dietary fibre which may be important in the physiological role of dietary fibre and its relation to health.     

The effect of provision of the first-limiting amino acid, gastrointestinal microbial activity and the level of nitrogen intake on protein utilization and energy digestibility in rats

1. The present work with growing rats was undertaken to study the effect of protein quality, gastrointestinal microbial activity and the level of nitrogen intake on protein utilization and energy digestibility. The experiment involved a total of thirty-six dietary treatments in a 9 x 4 factorial design, with five rats per treatment. The thirty-six diets resulted from nine protein sources. Each diet was composed of a basal N-free mixture plus minerals and vitamins, with N sources added at the expense of the N-free mixture to provide 15.0 gN/kg dry matter (DM) in the first three protein-addition treatments and 30.0 gN/kg DM in the fourth protein-addition treatment. The nine protein sources were soya-bean meal, casein, wheat gluten, skim-milk powder, meat-and-bone meal, wheat bran, barley, wheat and cooked brown beans (Phaseolus vulgaris). The four formulations for each protein source incorporated the protein unsupplemented at 15.0 gN/kg DM, unsupplemented at 30.0 gN/kg DM, or supplemented at 15.0 gN/kg DM with the estimated first-limiting amino acid or the antibiotic Nebacitin. 2. With all protein sources, the inclusion of the first-limiting amino acid had no effect on either protein or energy digestibility. 3. The microbial activity in the digestive tract affected protein utilization and energy digestibility to a different degree depending primarily on the level and type of dietary fibre. True protein digestibility (TD) of skim-milk powder and brown beans, both rich in easily-fermentable energy, increased from 0.959 to 1.000 and from 0.680 to 0.777 respectively by the addition of Nebacitin. TD of the other protein sources was only marginally affected by the antibiotic treatment. Only with brown beans was the biological value (BV) markedly affected by Nebacitin with an increase from 0.482 to 0.557 by the treatment. Energy digestibility was significantly lower in rats given antibiotic with soya-bean meal, wheat bran, barley, wheat and brown beans. 4. The effect of level of N intake on protein utilization was dependent on both protein quality and the fibre concentration of the diet. Protein sources with high BV were more affected than proteins of lower BV. It was concluded that TD is not always independent of dietary protein concentration.     

Binding of iron by three starchy legumes in the presence of iron alone,with calcium or with calcium,zinc, magnesium and copper

Iron (Fe) binding capacity of butter beans, broad beans and lentils in the raw, cooked and fibre-rich fraction (FRF) forms; with Fe addition alone, with calcium (Ca) and with Ca, zinc (Zn), magnesium (Mg) and copper (Cu) at concentrations based on physiological and dietary considerations, was investigated in conditions simulating the small intestine. The FRFs were found to bind significantly more Fe, indicating the substantial role of dietary fibre in Fe binding. More Fe was bound to legumes studied when Fe was added together with Ca than separate Fe addition. Further more Fe binding was noted in the presence of Ca, Zn, Mg and Cu.     

30 g of fibre a day: An achievable recommendation?

The Scientific Advisory Committee on Nutrition (SACN) recently published its draft report on Carbohydrates and Health, in which new recommended intakes for fibre were proposed for children and adults, following an in‐depth review of the scientific evidence base. The recommendation for the adult population of 30 g/day, measured by the Association of Official Analytical Chemists' (AOAC) method, is somewhat higher than current recommendation and, according to intakes reported in the recent National Diet and Nutrition Survey, would require men to increase their fibre intakes by around 50% and women by 75%. This paper discusses current fibre intakes in the UK and describes the main contributors of fibre to the diet. Simple dietary modelling was carried out to investigate the feasibility of the recommendation by SACN in the context of other nutrient recommendations and food‐based guidelines. This demonstrated that it is possible to consume 30 g of AOAC fibre a day in the context of a healthy diet that meets other dietary recommendations if all meals are based on starchy foods (including mainly wholegrain options and potatoes with their skins), high fibre snacks are selected and the diet is rich in fruit and vegetables (around 8 portions daily). Barriers to meeting the recommendation are considered, including the negative consumer perceptions of starchy foods and a lack of awareness regarding the health benefits of fibre, particularly those beyond bowel health. Labelling regulations may pose further challenges, as there is not an agreed definition for ‘wholegrain’ and the European Food Safety Authority (EFSA) has concluded that there is, as yet, insufficient evidence to warrant an official health claim for dietary fibre (as a general food ingredient). Although feasible in the context of other dietary guidelines, the required dietary pattern to achieve 30 g of AOAC fibre a day is not reflective of average diets in the UK. Achieving this population recommendation will therefore be a considerable challenge. Co‐operation and collaboration is likely to be required from a number of stakeholders including the food industry, health professionals and regulatory bodies to raise awareness of the benefits of dietary fibre and encourage greater consumption of higher fibre foods. Innovative high fibre ingredients may also be needed to make it easier for consumers to boost their fibre intakes.     

A comparison of dietary fibre intakes in South Wales estimated from a questionnaire and weighed dietary records

Intakes of dietary fibre in a random sample of 102 women living in a large South Wales village were measured by a four-day weighed dietary record and by questionnaire. The mean weekly intake of total dietary fibre calculated from the weighed records was 105.8g and that computed from questionnaires was 101.8g. The two methods indicated good agreement (r=0.51). Mean weekly cereal fibre intakes from weighed records and questionnaires were 46.6g and 46.4g respectively (r=0.69), and mean weekly fruit and vegetable fibre intakes were 59.2g and 55.4g respectively (r=0.34). The correlations were also good among a smaller group of 38 ‘health food shoppers’, consuming a high fibre diet. The questionnaire may therefore be used to obtain reliable data on dietary fibre intakes in free-living populations.     

Do beans and oat bran add to the effectiveness of a low-fat diet?

The effects of consuming an increased amount of soluble fibre as oat bran or beans were examined in 40 free-living hypercholesterolaemic men and women. The subjects were initially established on a low-fat background diet (29% of energy from fat) and then 55 g low-fibre oat bran, 55 g high-fibre oat bran or 80 g mixed cooked beans were added to their diet in random order for 6 week periods. Body weight and overall composition of the diet did not change. Plasma cholesterol and low-density lipoprotein cholesterol (LDL-C) were unchanged. High-density lipoprotein cholesterol (HDL-C) was significantly higher on all three intervention diets than on the lower fibre run-in diet. Supplementation of a moderately low-fat diet with palatable quantities of oat bran or beans without changing the overall fat intake does not appear to significantly lower cholesterol but may have a benefit by increasing HDL-C and reducing the ratio of LDL-C to HDL-C.     

Dietary fibre content and nutrient claims relative to the faecal bulking efficacy of breakfast cereals

The correspondence between the dietary fibre contents of 28 breakfast cereals and their faecal bulking efficacies was measured and used to assess criterion values controlling nutrient claims for dietary fibre. A valid, standardised rat assay was used to measure faecal bulking efficacy as the content of wheat bran equivalents for faecal bulk (WBEfb) in the cereals. Regression analysis of WBEfb content against dietary fibre content allowed the adequacy of criterion fibre values for claims of 'source of fibre,' 'high in fibre' and 'very high in fibre' to be assessed relative to a daily reference requirement of 63 WBEfb, based on human data. Faecal bulking by breakfast cereals was much lower than implied by the dietary fibre claims associated with them. Many more were claimed to be 'high' or 'very high' in dietary fibre (n = 13) than were 'high' or 'very high' in faecal bulking efficacy (n = 4). Conversely, dietary fibre requirements per serving predicted from WBEfb requirements, as necessary to maintain adequate faecal bulk in the current Australian diet, were much higher (4.4 g) than the criterion fibre content (1.5 g) for the most modest claim, 'source of fibre'. After removing four high-bran cereals (>15% dietary fibre) from the analysis, a modest correlation of r = 0.62 between dietary fibre content and faecal bulk was obtained. It is concluded that, with respect to breakfast cereals, fibre values specified for nutrient claims are too low, dietary fibre content is not a reliable guide to faecal bulking efficacy and direct measures of faecal bulking capacity would be more useful than dietary fibre content in describing faecal bulking efficacy for evidence-based food choice.     

The digestion of pectin in the human gut and its effect on calcium absorption and large bowel function.

1. The effect of dietary fibre digestion in the human gut on its ability to alter bowel habit and impair mineral absorption has been investigated using the technique of metablic balance. 2. Five healthy male students were studied for 9 weeks under controlled dietary conditions and during the last 6 weeks they took 36 g pectin/d. Bowel habit, transit through the gut, faecal fibre excretion, calcium balance and faecal composition were measured. 3. During the control period only 15% of the dietary fibre ingested was excreted in the stools and when pectin was added to the diet there was no increase in fibre excretion. Stool frequency and mean transit time were unchanged by pectin but stool wet weight increased by 33% and faecal excretion increased (%) for fatty acids 80, nitrogen 47, total dry matter 28 and bile acids 35. Ca balance remained unchanged. 4. It may be concluded from these results that dietary fibre is largely metabolized in the human gut and dietary pectin completely so. This could explain its lack of effect on bowel habit and Ca balance. Other changes in the faeces may be related to an increase in bacterial mass.     

Daily dietary fibre intake of children, 2 to 3 years of age, living in Antwerp, Belgium

Daily dietary fibre intake of toddlers living in Antwerp (Belgium) have been calculated by weighted food records with subsequent conversion using computerised food composition tables. Dietary fibre intakes were also measured by the duplicate portion technique to evaluate the intakes of total, soluble and insoluble dietary fibre. Calculated dietary fibre intake was 10 ± 3 g/d. Measured intakes of total, soluble and insoluble dietary fibre were 14.5 ± 2.8 g/d, 6.0 ± 1.9 g/d, and 8.6 ± 1.6 g/d. Values about the fibre intakes between both methods differed for 31.7%. Bread contributed for more than one third of the total dietary fibre intake. The contributions of the other food groups were 15.1% for fruits, 14.5% for starchy foods and 13.9% for vegetables. The intakes of dietary fibre from bread by the toddlers in this study could be considered as high, whilst intakes from fruits and vegetables might be increased.     

The public health rationale for increasing dietary fibre: Health benefits with a focus on gut microbiota

Dietary fibre can be considered an essential macronutrient which feeds our gut microbiota, thus enabling the optimal functioning of this ‘virtual organ’. Recommended daily intakes range from 25 to 38 g/day. However, most individuals fail to reach this target and may consume <50% of the recommended amount. Poor dietary fibre intake is associated with an imbalanced gut microbiota leading to reduced diversity and increased abundance of non‐beneficial microorganisms at the expense of beneficial ones. Inflammation is a recognised physiological effect of such a microbiome structure and is a common denominator in several non‐communicable diseases associated with the Western lifestyle/developed world including obesity, cardiovascular disease, type 2 diabetes and colorectal cancer. Yet increasing dietary fibre intake can reduce risk factors associated with these diseases and may even improve prognosis. Several food manufacturers have now developed strategies to increase the fibre content of foods. However, research gaps still remain. In particular, dietary fibre is composed of various non‐digestible carbohydrates and the exact effect of each of these on the gut microbiota is currently unknown. The links between dietary fibre and disease risk are generally associative. Understanding the biological mechanisms will require deeper insight into the impact of the gut microbiota on health. Furthermore, gut microbiota research has shown significant inter‐individual variation meaning that some individuals may not respond to certain dietary fibres owing to the absence of specific microorganisms. Understanding how the different fibre types influence the microbiota, combined with knowledge of an individual’s microbiota, should lead to fibre‐led, microbiota‐based dietary advice.     

Evaluation of dietary quality of adolescents using Healthy Eating Index

The aim of this study was to use the Healthy Eating Index-2005 (HEI-2005) to assess diet quality and determine the relationship between the HEI-2005 and the energy and nutrient intakes of adolescents. A cross-sectional study was conducted on 1,104 healthy adolescents, mean age of 15.8 ± 1.24 years. Dietary intake was measured with the 24-hour dietary recall method, and dietary quality was assessed by means of the HEI-2005. Diet quality scores ranged from 23.7 to 77.5. The mean score was found to be 51.5 ± 9.07 according to the HEI-2005. There were no differences according to gender; 42.8% had a poor diet and 57.2% had a diet that needs improvement. No subjects had a "good diet". Lower mean subgroup scores were found for whole grains, total vegetables, total fruits, dairy products, and meat and beans. Fruits and vegetables scores were significantly high in girls, whereas sodium, oil, and meat and beans scores were significantly high in boys. Total HEI-2005 scores were increasingly associated with parental education level when age and gender were adjusted. There was a negative correlation between HEI-2005 scores and age, total energy intake, and fat intake. Positive correlation was only observed in the HEI-2005 scores for protein and dietary fibre intakes. Consequently, the overall diet quality and nutritional habits of Turkish adolescents need modification and improvement. In the family, measures should be initiated by the government, including advertisements and campaigns.     

Dietary fibre in the diets of urban and rural yoruba nigerian women

Dietary fibre (DF)-measured as neutral detergent fibre — as well as crude fibre (CF) and energy and mineral intake of a group of 250 urban and 50 rural Yoruba Nigerian women were determined. The contribution of different food sources to the total dietary fibre intake as well as the relationship between dietary fibre and the nutrient (energy, zinc, copper, iron and calcium) content of the diets was also determined. Total DF intake of 62.0 g/day by urban women was significantly (P<0.001) lower than the value of 69.3 g/day for rural subjects. CF intake was similarly different. Wide variations were observed in the contribution of different foods to the total DF intake in each of the two groups. Very high and significant correlations were observed between dietary fibre intake and intake of other nutrients. The correlations between dietary fibre and energy intake suggest that the satisfaction of energy demands in this environment is accompanied by a higher intake of dietary fibre. The high correlation observed between dietary fibre and mineral content of diets consumed, may be of health importance.