Faecal bulking index and wheat bran equivalents

Faecal bulking capacity is an important property of foods because bulk in the large bowel has a crucial role in large bowel function and health (Table 7.6). The direct relationship between faecal bulk and 'regularity' is of concern to a large

Table 7.6 Putative links between properties and effects of bulk in the large intestine


Consequent effect


Bulk transfer.

Bulk transfer

Toxin removal, colonic exposure reduced, decreased transit time.

Replenishment of substrates for fermentation - decreased colon cancer



Less protein putrefaction to harmful nitrogenous products - decreased

transit time

colon cancer risk.

Less time for dehydration and stool hardening.


Diluted colon contents, stool softening, pressure distribution -

water load

decreased risk of diverticulosis and haemorrhoids.


Provides substrates for bacterial growth. Butyrate produced by

fermentaion protects against colorectal cancer. Short chain fatty acid

production decreases pH and solubility of carcinogenic bile acids.


Reduced toxin/carcinogen activity.


Reduces risk of diverticulosis and haemorrhoids by reducing localised


pressure points.


Stimulates defaecation, preventing stagnation.


Comfort and continued flow, sense of well-being.

proportion of the population, who assume that dietary fibre is the food component responsible,67 a point well used by food marketers. Countless foods and supplements are promoted as containing dietary fibre, implying that they will promote regularity, but with no supporting efficacy data other than dietary fibre content.

Dietary fibre values can, in fact, be a poor guide to faecal bulking.34,68 Faecal bulk is a result of multiple interactions between the food, the host, and the gut ecosystem - food composition, digestion, endogenous secretions, fermentation, bacterial biomass, water-holding capacity, and particle structure will all play a role.69 Materials that do not analyse as dietary fibre contribute to faecal bulk,46 and the combined loss of polysaccharide and its water-holding capacity can be only partly compensated for by bacterial growth.70 Pectin, for example, is a much less effective faecal bulker than psyllium gum,34 because it is readily fermented, whereas psyllium is fermentation-resistant and remains highly hydrated in the colon. Both pectin and psyllium are non-starch polysaccharides, and therefore dietary fibre, but dietary fibre analysis gives no indication of the important differences in their physiological actions.

7.7.1 End-points

Normal large bowel function and health are general end-points which, it is almost universally agreed, depend on a supply of bulk to the distal colon.69,71 More specific effects of bulk in the colon were summarised in Table 7.6. Colonic bulk has been related to a number of health end-points in a number of ways. It is a direct stimulus to defecation, dilutes various toxins and distributes intracolonic pressure, reducing the risk of diverticulosis.69,72 The defecation that it induces allows movement of fermenting material into the distal colon, where it produces butyrate, thought to protect against colorectal cancer.73 Replenishment of carbohydrates in the distal colon may also reduce formation of carcinogenic nitrogenous compounds, formed when proteins are used as a carbon source in fermentation after exhaustion of carbohydrate substrates.74

Simple constipation resulting from an inadequate supply of moist bulk to the distal colon is one of the most widespread consequences of Western eating pat-terns.75 If only to prevent constipation, faecal bulking efficacy deserves attention.

7.7.2 Markers

Distal colonic bulk is a highly relevant marker of large bowel health because of the number of direct and indirect effects that it has (Table 7.6), and because it is a major factor in laxation.

7.7.3 Current indices

Dietary fibre has traditionally been relied on to indicate the potential of a food to promote regularity of bowel function. Indeed, fibre analysis for human foods originated from the physiological concept of dietary fibre as roughage - plant cell wall material not digested by human enzymes in the gut, and responsible for stool bulk.76 The plant cell wall still remains central to most definitions of dietary fibre and corresponding methods of analysis,77 although, the definition is being extended to include added non-digestible polysaccharides and oligosaccharides that have beneficial effects.78,79 But 'beneficial effects' specified as part of the definition of dietary fibre include blood cholesterol-lowering and blood glucose-lowering effects, and are not confined to those associated with faecal bulking, so the definition has become too broad to be useful. In fact, dietary fibre analysis has never been congruent with the original physiological concept of dietary fibre as faecal bulking 'roughage', because it measures materials resistant to digestion by foregut proteases and amylases, while faecal bulk depends also on resistance to hind gut fermentation. Food components that are both resistant to digestion and to fermentation are not discretely analysed in fibre analysis.

There is an overall statistical relationship between dietary fibre intake and faecal bulk,80 but because of the large amount of variability in foods of low fibre content, fibre cannot be used as a guide to choosing any individual food for bulk, as the results in Fig. 7.3 show. Each food needs to be individually tested before it can be chosen for its faecal bulking efficacy.

  1. 7.3 Increase in hydrated faecal weight (FHW) induced by breakfast cereals fed to rats at 50% of the diet. The results show that dietary fibre content is not a reliable predictor of faecal bulking in most breakfast cereals.
  2. 7.4 Relevant indices

The faecal bulking index (FBI) has been developed as an index of the distal colonic bulking efficacy of a food.34 FBI is defined as the increment in hydrated faecal mass induced by a food as a percentage of the increase due to a wheat bran reference.

Increase in weight of rehydrated faeces per g of test food

Increase in weight of rehydrated faeces per g of reference food

FBI allows foods to be ranked by relative bulking efficacy, on an equal weight basis. Ranking by FBI of a representative sample of Australasian breakfast cereals is shown in Fig. 7.4.

Cornflakes Puffed Rice Nutrigrain Chex Vita Crunch Special K Oat Bran (Flemming's) Puffed Wheat Berry Berry Nice Rolled Oats (Flemming's) Fruitful porridge Sustain Just Right Creamoata Multiflakes Nut Feast Miniwheats Kornies Fruity Bix Sports Plus Vita Brits Rolled Oats (Pam's) Muesli Wheat Bix Sultana Bran Bran Flakes All Bran San Bran

20 40 60 Faecal bulking index

Fig. 7.4 Faceal bulking indices (FBI) of a representative selection of Australasian breakfast cereals. FBI gives the increment in faecal bulk indiced by a food relative to that induced by an equal weight of wheat bran.

To measure FBI, rats are fed a nutritionally complete baseline diet containing 5% mixed dietary fibre and 50% sucrose, and the increase in hydrated faecal bulk when a proportion of the sucrose is replaced by a test food is compared with the increment induced by an equal weight of wheat bran reference. In the case of the breakfast cereals, all of the sucrose was replaced by cereal.

7.7.5 Valid measurement

A crude estimate of faecal bulking would be resistance to fermentation by colonic bacteria in vitro.8 But, because of the complex of host and food factors determining faecal bulk, it is best measured in vivo, and ideally in humans. However, for the purposes of product screening in food processing, an animal model giving a valid representation of human responses would be expedient, to avoid the difficulties of human research. The laboratory rat gives a reasonable prediction of the relative effects of foods on faecal bulk,34,82 because, like humans, it is mono-gastric, and food residues from the small intestine undergo extensive mixed bacterial fermentation in the hind gut, where extent of digestion will be limited more by substrate characteristics than by the availability of microbes. As in humans, dominant factors determining faecal dry matter are the mass of material resistant to mixed bacterial fermentation, bacterial mass, and endogenous secretions.81

Transit time in the rat is shorter than in man,83 the rat concentrates faecal dry matter as pellets, and is a caecal rather than a colonic fermenter. Its validity as a model is improved by using large rats (>250 g), preadapting them to mixed dietary fibre, and making sure that test foods are fed at realistic levels to avoid overloading the gut or exceeding normal fermentation capacity. The problem of faecal dehydration is overcome if the rat faecal pellets are allowed to imbibe water passively to their full water-holding capacity, when their water content rises to that of fresh human stools, and the increase in faecal bulk per gram of wheat bran fibre is almost identical to that measured in humans.84

The FBI assay gives a relatively quick in vivo measure of relative faecal bulking efficacy. However, as material entering the colon depends on diet composition, not on one component of it, absolute effects may be influenced by other foods in the diet.

7.7.6 Practical units

FBI is a percentage based on comparison of equal weights of foods, so, like GI, it is not a function of food quantity. This problem is addressed by expressing faecal bulking efficacy in terms of content of equivalents to a familiar faecal bulking reference material, so that choice of food type (content), and quantity, can be a guide in choosing foods.42

To be consumer-friendly such a reference should be:

  • Widely available.
  • Familiar/identifiable - well known to consumers.
  • Understood - known for its effectiveness.
  • Relevant - occurring widely in the normal diet.
  • Constant - not varying in the relevant properties.
  • Effective - exhibiting the property of interest to at least a moderately high degree.

Hard red wheat bran is a relevant reference for faecal bulking: it is widely available, familiar, well known for its bulking effects, widely consumed, and although it may vary between batches, it is available as an American Association of Analytical Chemists reference material for dietary fibre analysis.

With wheat bran as the reference (FBIwheat bran = 100), faecal bulking efficacy may be expressed as the weight of wheat bran that would contribute the same faecal bulk as the given quantity of food, that is, as the content of wheat bran equivalents for faecal bulk (WBEfb) in the food.

WBEfb in a weight wA of Food A, with an FBI value of FBIA is:

The faecal bulking efficacies of some of the breakfast cereals in Fig. 7.4, expressed as WBEfb per serving, are shown in Table 7.7.

WBEfb content allows direct comparison of different amounts of different products and can therefore be useful to both food processors and consumers. WBEfb allows the potential contributions to faecal bulk by food in a meal or diet to be monitored, in theory, by adding the WBEfb contributions of each food. Then, based on a desired daily WBEfb intake, any shortfall can be determined by difference, and remedied with an appropriate food or bulking supplement.

Individual WBEfb requirements will vary greatly, but once established, cumulative intake of WBEfb per day could be used to ensure that dietary goals are met. An average adult daily reference value for faecal bulking (DRVfb) of 67.6 WBEfb per day has been calculated,84 from associations between faecal weight and protection against large bowel disease measured in epidemiological studies.80,85

7.7.7 Application of FBI assay to product screening

An evaluation of the faecal bulking efficacy of Australasian breakfast cereals is an example of how the FBI assay might be used in product screening.82 The results are shown in Fig. 7.5. Faecal bulking efficacies, as wheat bran equivalents per serving, were classified according to associated nutrient claims for dietary fibre. The results show that most breakfast cereals provided less than the average bulk per serving required to provide the DRVfb on current intakes of dietary fibre sources (10 servings per day; 6.7 WBEfb per serving required), or on recommended intakes (16 servings per day; 4.2 WBEfb per serving required).

It was concluded that nutrient claims for dietary fibre in most breakfast cereals probably exaggerated their faecal bulking efficacy, although the findings would be best confirmed in human trials with some selections of the cereals.

Table 7.7 Relative faecal bulking efficacy of breakfast cereals as faecal bulking indices (FBI), and as wheat bran equivalents for faecal bulk (WBEfb) per serving

FBI (= WBEfb/100g)

Serving size (g)

WBEfb/serving (g)

All Bran




Berry Berry Nice




Bran flakes
















Fruitful porridge




Fruity Bix




Just right




















Nut Feast








Oat Bran




Puffed rice




Puffed wheat




Rolled Oats (Fleming's)




Rolled Oats (Pam's)




San Bran




Special K




Sports plus




Sultana bran








Vita Brits




Vita crunch




Wheat biscuits




Wheat bran (reference)




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