Fibre is an interesting macronutrient because until recently it was largely neglected in the nutritional literature, when compared to the other major macronutrients. In this regard, fibre was seen as a component of the diet for providing bulk to food and little else. However, increasingly novel nutritional roles are being discovered for fibre and this has expanded interest it its role in human nutrition. Fibre is also interesting because of the heterogeneity that it possesses. In fact of all the macronutrients in the human diet, fibre perhaps displayes the largest heterogeneity of them all, and in this regard generalisations about its physiological and biochemical functions are difficult without qualifying statements about the type of fibre being discussed. The health benefits of fibre have resulted in the production of a number of fibre supplements that are derived from various plant sources, one of the most common being wheat bran fibre. Such fibre supplements have been extensively investigated for their physiological effects.
One thing that has emerged from the nutritional literature is that dietary fibre supplements appear not to possess all of the benefits of fibre when taken in its whole food form. In other words, processing the fibre to create the supplement appears to remove some of the original properties of the fibre that nullifies some of its health benefits. A number of factors have been identified that may account for this phenomenon, but there appears to be no single reason for these modified effects. That being said, the original location of the fibre, surrounding the nutrients within the cell, may be the overriding reason for the lack of transfer of effects from whole food fibre to processed supplemental fibre. In addition, the reduction in the particle size of the crude fibre extract ingested may play a role in determining the physiological effects, particularly those for gut transit times and water holding capacity. In this regard the effect of fibre particle size of gut transit times has been investigated for wheat bran fibre.
For example1, researchers have investigated the effects of fibre particle size on gut transit times in healthy volunteers. The subjects consumed a low fibre diet supplemented with either 32 grams of a coarse ground or fine ground wheat bran fibre. The results showed that consumption of the course bran fibre produced significantly shorter transit times as well as significantly higher (14 %) faecal wet and dry weights compared to the fine bran fibre. This was reflected in a higher water content of the faeces of the subjects consuming the course bran, when compared to the subjects consuming the fine bran (75.2 and 72.3 %, respectively). The hemicellulose digestibility in the course and fine bran diets was similar (50 and 54 %, respectively), but the digestibility of the cellulose during the coarse bran diet was only 6 %, compared to 23 % in the fine bran diet. Therefore fine bran is less effective at promoting transit through the gut, has less water holding capacity and results in less digestion of cellulose in the colon through bacteria fermentation.
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