Fibre is a heterogeneous group of non-digestible plant polysaccharides. Evidence is mounting that fibre is an essential nutrient in man. In this regard, diets absent of a minimum level of fibre lead to specific metabolic changes including blood sugar and insulin dysfunction, and this causes a raft of downstream metabolic aberrations that are associated with Western lifestyle diseases. Fibre is therefore likely essential for good health. However it is not known which type of fibre or how much of it benefits health the most, and in this regard there are no real recommendations for fibre intake for Westerners, other than to increase levels. In very high levels dietary fibre may affect digestion in other way, because fibre appear able to increase the excretion on many nutrients including vitamin, minerals, lipids and nitrogen (protein). Many studies have investigated the effects of fibre on minerals, as the phytic acid component of fibre appears effective at binding certain divalent metals. The ability of fibre to cause excretion of lipids is also well reported.
The effects of fibre on nitrogen excretion is less well studied, but generally increases in dietary fibre increase faecal excretion of nitrogen. For example, in a study conducted on healthy subjects in Cameroon1, the nitrogen losses of subjects were assessed following administration of high fibre diets. Over the course of 11 days the subjects received diets of 3.3, 4.8 or 5.4 grams of fibre per 100 grams of dry matter in the diet. The fibre was in the form of sorghum meal, a staple of the individual’s normal diet. The nitrogen losses of the subjects were then monitored to assess the effects of the fibre on the total nitrogen losses. As expected, increases in dietary fibre were associated with increases in faecal nitrogen excretion. In particular, the apparent digestibility of the nitrogen from the sorghum based diets dropped from 65.4 to 60.5 to 56.9 % within increasing fibre intakes. However, as faecal nitrogen losses increased, urinary nitrogen losses decreased to compensate, such that nitrogen balance was not affected despite the poor quality nitrogen source.
Dr Robert Barrington’s Nutritional Recommendation: High fibre diets provide health benefits because they allow the control of blood sugar. The amount and type of fibre required for optimal health is not known. Therefore a range of dietary fibres should be included in the diet including soluble and insoluble types. Generally legumes, oats, vegetables and fruit contain high amounts of soluble fibre, whereas wheat, rice and other cereals contain high amounts of insoluble fibre. The upper limit for benefits of fibre intake may be as high as 60 to 80 grams per day, as this is the amount of fibre found in the diets of individuals who consume traditional diets. Fibre appears only to become essential nutritionally when starches are consumed, as some traditional populations such as the Eskimos and Masaai maintain health with low fibre intakes, but do not eat starches. Fibre increases nitrogen losses in the faeces, but these losses are recovered by a reduction in nitrogen loss from the urine.
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