Fibre is not officially an essential component of the diet. However, evidence has been mounting for a number of decades to show that carbohydrate containing low fibre diets cause disease. In particular, carbohydrate diets devoid of fibre are thought to cause aberrations in normal blood glucose regulation and insulin homeostasis. Further, reintroduction of fibre to the diet reverses this deterioration in glycaemic control. As fibre is not produced endogenously, it therefore falls under the definition of a conditionally essential dietary component. The mechanism by which fibre is able to exert beneficial glycaemic effects is not fully understood, but may relate to the influence fibre has on digestions and absorption rates. For example, soluble fibre can form a viscous gel in the gut which inhibits to some extent the chyme within the gut interacting with the enterocytes, and in doing so decreases the absorption rates of the glucose significantly. Fibre in the gut may influence other digestive processes including the inhibition of digestive enzymes.
Much of the work of fibre in human nutrition has been performed in vitro. This is largely because it is difficult to study the mechanistic factors at play when the fiber is contained within an animal, particularly a human animal. However such in vitro work, much of it performed decades ago, has produced a plethora of useful data. For example, in one study1, the effects of dietary fibre from alfalfa, oat bran, pectin, cellulose, wheat bran and xylan on digestive enzyme function were studied using an in vitro model of digestion. The dietary fibres were incubated with pancreatic juice and the activity of lipase, amylase, trypsin and chymotrypsin was analysed. Incubation with cellulose and xylose caused a significant decrease in the activity of the pancreatic enzymes. Wheat bran and oat bran caused a decrease in the activity of amylase and chymotrypsin, while alfalfa decreased trypsin and chymotrypsin activity. Only pectin caused an increase in enzyme activity, with amylase and chymotrypsin activity increasing significantly.
It should be remembered that this study was performed in vitro, and so the results may not be directly representative of the effects of fibre in the gut. However, the authors reported that previous animal experiments performed in the same laboratory showed that feeding cellulose to rats decreased digestive enzyme activity, while feeding pectin increased digestive enzyme activity. Other studies in rats have shown that cellulose and pectin cause decreases in the activity of leucine aminopeptidase2. Therefore the in vitro results of this study are supported by evidence from animal studies. Mechanistically, it is unclear as to how fiber can affect digestive enzyme function. The most obvious explanation relates to the physical barrier that fibre can form between an enzyme and its substrate. Further, many dietary fibres contain substances that act as enzyme inhibitors, such as phytic acid. However, it is less easy to explain the ability of pectin to increase digestive enzyme activity, although the provision of a catalytic matrix is one possibility.
Dr Robert Barrington’s Nutritional Recommendation: The observation that the absence of fibre from carbohydrate containing diets leads to a clear set of deficiency symptoms that are reversed upon reintroduction of the fibre suggest that fibre is conditionally essential to the health. As with many components of plants, dietary fibre is a diverse group of structurally different examples. These dietary fibres appear to have overlapping but synergistic effects on gut physiology and human metabolic regulation. No one type of fibre appears to produce all of the benefits of the whole group, and as such recommendations to consume a range of fibres in order to derive all of the health seem prudent. Food rich in soluble fibre include fruits, vegetables and oats, whereas wheat and other cereal grains including rye, barley, rice, spelt, millet and sorghum and rich sources of insoluble fibre. Oligomers of fructose (fructooligosaccharides; FOS) and galactose (galactooligosaccharides; GOS) also have important health effects in humans gut function.
RdB
