Traditionally fibre has been seen as roughage providing bulk to the food and nothing more. However, recently research has shown that fibre is likely an essential component of the human diet. Fibre is actually a diverse group of carbohydrates that can be broadly split into two groups, Soluble fibre has the properties that it absorbs water and forms a gel-like substance, whereas insoluble fibre has much less capacity to absorb water in this way. However, other classifications exist and the simplistic soluble-insoluble fibre mantra does a disservice in some respects to the understanding of the physiological functions of this complex group of molecules. Soluble fibres have been well researched for their ability to benefit postprandial glucose levels in humans. In this regard soluble fibres have taken most of the plaudits for their beneficial glycaemic effect, which is unfair as insoluble fibre may also have benefits. Bran fibre is an insoluble fibre found in cereal grains that may have similar blood glucose lowering effects to the soluble fibres.
For example, in one study1, researchers assessed the long term effects of the addition of 20 grams of wheat bran containing 11 grams of fibre to the diet of 5 healthy subject. The subjects consumed the fibre for 7 weeks, incorporated in bread, and the subjects were free living and no other aspects of their lives were controlled. Baseline measurements were taken to assess the blood glucose and insulin responses to a test meal that contained 15 % protein, 30 % lipids and 55 % carbohydrates. Further assessments of blood glucose and insulin responses to the same test meal were taken at day 10, day 24 and day 45 following the commencement of consumption of the fibre supplement. The addition of bran lead to progressive reductions in the meal-induced glucose response to these test meals over the duration of the study. However, the insulin response remained unchanged during the first three test meals, and then increase during the last test meal. This suggest that the benefit to glucose levels was not as a result of changes to the release of insulin.
It has been hypothesised that fibre can slow the release of starch from the stomach and this may be a contributory factor in the beneficial effects on glycaemic response seen with fibre consumption. In addition, fibre may increase the speed of transit of food through the intestine and this may in term reduce the contact of starch with digestive enzymes. However, this does not explain over time the gradual improvement in the disposal of glucose seen following fibre ingestion. While fibre may indeed account for changes in transit speed, how does the magnitude of this effect become greater over time? It cannot, and so some other mechanisms, probably biochemical in nature, is the likely explanation for this phenomenon. Another possibility is that fibre can change the composition of the pancreatic juice, and that release or production of enzymes such as amylase are reduced. However, this does not explain the increase secretion of insulin seen after day 45 of the study. The complex nature of fibre is still to be uncovered, yet is clearly is essential for good health.
Dr Robert Barrington’s Nutritional Advice: Fibre is a diverse group of carbohydrates and the full mechanisms by which it influences good health are not fully understood. Because fibres can have differing effects, a range of dietary fibre from a wide selection of plant foods should be incorporated into a healthy high quality diet.
RdB
