Increasingly fibre is being seen as essential to human health. This is because the fibre contained within plant foods may be required for the correct digestion and absorption of the starch also within the plant material. In particular, the fibre within plant foods may modulate the rate at which starch derived glucose is absorbed, and thus can be said to be a glycaemic modulating factor. Generally whole plant foods with their natural fibre intact have low glycaemic indices and those which have had their fibre removed through a refining process have high glycaemic indices. However, the presence of fibre alone is not enough to causes these beneficial glycaemic effects. Other factors are at play, and these result in a complex interaction between fibre, starch and the digestive process. It is thought that the fibrous plant cell wall must still be partially intact for the most beneficial glycaemic effects to be seen. This is because the cellulose cells walls act as a physical barrier to the digestive process.
For example, in one study1 a group of researchers assessed the biochemical response to a number of carbohydrate sources. Six subjects with type 2 diabetes and six healthy subjects consumed a number of carbohydrate foods included glucose, unpolished brown rice, or ground brown rice, each providing 75 grams of carbohydrate. As expected the brown rice exerted a significantly lower blood glucose response following ingestion of the foods, and this was reflected in lower insulin and gastric inhibitory peptide (GIP; also called glucose dependent insulinotropic peptide) responses. All the subjects responded in a similar way, but the responses of the type 2 diabetics were elevated in comparison to those of the healthy subjects. Interestingly the ground brown rice produced a blood glucose, insulin and GIP response that was no different to that of glucose, suggesting that the presence of fibre within the food played no part in modulating the glycaemic effect once it lost its original plant form.
Data such as these highlight the requirement for the fibre within plant foods to be in its original physical form in order for it to have beneficial glycaemic effects. This may explain the superior glycaemic effects of legumes in comparison to other starch sources such as grains. Legumes are usually eaten in their original whole plant form and as such the cell walls remain partially intact. In contrast grains are often milled into flour and used in a processed manner. One exception to the use of grains as flour is oats, which are often simply rolled before consumption as porridge. Interestingly oats show lower glycaemic responses that most other grains and in this respect are more closely similar to legumes. This may also explain the weight loss effects of incorporating oats into the diet. Lastly, adding supplements of fibre to other foods does not generally improve the glycaemic response, largely because the fibre is in a powdered and refined form and the physical barrier properties it used to possess have been destroyed.
Dr Robert Barrington’s Nutritional Recommendation: Eating plant foods in their least processed forms provides superior glycaemic effects. Refined foods, even if containing added fibre often have high glycaemic indices and thus produce unfavourable glycaemic responses. Foods with high glycaemic indices produce rapid rises in blood sugar and this overloads the liver with nutrients causing insulin resistance, weight gain, type 2 diabetes and cardiovascular disease. Picking foods with the lowest glycaemic indices is therefore the healthy option.
RdB
