While the traditional diets from different geographical regions may be quite different, they all have no sugar or refined starches as their defining factor. The nutritional sciences has investigated the Mediterranean diet with regard its health effects, and generally the results of such investigations show that regular long term consumption reduces the risk of disease and being overweight. Its is no coincidence that these two outcomes are reduced concomitantly because they are intrinsically linked to an underlying disease state characterised by metabolic dysfunction. Increasingly Western processed foods, are being seen as the cause of this dysfunction, and traditional diets such as the Mediterranean diet are seen as the cure. Traditional diets contain only whole foods and in particular their starches retain their fibrous plant material that is required for their correct digestion and metabolism. The modification of digestive processes by fibreless refined starched is now thought to cause blood sugar dysfunction and disease.
However, there exists a disconnect between the fibre content of foods and the glycaemic index. Many foods with high glycaemic indexes contain more fibre than those with low glycaemic indexes. Starch digestibility is therefore not dictated solely by the fibre content of the food. Studies assessing the rate of digestion of different starches are therefore of interest. Many studies have been performed on the in vitro digestion rates of different foods, and these studies are useful because they allow a detail understanding of the possible changes to the digestion and absorption rates of foods. For example, comparisons of the digestibility of a number of processed factory foods such as instant rice, corn chips, corn flakes, rice bubbles, instant potato and potato crisps have been compared to home cooked foods such as boiled rice, sweet corn and potato1. Incubation of the foods with human saliva and porcine pancreatic juice for 3 hours resulted in a significantly higher percentage of starch digestion in the processed foods compared to the home cooked foods.
A follow up experiment fed healthy volunteers the foods, and in all but one case (potato crisps) the processed foods produced a significantly higher glycaemic response compared to the home cooked foods. While not a direct comparison of similar foods, this study was designed to compare the sorts of foods that might be purchased and eaten by people with different attitudes towards diet. Those who are inclined to eat processed ready to eat foods tend to do so routinely, and this may affect disease outcomes. The similar glycaemic responses to potato chips and home cooked potato is interesting, and may be explained by the fat content of the crisps which may alter transit times through the gastrointestinal tract. In addition, even whole potato has quite a high glycaemic index which would diminish the difference seen between home cooked and processed variations. The home cooked rice in this study was not whole grain but refined white rice (Mahatma long grain), supporting the view that factors other than fibre can affect starch digestibility.
Factory processing of ready to eat foods differs from that of home cooking. For example processing techniques such as explosion puffing, extrusion cooking and instantisation make use of very high temperatures and pressure that would not be available in home cooking. Use of these cooking methods produces a range of snacks and foods with long shelf lives that are ready to eat, but at the same time the techniques may alter the digestibility rate of the starches contained within. While fibre may affect the digestibility rate of some foods, particularly oats and legumes, other studies have shown no effects on glycaemic responses between high fibre and low fibre meals. Many studies have shown that fibre itself is not beneficial unless the original cell walls are intact or partially intact as this inhibits the action of digestive enzymes. The inability of fibre supplements to alter postprandial glycaemia also suggest that fibre per se is not the arbiter of digestive rate. Care should therefore be taken when comparing starch digestion rates.
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