Dietary Fibre and GI

The glycaemic index is a means of classifying carbohydrate rich foods and various carbohydrates according to their effect on blood sugar levels after a meal.

These data are frustrating for health professionals. For example it is known that obesity can, for the most part, be prevented. It is strongly linked to personal lifestyle, in particular energy consumption and the level of physical activity undertaken. Current dietary recommendations suggest decreases in both saturated fat and free sugar consumption, while at the same time increasing consumption of fruit and vegetables, and dietary fibres with an objective of obtaining 55-75% of daily energy from complex carbohydrates (e.g. starch) [2]. As part of a dietary programme to reduce the risk of these metabolic diseases, it has been suggested that low glycaemic index (GI) foods may make a significant role.

GI was defined as the ‘incremental area under the blood glucose response curve elicited by 50g available carbohydrate portion of a test food expressed as a percentage of the response elicited by the same amount of carbohydrate from a reference food taken by the same subject, during a given period of time after ingestion’ [3]. Thus the reference food is given the value 100, and the lower the response, the lower the GI.

Based on the values of the GI, foods have been classified as [4]:

• low GI foods (GI <55 )

• intermediate GI foods (GI 55-70)

• high GI foods (GI>70).

This simple method provides an effective means to compare the rate of glucose assimilation following the ingestion of various foods. It has also demonstrated that the effect of a carbohydrate food on blood glucose level is determined by much more than just its chemical composition. Some factors that influence glycaemic response to foods are presented in table 1.

The short-term biological and health effects of low GI foods or diets have been extensively investigated [8] [9]. Dietary intervention and epidemiological studies have highlighted possible benefits of low/medium GI foods to health in terms of diabetes [10, 11], [12], cancer [13], appetite control [14], body fat [15], and sports performance [5]. During the past 20 years, the GI of a variety of foods has been determined, and summary tables of the GI of over of 1000 different food items are available from both the scientific literature and other educational media.

Although the GI concept seems to be relatively well established, there is no consensus regarding its use in guiding the food choices of diabetics, obese people or other at-risk groups. The debate evolves around of number of factors. These include:

• The ability to predict GI values of meals based on GI determinations for individual foods.

• The methodology used in determining the GI value of foods.

Another area of ongoing debate concerns identifying a simpler way of expressing/communicating the impact of foods on blood sugar levels.

Various parameters have been proposed, amongst which are:

• Glycaemic load (GL) – the GI value of a food product multiplied by the amount of carbohydrate [16]. The concept takes into account both the quality and quantity of carbohydrate since they both affect the level of blood sugar after eating [17].

• Glycaemic glucose equivalent (GGE) – the theoretical weight of glucose that would induce a glycaemic response equivalent to that of a given amount of food [18].

The most suitable parameter to be used for labelling has yet to be agreed. Part of the reason is related to concerns about the relevance of informing the consumers on the GI of foods. While it is clearly understood that low GI foods are associated with several health benefits mentioned above, it is also evident that low GI foods/diets alone do not confer per se overall health benefits. For example a high fat carbohydrate food may have a low GI, but it has at the same time a high energy density – detrimental in the context of obesity or other chronic diseases. Therefore low GI foods/diets should not be looked at in isolation, but placed within the context of standard recommendations for a healthy diet.

Despite this lack of consensus, food manufacturers and retailers have started to promote the GI concept (e.g. Australia – the GI symbol program). Some countries have also started to allow a health claim related to ‘maintaining a healthy blood glucose level’ (e.g. Sweden), while others have started to adapt their basic food law for the GI labelling of products (e.g. South Africa – the ‘GISFA Symbol’).

Within the UK, some food retailers and manufacturers have appeared to also start embracing the GI concept. Food manufacturers have also shown interest in finding ways to lower the GI of existing food products. Examples from the UK include a low GI white bread and a fibre enriched white bread. These products have widened the range of high fibre and/or low GI bread products already available on the market such as wholegrain bread, rye and pitta bread. Nevertheless, the drive for developing low GI foods is not shared across the food industry. Some are looking at the GI concept more sceptically. A survey commissioned by the Flour Advisory Bureau, UK, revealed that, ‘66% of consumers do not understand the GI concept and therefore can not apply it when trying to select a healthy balanced diet’. Moreover, ‘consumers were so confused by the GI system, that they are at risk of selecting a diet too high in fat and too low in starchy carbohydrates’ [19].

Development of foods, which combine the health benefits associated with low/medium GI products together with those of DF, provides an opportunity for food manufacturers to launch new products onto the market. Inclusion of cereal beta-glucans or guar gum has been shown to lower the GI of foods.

Interest in using cereal beta-glucans as an ingredient in foods has increased since the US Food and Drug Administration (FDA) approved the health claims for oat beta-glucan (0.75g per serving) on the assumption that 4 servings per day would reduce the risk of cardiovascular disease [26, 27]. This claim was more recently followed by Joint Health Claims Initiative, UK who approved last year a claim related to consumption of oat beta-glucans [28]. Beneficial health effects are thought to be associated with barley beta-glucans as well. Several research studies have substantiated this by reporting effects on lowering both blood glucose [29] [30] and cholesterol levels [31] in humans. Development of functional foods is increasing, as shown by the existence of several beta-glucan concentrates available commercially that can be added to different foods (table 2). Enriching foods with DF, while potentially of nutritional value, presents technological challenges [32], in particular in arriving at a product, which is acceptable to the consumer. It is essential therefore, that we understand at a fundamental level how different types of DF interact with other food components and how these interactions influence not only product attributes (consumer acceptability), but also any potential physiological effects.t