Glycemic Index - the basics

Glycemic Index (GI) is a measurement that expresses a food's ability to give rise to an increase in blood glucose levels. It relates specifically to the increase brought about by consuming a quantity of food containing a fixed amount of available carbohydrate (traditionally 50g or 25g).

This measurement has assumed significance as the balance of medical opinion is now concluding that a diet comprising food with a high GI is implicated as a major risk factor in diseases such as diabetes, and in the promotion of medical conditions related to obesity.

The principal medical concern is that sharp rises in blood glucose lead to sharp rises in insulin. Of itself, this sharp rise in insulin is unwelcome, but there is the subsequent risk that the consumer develops insulin resistance. Both insulin resistance and insulin-like growth factors have been implicated in cancers of the colon, breast, and prostate.

Not surprisingly, many food producers are now attempting to lower the GI value of their products in support of consumer preferences for a more healthy diet. The UK market is a little behind countries such as Australia, where the GI Symbol Programme was launched in 2002. Here, foods can carry the GI symbol, and their GI value, provided they have been tested in an appropriate manner. Similarly, the National Food Administration of Sweden has formally endorsed labelling of foods based on their GI values.

GI Testing

A food's GI value can only be measured by testing the blood of actual volunteers following consumption of the product in carefully controlled conditions. This control includes a degree of medical supervision. At RSSL, measured portions of the food under test are fed to a panel of healthy volunteers after an overnight fast. Finger-prick blood samples are taken at 15 minute intervals over the next 2 hours, and their blood glucose levels are measured. These results give a blood sugar response curve for the 2 hour period. This can be compared with the curve derived from consumption of a glucose standard by the same volunteer. The average of the GI ratings from all volunteers is considered to be the GI of that food.

Hence the GI is a numerical value given to foods based on their measured potential to raise blood glucose levels relative to the standard.

There are other measures that are also sometimes calculated. Glycemic Load (GL) another dimensionless number that relates the GI of a food item to the amount of available carbohydrate it contains in an average serving, rather than the arbitrary portion containing a fixed quantity of available carbohydrate.

For products containing ingredients such as sugar alcohols (polyols) or resistant starch (which do not analyse as available carbohydrate) it is difficult to measure their GI.  However, it is possible to measure their Glycemic Response (GR).  GR compares the potential of a product to raise blood glucose with a glucose reference.  The results would need to be declared/phrased differently from GI data, in that a phrase such as "a serving of this product raises blood glucose to x% of the increase from a 50g serving of glucose" would need to be used rather than a number alone.

It is not legitimate to assume that the GI (or GL/GR) value of one brand of product will match that of a different brand. Nor is it legitimate to conclude that the GI of a mixed meal will represent an average of its constituent ingredients. Many dietary factors affect the body's ability to metabolise glucose and insulin. They include major constituents such as fat and fibre, as well as metal ions, vitamins and minerals, various hormonal interactions, and an individual's weight and physical activity.

Hence, the only safe route to providing a GI value is to have it measured for each product. Similarly the only safe way to adjust a product's GI value is to reformulate it, and test that the reformulation has achieved the desired effect.

Adjusting GI

As with any formulation change, the process of adjusting the GI is seldom straightforward. It is not always easy to replace a 'high' GI ingredient with a 'medium' or 'low' GI carbohydrate without it affecting the sensory characteristics of the product.  Ingredients such as sugar alcohols, fructose, polydextrose and resistant starches have a low glycemic effect.  These ingredients can be used extensively to completely or partially replace sucrose, glucose and high GI polysaccharides in a wide range of food products.  Also, a growing number of ingredients, such as oat betaglucans and other soluble fibres, which are reported to delay gastric emptying and therefore attenuate blood glucose responses, could also be used.

Growing trend

There is clearly a momentum building in favour of lowering our consumption of foods with a high GI or GL value. That said, it would be wrong to argue that low GI foods are a panacea to all of the industrialised world's health problems. It would certainly be wrong to select our diet on the basis of GI values alone. Nonetheless there is every possibility that GI values may soon become a common feature of food labels, and consumers will come to accept a low GI value as a healthier option. 'Low GI' may soon be as prevalent on food labels as the now commonplace 'low fat' and 'low salt'.