The Glycemic Index and pulses

New ways to measure just how our blood sugar will react to various foods has provided insights into the prevention and control of conditions such as obesity, diabetes and heart disease and has also revolutionised diet in sport and the foods eaten in the hours before physical exertion in sport.

Twenty years have passed since the first index of the relative glycemic effects of carbohydrate exchanges from 51 foods was published by Jenkins et al in the American Journal of Clinical Nutrition. Per gram of carbohydrate, foods with a high glycemic index (GI) produce a higher peak in postprandial blood glucose and a greater overall blood glucose response during the first 2 h after consumption than do foods with a low GI. Despite controversial beginnings, the GI is now widely recognized as a reliable, physiologically based classification of foods according to their postprandial glycemic effect.

In 1997 a committee of experts was brought together by the Food and Agriculture Organization (FAO) of the United Nations and the World Health Organization (WHO) to review the available research evidence regarding the importance of carbohydrates in human nutrition and health. The committee endorsed the use of the GI method for classifying carbohydrate rich foods and recommended that the GI values of foods be used in conjunction with information about food composition to guide food choices. To promote good health, the committee advocated the consumption of a high-carbohydrate diet (55% of energy from carbohydrate), with the bulk of carbohydrate-containing foods being rich in nonstarch polysaccharides with a low GI.

In Australia, official dietary guidelines for healthy elderly people specifically recommend the consumption of low-GI cereal foods for good health, and a GI trademark certification program is in place to put GI values on food labels as a means of helping consumers to select low-GI foods. Commercial GI testing of foods for the food industry is currently conducted by many laboratories around the world.

Several large-scale, observational studies from Harvard University (Cambridge, MA) indicate that the long-term consumption of a diet with a high glycemic load is a significant independent predictor of the risk of developing type 2 diabetes and cardiovascular disease. More recently, evidence has been accumulating that a low-GI diet might also protect against the development of obesity, colon cancer, and breast cancer.

The Glycemic index (GI)

The glycemic index (GI) is a way of assessing measuring the effects of food on blood sugar levels. After When a carbohydrate food is eaten there is a rise, and subsequent decrease, in blood sugar (glucose). This phenomenon is level known as the glycemic response. The glycemic response to the ingestion of effect of different carbohydrate-containing foods can be compared to the response on the glycemic response of the body is classified according to a standard food, such as white bread or glucose. This resulting value measurement is called the glycemic index of that food.

A ranking on a scale up to 100 rates the increase in blood sugar levels after eating a portion of food providing 50 grams of carbohydrate. A 50-gram portion of pure glucose is the standard that is now accepted for use as the basis of comparison. Foods that break down quickly during digestion, for example rice, have the highest glycemic indices. They raise blood sugar levels higher and more quickly than foods with low G.I.s. Low G.I. foods, such as beans, break down more slowly, releasing glucose gradually into the bloodstream.

GI surprises

The finding of the glycemic index resulted in a few surprises. Originally it was believed that "complex" carbohydrate foods, such as bread, rice and potatoes, were digested slowly, causing a gradual increase in blood sugar levels. However, it is now known that many starchy foods are digested very rapidly and absorbed quickly. They are high G.I. foods. On the other hand, moderate amounts of many “sugary” foods (those high in sucrose), such as confectionery, do not generally produce dramatic increases in blood sugar as has always been thought. Foods containing sucrose actually show quite low-to-moderate blood sugar (glucose) responses - lower than foods like rice.

Implications for health

The finding of the glycemic response may have implications for several conditions including diabetes, overweight and obesity, and heart disease as results from some studies suggest that foods with a low GI factor could help to control hunger, appetite and blood sugar levels.

One of the most important implications of the GI factor is in the area of diabetes. A higher Iintake of low rather than high G.I. foods results in slower digestion of starches and sugars and slower absorption of sugar into the bloodstream. This in turn may help to regulate blood sugar levels although long-term studies on the overall health benefits are not yet available. In the past, people with diabetes were told to avoid sugar as it was thought to raise blood sugar levels rapidly. Most current recommendations allow a modest amount of sugar as the inclusion of sugar with a meal has little impact on either blood sugar (glucose) or insulin concentrations.

Weight reduction is often important for people with non-insulin dependent diabetes and low GI foods may help by promoting satiety. A diet that is comprised largely of carbohydrate-rich, low GI foods also tends to be low in fat, which may benefit weight control.

A diet with mostly low GI foods may also help reduce the risk of other diseases, such as heart disease and hypertension, although results are not conclusive. Insulin, a hormone needed for carbohydrate metabolism, has a profound effect on the incidence of many diseases. It is thought that high insulin levels are one of many factors in the development of heart disease and hypertension. The consumption of a diet rich high in low GI foods will help to lower raised insulin levels.

The GI factor is also relevant for optimum sports performance. For athletes, high and moderately-high GI foods can be used to replenish energy stores faster than low GI foods when taken immediately after an event. One study has suggested that low GI foods can help to prolong endurance but this has not been replicated by other researches.

The Glycemic Index of Some Common Foods using glucose as standard

From: Foster-Powell, K., Holt, S.H.A., Brand-Miller, J.C. 2002. International tables of glycemic index and glycemic load values. Am J Clin Nutr, 76:5-56. Foster-Powell, K and Brand-Miller, 1995. International tables of glycemic index. Amer. J. Clinical Nutrition, 62 (supp) 871-93