The glycemic index - what is it, and what does it mean for the future of the humble carbohydrate?
TO SUSTAIN life and maintain normal physiological function, we require energy to fuel the various cells and tissues of the body.
Energy is consumed in the diet in the form of key macronutrients such as carbohydrate, fat and protein as well as in a number of micronutrients and essential vitamins and minerals which can usually be obtained from a healthy, balanced diet.
The internationally-recognised guidelines, taking into account personal circumstances and individual needs, recommend that the daily calorie intake of a healthy diet should be high in carbohydrates (45-65 per cent), moderate to low in fat (20-35 per cent) and adequate in protein (10-15 per cent).
Carbohydrates are a vital nutrient for energy production in the body and are stored as sugar (glucose) in the blood and as glycogen in the muscles and liver.
Effectively, carbohydrate acts as a "high-grade" fuel, which is particularly important to sustain more intensive everyday activity as well as during most forms of physical exercise.
The downside is that carbohydrate stores in the body are only limited, whereas fat deposits for the average person are plentiful and can sustain activities associated with daily living almost indefinitely, so long as they are of a sufficiently low intensity.
Carbohydrates come from a diverse source of food groups such as sugars and starches and, depending on their origins, have varying effects on energy levels within the body.
The different forms of carbohydrate were, until recent times, referred to as either "complex" (starches) or "simple" (sugars). The notion was that simple carbohydrates were digested and absorbed more rapidly, while the energy released from complex carbohydrates was more gradual.
However, this is not always the case and in fact certain complex carbohydrate food types may actually elevate blood glucose more rapidly than some typically classified as simple carbohydrates.
The traditional view was that carbohydrates in their complex form were good, and, in their simple version, they were bad. This concept has been since found to be far too simplistic.
To overcome this confusion, the principle of the "glycemic index" (or GI) was developed.
Glycemic index quite simply is a measure of the effects of carbohydrates on blood sugar levels and how long it takes for the glucose from a food to be absorbed into the blood.
Carbohydrates that break down quickly during digestion, leading to a rapid rise in blood glucose have a high GI, whereas carbohydrates that break down more slowly, leading to only a gradual release of glucose into the blood, have a low GI.
The glycemic index is based on a numerical scale of 1 to 100 with high-GI food classified as 70 or greater; medium-GI food in the range of 56-69 and low-GI foods of 55 or less.
It is now therefore possible to classify all food types based on their glycemic index, which may have major implication for optimising nutritional intake for a healthy and active lifestyle.
Next week, we will look at examples of high and low GI foods as well as the importance of the glycemic index for disease control, weight management and sports performance.
• Dr Giles Warrington is a sport and exercise physiologist and lecturer in the School of Health and Human Performance at DCU