Tools like the Glycemic Index (GI) and Glycemic Load (GL) are used to describe and compare the effect of carbohydrates and carbohydrate foods on blood sugar levels. The Glycemic Index concept was first introduced by Jenkins and co-workers in the early 1980th. The researchers used the Glycemic Index as a tool to illustrate that the same amount of carbohydrate (50 g) consumed with different foods can elicit very different blood glucose responses. For instance, starch in lentils or pasta has a much lower effect on blood sugar levels than for instance starch in boiled potatoes or white bread, which behave more like traditional sugars in terms of a fast and high, pronounced blood glucose response. And also sugars cover the entire range from fast and high glycemic (e.g. glucose) to low and slow blood glucose responses (e.g. isomaltulose).
Figure: Blood glucose curves of different foods providing 50 g carbohydrate (adopted from Jenkins et al 1981)
Glycemic Index (GI)
The Glycemic Index is defined as the incremental area under the blood glucose response curve (IAUC) after a standard amount of a carbohydrate (typically 50 g) from a test food, relative to that of the same amount of carbohydrate from a control food (i.e. glucose or white bread), consumed by the same person on two different occasions.
Glycemic Index values have been determined for various foods and carbohydrates since then. GI values have been collected in comprehensive GI databases, for instance by Sydney University, and applied for consumer information, labelling and claims, or the ranking of foods into high GI, medium GI and low GI foods. Carbohydrate foods with a GI of 70 and more are classified as “high glycemic”, those with a GI between 56 and 69 are “medium glycemic”, and carbohydrate foods with a GI of 55 and less are classified as “low glycemic.” For isomaltulose, a GI value of 32 was determined at Sydney University, and hence it falls into the category of “low GI” foods.
When applying the GI concept to food choices, it is important to realize that the GI compares equal amounts of carbohydrate, not equal amount of food. Per gram of carbohydrate, foods with a high GI induce a higher blood glucose response than foods with a low GI. According to FAO/WHO (2003) or the ISO Standard for the determination of the Glycemic Index, the carbohydrate portion commonly contains 50 g glycemic (available) carbohydrates such as sugars and starches, whereas polyols and dietary fibers are not considered.
Glycemic Load (GL)
The term “glycemic load” was introduced in 1997. While the Glycemic Index (GI) describes the quality of carbohydrates with respect to their blood glucose effect, the quantity per serving is expressed by the Glycemic Load (GL); thus the GL provides a measure of total glycemic response to a food or meal. The GL is calculated by multiplying the amount of carbohydrates contained in a specific serving size times the GI value of that food divided by 100. One unit of GL corresponds approximately to the glycemic effect of 1 gram glucose.
Insulin Index (II)
The insulin index is calculated in the same way as the Glycemic Index, with reference to the areas under the insulin curves.
Glycemic Response (GR)
According to FAO/WHO, the glycemic response is defined as the positive incremental area under the blood glucose response curve.
Practical Use and Physiological Relevance
Tools like the Glycemic Index and Glycemic Load have their limitations for dietary planning and should be applied with common sense. They can help to compare the quality of food carbohydrates and give guidance for lower glycemic food choices within a food category (e.g. among baked good or among breakfast cereals), while however, they are not suitable as first-line guidance without considering the overall nutritional composition of a food (e.g. a chocolate would have a lower GI with 42 – 49 (yet 30 g carbs per 50 g serving, GL: 14) than a water melon with a GI of 72 to 80 (yet only 6 g carb per 120 g serving, GL: 4). Moreover, they can help to compare the glycemic properties of different sugars and ‘available carbohydrates’ (e.g. low GI isomaltulose (GI: 32), medium GI sucrose (GI: 65), and high GI glucose (GI: 100)), while food products in which sugar reduction goes along with fiber enrichment would not show a difference, since the Glycemic Index refers to the 50 g ‘available carbohydrate’ only, excluding fibers.
From nutritional perspective, dietary planning for a carbohydrate-based diet and high fiber intake comes first, while carbohydrate food choices should then give preference to those with lower glycemic properties. What matters from physiological perspective is whether a carbohydrate food has a high or a low effect on blood sugar levels and insulin release. Food options with lower effect on blood sugar levels help to lower blood sugar fluctuations and insulin release over the day. Insulin has the role of a “storage hormone”. Therefore, keeping blood sugar levels and insulin low creates a more favorable metabolic profile for the prevention and management of metabolic diseases such as overweight and obesity, diabetes mellitus and cardiovascular diseases.
References
FAO/WHO (1998) Carbohydrates in human nutrition: Report of a Joint FAO/WHO Expert Consultation, Rome, 14-18 April 1997. FAO Food and Nutrition Paper No. 66.
ISO Norm 26642:2010 – International Standard for the Determination of the glycaemic index (GI) in food products. Link: http://www.iso.org/iso/catalogue_detail.htm?csnumber=43633
Jenkins DJ, Wolever TM, Taylor RH et al (1981) Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr 34(3):362–366.