Shilpa Joshi, RD February / 11 / 2019
Generally carbohydrates are classified as simple and complex. According to this classification, simple forms of carbohydrate are digested quickly whereas complex are digested slowly. As scientists dove deeper into studying carbohydrates, our understanding has become more refined. It is classified by how quickly carbohydrate are digested and absorbed into the blood as glucose. This is described by a term known as glycemic index. Due to increasing popularity of nutrition science, scientific terms like glycemic index have become a common term. Often, such terms are used with little understanding, leading to some confusion. Hence, interpreting glycemic index to make wise food choices either to improve health or fitness is the need of the hour.
As defined by Jenkins et al (in 1981) glycemic index is the ranking of foods based on the blood glucose response after the consumption of the food compared with the reference food (1). According to postprandial response foods are classified as:
An important determinant of glycemic response is the rate of food digestion (1). Foods which are quickly digested and absorbed have high glycemic index, and ones with low glycemic index are those which are slowly digested and slowly absorbed. Therefore, high fibre foods have low glycemic index compared to refined low fibre foods. Eg. Fruits like guavas and apples have a lower glycemic index than bananas and mangoes.
Foods which are rich in fat and protein have low glycemic index compared to Rice or Roti (cereals) (1). Simply because the rate of digestion of fat and protein is slower than foods which are rich sources of carbohydrates. Eg Pulses like moong, moth, channa are rich sources of protein and have lower glycemic index than wheat and rice. Nuts like almonds, walnuts, pistachio, cashew which are rich source of fat also have a lower GI.
It is interesting to note that natural food processes like fruit ripening changes its glycemic index. Unripe bananas have low glycemic index as compared to ripe banana. As the banana ripens, starch is broken down to glucose changing its glycemic index from low to high. Similarly industrial food processing has an effect on glycemic response (1). Polishing the rice, puffing the cereals, that are then quickly digested, hence have high glycemic index (1). Roti has lower glycemic index compared to bread ( as they are essentially made from maida) and other refined cereals products. White rice, surti Kolam, have high glycemic index compared to brown rice, red rice (1). Depending on the degree of polishing all the rice varieties have different glycemic index. So, understand one kind of food does not have a same glycemic index, but range of glycemic index, which are influenced by many factors, described above.
However, the glycemic index has some limitations, for instance it does not consider the total amount of food consumed. For example watermelon has high glycemic index of 76. But slice of a watermelon provides only 6-8gm of carbohydrate (2). Resulting in a modest rise in blood glucose. Therefore, foods having high glycemic index do not necessarily increase blood glucose if consumed in small quantity.
In theory, low glycemic index foods are supposed to be high in fibre, thereby having a number of health benefits. But surprisingly, some low fibre and no fibre foods also have low glycemic index. Glycemic index of Ice cream, puri, chocolate, banana chips is lower than whole grains and fruits due to the high content of fat, but certainly do not have any health benefits.
Glycemic index is a double edged sword. Therefore, relying completely on the glycemic index to determine the portion size of food is misleading. Each of us have differing daily glycemic response, depending upon meal composition, the amount eaten, added fat, the way food is prepared, the temperature of the meal (hot or cold). The concept of glycemic index will give you insight about general health advice and guide you to choose healthy foods.