Glycemic Index Scale or GI scale
In very technical terms if we plot blood glucose response curve (curve showing increase in glucose after ingestion of fixed quantity of food), then area under two hour blood response curve is Glycemic index of food. For comparative purposes, this area under curve found as percent of area under curve of standard food as it gives a relative ranking. Glycemic scale thus depends on the reference or standard food used for the purpose. In general, staple food is used as standard. If Glucose is used as reference, Glycemic index thus can be on scale from 1 to 100. (Image illustration – University of Sydney)
Various methods of cooking or initial food processing have various effects on Glycemic index of food. Hence its very difficult to attribute single GI score for a food. Using this GI scale we can separate Carbohydrate containing food into various categories. In general we have three categories: High Glycemic Index Foods, Intermediate Glycemic Index Food and Low Glycemic Index food
High Glycemic Index food: This high Glycemic index food have GI more than 70 and are known to cause rapid rise in blood sugar levels.
Moderate or Intermediate Glycemic food: Intermediate Glycemic food have GI between 55 to 70 and are moderate in nature with respect to increasing sugar levels.
Low Glycemic Index Food: These low glycemic index food cause slow rise in blood sugar and have GI less than 55. Glycemic Index evolved out of research on diabetes. Low Glycemic index food is considered good for people suffering from diabetes. Low GI food offer controlled release of glucose in blood as compared to high GI food. Such low GI food includes oats, beans, puluses etc. With slower rate of digestion and absorption of food carbohydrates, equates to lesser insulin demand (low insulin index) and long term improvement in blood sugar control and blood lipids.
Low GI diet
It was Dr. David Jenkins from University of Toronto who was first proponent of Low Glycemic Index Diet. Subsequently many nutritionists promoted low GI food. Low GI diet apart from health benefits on diabetes, also have health benefits for weight control through ability to control appetite and delaying hunger. Low GI diets help reduce levels of body insulin and offer less insulin resistance.
Books like Montignac Method, The South Beach Diet, the Atkins Diets, The Zone Diet and others all promoted the same idea. Harvard School of Public Health suggests that there is strong correlation between GI of overall diet and risks of diseases like Type 2 Diabetes and coronary heart disease. Various renowned organizations like World Health Organization (WHO) and Food and Agriculture Organization recommends use of low GI diet for reduction of lifestyle disorders like obesity, heart diseases and diabetes.
According to GI unit of University of Sydney, Low GI diet helps to reduce and manage weight. It improves diabetes management and body sensitivity to insulin. Low GI diet reduces risk of heart diseases. It helps prolong physical endurance and refuel carbohydrate stores after exercise. (Reference: Human Nutrition Unit, School of Molecular and Microbial Biosciences, University of Sydney)
Methods of cooking do have impact on GI of food. Whole grain cereals have lower GI than processed or milled cereals. Some studies show that adding vinegar or lemon juice is said to lower glycemic index by 25%. Adding cereals, fresh vegetables and fruits can lower over GI of diet.
Various studies show that GI diet program have health advantages over low carbohydrate or low carb diet. GI diet program follow approach of having nutritious carbohydrate rich food but with low GI value for better control on blood glucose levels.
Along with Glycemic index another ranking system followed is Glycemic load. Glycemic index is for fixed portion size. High Glycemic index food consumed in small quantities have same impact on blood sugar levels as low Glycemic index food in large quantities. Glycemic load is a ranking system for carbohydrate content in food portions based on their glycemic index (GI) and the portion size. Glycemic load or GL combines both the quality and quantity of carbohydrate in one ‘number’. It’s the best way to predict blood glucose values of different types and amounts of food. The formula is: GL = (GI x the amount of available carbohydrate) divided by 100. Peak Glucose levels in blood is important parameter in fighting diabetes, hence use of Glycemic Load as ranking system is also essential.