Glycemic index is a scale that ranks carbohydrates by how much they raise blood glucose levels compared to a reference food. A GI value tells you only how rapidly a particular carbohydrate turns into sugar. Not all carbohydrates act the same, some are quickly broken down in the intestine, causing the blood sugar level to rise rapidly. Such carbohydrates have a high glycemic index (GI). A low GI is between 0-55, a moderate GI is between 56-69 and a high GI is 70 and above. Some of the factors that influence GI ranking are type of starch, cooking, physical entrapment, viscosity of fibre, acid content, sugar content, fat content etc. Types of starch are like amylose which absorbs less water, molecules form tight clumps and amylopectin which absorbs more water and molecules are more open. Viscous, soluble fibres transform intestinal contents into gel-like matter that slows down enzymatic activity on starch. Apples have a low GI of 40 while whole wheat bread has a higher GI of 73. Acid slows down gastric emptying, and thus, slows down the digestion of starch. Sourdough wheat bread has a low GI of 54 while wonder white bread has a high GI of 73. Most refined, processed starches and fruits foods result in a higher glycemic index. Low GI diets lowers cholesterol levels, weight, heart disease risk and type 2 diabetes mellitus risk.
Glycemic load measures the degree of glycemic response and insulin demand produced by a specific amount of a specific food. Glycemic load reflects both the quality and the quantity of dietary carbohydrates.
GL=GI/100*CHO (grams) per serving
Individual food portion and whole day food portion have different GL ranking values. Sometimes even a low GI food can have a high GL if the portion is big enough. And, a high GI food can have a lower GL if the portion is controlled. Glycemic Index ranks carbohydrates based on their immediate blood glucose response hence GI=glycemic quality while Glycemic Load helps predict blood glucose response to specific amount of specific carbohydrate food quality therefore GL= Glycemic quantity.
If we look at the GI and GL values of some foods we see the following comparison. Avoiding carrots because of their GI ranking would be a big mistake, particularly given all the vitamins and minerals they contain and the low GL of each serving. The GI of potatoes, on the other hand, is not a misleading measure because potatoes are carbohydrate-dense. Their GL is also fairly high.
Some dietary guidelines according to GL and GI index would be to balance calories in with calories out. Eat balanced diet with variety of nutrient-dense foods and beverages. Consume 2 cups fruit, 2% cups vegetables per day (2,000 calories intake). Choose whole grains for at least half of daily grain consumption. Some caution should be observed like not focusing exclusively on achieving a low glycemic load diet with all low glycemic index food choices. Result could be high fat low carbohydrate low fibre calorie dense diet, instead aim for a well-balanced diet that includes low glycemic index carbohydrates. Use glycemic load as a guide for controlling portions.