The Glycemic Index (Gi) is a tool that measures how carbohydrates affect blood glucose levels. It then ranks the quality of carbohydrates based on this score.

Carbohydrates are an essential part of our diet since they provide fuel for the brain, most other organs and muscles during exercise. However, not all carbohydrate foods are equal.

High GI carbohydrates cause blood sugar levels to spike and then crash, whereas low GI foods are digested and absorbed more slowly. This slow release of glucose into the bloodstream is proven to be much more beneficial for the body – from improving energy levels to managing weight, diabetes and other health concerns.

How is GI measured?
Understanding the GI value of food can help you make healthier food choices and improve your overall health in the long run.

The GI value of foods is calculated in a food laboratory using valid scientific methods. There are three classifications for GI:

Individual food portion:
Low: 55 or less
Mid: 56 – 69
High: 70+

Foods with a high GI score contain carbohydrate that is rapidly digested and produces a sharp rise and fall in the level of blood glucose. In contrast, foods with a low GI score contain slowly digested carbohydrates which produces a more gradual and relatively low rise in the level of blood glucose and insulin levels. GI values of foods must be measured using valid scientific methods. It cannot be guessed by looking at the composition of the food or the nutrition information panel on food packaging.

Foods with a high GI score contain rapidly digested carbohydrates, which produces a large rapid rise and fall in the level of blood glucose. In contrast, foods with a low GI score contain slowly digested carbohydrates, which produces a gradual, relatively low rise in the level of blood glucose.

What is the glycemic response?
After eating a meal, any digestible or available carbohydrates in the meal are absorbed into the bloodstream, producing an increase in blood glucose concentration.  

The rise and fall in glucose can be detected using a blood glucose meter.  However, because glucose (glycemic) responses vary markedly from day to day for all sorts of reasons, it is almost impossible for you to work out the GI of a food. The GI is the average glycemic response (GR) elicited by a portion of food containing 50 g of available carbohydrate expressed as a percentage of the GR elicited by 50 grams of the reference carbohydrate (e.g., 50 g of glucose). The GI is therefore a ranking of the glycemic potential of the carbohydrate embedded in different foods.  It is a characteristic of food, not a person.  

More slowly digestible carbohydrates and minimally processed starchy foods produce a different response compared to rapidly digestible carbohydrates. They produce a slower and more prolonged increase in blood glucose levels, rising to a lower peak.

Other factors that contribute to the glycemic response include the amount of food consumed, how much the food is processed and even how the food has been prepared. For example, pasta that is cooked al dente has a slower glycemic response than pasta that is overcooked.

What about glycemic load?
Your blood glucose levels rise and fall after you eat a meal containing carbohydrates. How high it rises and how long it stays high depends on the quality of the carbohydrate (the GI) as well as the quantity.  Glycemic Load (or GL) combines both the quantity and quality of carbohydrates. The formula for calculating the GL of a particular food or meal is:

Glycemic Load = GI (%) x Carbohydrate (grams) content per portion ÷ 100

For example, a single apple has a GI of 38 and contains 13 grams of carbohydrates.
GL= 38 x 13/100 = 5g
So an apple provides an equivalent of 5 g of glucose.

A potato has a GI of 85 and contains 14 grams of carbohydrate
GL=85 x14/100 = 12g
So a potato provides an equivalent of 12 g of glucose.

We can therefore predict that the potato will have more than double the glycemic impact of an apple (12 vs 6). Similar to the glycemic index, the glycemic load of a food can be classified as low, medium, or high:

Low: 10g or less
Medium: 11 – 19g
High: 20g or more

The GL of a mixed meal or diet can simply be calculated by summing together the GL values for each ingredient or component. For example, if breakfast was composed of 2 wheat biscuits (GL = 15), ½ a cup of milk (GL = 4) and 2 teaspoons of sugar (GL = 6), its overall GL would be 25 g (15 g + 4 g + 6 g).

For the whole day, a target GL of 100 g has been recommended for people consuming 8,400 kJ (2000 calories).

For people consuming 3 meals per day, a low GL meal would have a GL ≤ 33 g. For optimal health, you should aim to keep your daily glycemic load under 100.

Presence of fibre
Fibre (either in the carbohydrate itself or in the stomach) protects the starchy carbohydrate from rapid attack by digestive enzymes and slows digestion in the digestive tract. Both of these consequences ultimately slow down the conversion of carbohydrates to glucose.

Presence of fat and/or acid
The more fat or acid a carbohydrate food contains, (or the more fat or acid in the stomach during digestion), the slower the carbohydrate food is converted to glucose and absorbed into the bloodstream. The presence of fat and/or acid delays the emptying of the stomach, so adding vinegar, lemon juice or pickles to food will help lower the GI of a meal. Fermenting foods or the sourdough method of baking bread also lowers the GI.

Mixed meals and the effect of extra protein and fat on blood glucose response
Eaten alone, protein and fat have little effect on blood glucose levels, but that’s not to say they don’t affect your blood glucose response when they are combined with carbohydrate-rich food. Protein will stimulate additional insulin secretion, resulting in lower blood glucose levels. Protein and fat both tend to delay stomach emptying, thereby slowing the rate at which carbohydrates can be digested and absorbed.