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What is the Glycemic Index

Carbohydrate is an essential part of our diets, but not all carbohydrate foods are equal. The Glycemic Index (GI) is a relative ranking of carbohydrate in foods according to how they affect blood glucose levels. Carbohydrates with a low GI value (55 or less) are more slowly digested, absorbed and metabolised and cause a lower and slower rise in blood glucose and, therefore usually, insulin levels.

Low GI vs High GI curve

There are three ratings for GI:

In individual portions:

Low = GI value 55 or less

Medium = GI value of 56 – 69 inclusive

High = GI 70 or more

So why do we need good quality Low GI carbohydrates?

Carbohydrates are an essential nutrient 

You need carbs as they break down into glucose in your body providing the

  • main fuel for our brains and nervous systems,
  • preferred source of fuel for most organs and our muscles during exercise.

Consuming good quality carbohydrates aka Low GI ones help to facilitate the management of diabetesweight loss and weight loss management and reducing the risk of developing type 2 diabetes, diabetes complications and other chronic lifestyle diseases.  In fact a low GI diet provides health benefits for everybody across all stages of life.

A low GI diet is not a fad diet but a way of eating that is sustainable in the long term and backed by over 30 years of scientific evidence. To get you started we have developed a Low GI Shopping List and you can get easy tips on how to go low GI here. Remember to look out for the GI Symbol on products – Making healthier choices easy.

How is the GI measured?

The glycemic index (GI) is a measure of the power of foods (or specifically the carbohydrate in a food) to raise blood sugar (glucose) levels after being eaten. The GI values of foods must be measured using valid scientific methods. It cannot be guessed by looking at the composition of the food. Currently, only a few nutrition research groups around the world provide a legitimate testing service.

The GI value of a food is determined by feeding 10 or more healthy people a portion of the food containing 50 grams of digestible (available) carbohydrate and then measuring the effect on their blood glucose levels over the next two hours. For each person, the area under their two-hour blood glucose response (glucose AUC) for this food is then measured. On another occasion, the same 10 people consume an equal-carbohydrate portion of glucose sugar (the reference food) and their two-hour blood glucose response is also measured. A GI value for the test food is then calculated for each person by dividing their glucose AUC for the test food by their glucose AUC for the reference food. The final GI value for the test food is the average GI value for the 10 people.

Foods with a high GI score contain rapidly digested carbohydrate, 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 carbohydrate, which produces a gradual, relatively low rise in the level of blood glucose.

What affects the actual GI value?

Several factors influence how fast a particular carbohydrate food raises blood sugar. These factors include: the chemical and physical structure of the carbohydrate-food in question; how refined the carb is; how the carb is cooked and also the presence of other substances which reduce either the potency of the body’s digestive enzymes, or the speed of digestion.

Chemical Structure of the Carbohydrate
The body processes glucose very efficiently. (The glycemic index of glucose is 100.) But the body cannot easily metabolize fructose, a common monosaccharide in fruits, which is why fructose has a low GI of 23. Ordinary table sugar (sucrose), is a disaccharide made up of one molecule of glucose linked to one of fructose. Hence the glycemic index of table sugar is 65, midway between 23 and 100 in the medium-glycemic-index range.

Physical Structure of the Carbohydrate
The physical structure of the carbohydrate also affects its glycemic index value. For example, most breads are in the high range – not due to the chemical nature of wheat starch, but for two physical reasons.
(1) The fine particle size of wheat flour gives digestive enzymes great surface area to attack and metabolize the bread.
(2) The surface area of bread is also increased by its puffed-out, fluffy structure. Result? The glycemic value of bread is significantly raised by these structural attributes.

How Refined is the Carbohydrate
One of the most important factors that determines the glycemic index of carbohydrate foods, is how refined or processed the carbs are. In general, refined or processed carbs have had most of their ‘natural’ fiber and other ‘inconvenient’ constituents (eg. which may affect the food’s shelf-life) removed. Result? The carbohydrate is incapable of resisting the digestive enzymes and is rapidly metabolized into glucose.

How Carbohydrates are Cooked or Prepared
Pasta has a glycemic-index value of 40-50. This can be further reduced by cooking it less (al dente). This is because al dente pasta resists the effect of digestive enzymes more than regular cooked pasta and so has a lower GI.

Fiber Slows Down Metabolism of Carbs and Their Digestion
Fiber (either in the carbohydrate itself or in the stomach) protects the starchy carbohydrate from rapid attack by digestive enzymes, or slows digestion in the digestive tract. Either of these consequences will slow down the conversion of the carbohydrate to glucose.

Fat and/or Acid Slows Down Metabolism of Carbs and Their Digestion
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 retards the emptying of the stomach. Example: Vinegar, lemon juice, pickles, sourdough bread, etc., all contribute to lowering the GI of a meal.


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