The Glycemic Index (GI) ranks foods that contain carbohydrates based on the speed at which they raise blood glucose levels.
Carbohydrates cause blood glucose levels to rise, and the GI ranks them on a scale from 0 to 100 when that food is eaten on its own. The higher the value, the more rapid the rise in blood glucose.
The University of Sydney defines a low GI as 0 to 55, a medium GI as 55 to 70, and a high GI as 70 and above. Pure glucose is used as a reference food for the calculations, and is given a score of 100.
Low-GI foods are slowly broken down into glucose. They can help people to feel fuller for longer and aid weight loss because appetite is suppressed. Low-GI foods include some fruit and vegetables (such as grapefruit, apples, oranges, broccoli, cabbage and carrots), whole wheat bread and pulses.
High-GI foods break down into glucose quicker than low-GI foods. They are digested more rapidly, and can cause a person’s blood glucose levels to rise sharply (often referred to as blood sugar ‘spikes’). High-GI foods include sugary foods and drinks, white bread, white rice and potatoes.
Medium-GI foods don’t break down into glucose as quickly as high-GI foods, but they still have more of an effect on blood glucose levels than low-GI foods. Examples include basmati rice, couscous, bananas, mango and beetroot.
Although the GI was created to help people with diabetes manage their blood sugar levels, it can also be utilised to help achieve weight loss.
Why is the Glycemic Index important?
The GI is important because it acts as a reference point to help people with diabetes maintain good blood glucose levels.
Moreover, the GI can provide people with prediabetes useful information to prevent them developing type 2 diabetes later on.
By identifying which GI category certain foods fall under, people with diabetes can choose to avoid foods which will cause their blood glucose levels to rise rapidly.
High-GI foods can force the body to produce insulin to counter quick-acting carbohydrates, and this not only affects the regulation of blood sugar, but it can lead to people still feeling hungry afterwards.
Favouring low-GI foods can help you feel more satiated over a longer period of time, and less likely to feel hungry before your next meal.
How is the Glycemic Index measured?
At the time of writing, there are only a few centres around the world which provide legitimate testing services for measuring the GI of a food.  According to the University of Sydney, whose Human Nutrition Unit has played a major role in GI research, food can be measured on the GI by using this example:
Following an overnight (12 hour) fast, 10 healthy people are fed carefully measured portions, which each contain 50 grams of available carbohydrate – this means 50g of carbohydrate after fibre is excluded, as fibre is known as unavailable carbohydrate. If a food contains lower amounts of carbohydrate, then they are fed a portion that contains 25 grams of available carbohydrate, and the calculations are adjusted accordingly.
Over the next two hours, each person conducts a blood glucose test every 15-30 minutes. The blood samples produced indicate what effect the food has had, and are used to create a ‘blood glucose response curve’ for the two-hour period.
The incremental Area Under the Curve (iAUC) is then calculated. This represents the total rise in blood glucose levels following consumption of the test food.
Next, the iAUC for the test food is divided by the iAUC for the reference food (which is the same amount, of pure glucose), and then multiplied by 100.
The GI value given for each food will be the average of all 10 readings.
How Glycemic Load improves the Glycemic Index
The GI ranks carbs based on how high blood glucose levels rise, and for how long they remain high, based on the quality of the carbs.
Glycemic Load (GL) differs from this, in that it combines the quantity of carbs, as well as the quality, into one value. Put simply, it factors in portion size.
The GL is the most effective way to predict how your blood glucose will rise after eating certain amounts of certain foods. It is calculated as such:
GL [in grams] = (The amount of carbohydrate [in grams] x GI) divided by 100.
The University of Sydney defines a low GL as 0 to 10, a medium GL as 11 to 19, and a high GL as 20 and above.
It is important to remember that a range of factors should be taken into account when trying to work out how blood glucose will be affected by food. The NHS says that the amount of carbs eaten, rather than the GI rating, has a bigger impact on post-meal blood glucose levels. 
Where a food ranks on the GI does not signify whether or not it is healthy, though. For example, chocolate cake has a low GI, whereas watermelon has a high GI rating. It is possible that a low-GI diet is high in saturated fat, which is associated with high levels of LDL cholesterol, as well as obesity.
Overeating is a real possibility, as some foods with a low-GI and low-GL actually contain more calories than other foods. Therefore, you may end up eating too many calories and gaining weight.
Additionally, everyone is different, so there will be variations in blood glucose levels after consuming carbs. There may also be variations depending on the time of day, so it is not possible to rely completely on the GI and avoid measuring blood glucose levels.
Glycemic Index vs low-carb vs satiety
The GI is not the only way to limit your consumption of carbohydrates. Low-carb diets are proving increasingly popular as a way to control carbohydrate intake, and blood glucose levels. A low-carb diet focuses on ensuring you eat a healthy, balanced diet which contains no more than 150g of carbs each day, generally, to prevent blood glucose levels from spiking.
Low-carb diets can be a better choice than low-GI diets, which focus on the carb content of foods, but not the amount of food eaten. This is why portion control is important.
Satiety focuses on limiting the total number of calories you consume with each meal, which limits carb consumption at the same time. While this can help people manage their blood glucose levels, and lose body fat, it can also result in increased hunger.
Certain foods are better for satisfying hunger than others, however, and can be included in the diet to help counter this problem. Researchers at the University of Sydney, in Australia, created a Satiety Index, ranking foods from 0 to 100, to find out which foods would be best to include in our diets, to suppress hunger.
Lean meats, fish, raw fruits and plain, boiled potatoes were found to be high-satiety foods, while foods such as donuts, croissants and chocolate bars were found to be low-satiety foods.