The GI has demonstrated that different types of carbohydrates (e.g.: glucose, sucrose, fructose, starches, etc.) do not have the same impact on a person’s glycemia.
The GI calculates the rise in blood glucose (glycemia) following the ingestion of 50 g of carbohydrates from a food, compared to the rise in blood glucose following the ingestion of 50 g of carbohydrates from a control food (glucose or white bread).
Foods are grouped into three categories, on a scale of 0 to 100 (the classification used by Diabetes Canada):
- Low GI: 55 or less
- Medium GI: 56 to 69
- High GI: 70 or more
According to scientific literature, replacing high-GI foods with low-GI foods would help:
- Reduce hypoglycemia events in people with type 1 diabetes
- Improve glycemic control and blood lipids in people with type 1 and type 2 diabetes
However, for many reasons, Diabetes Québec does not promote the use of the GI for these reasons:
1. The GI does not take the amounts normally eaten into account.
The GI only calculates the glycemic response of 50 g of carbohydrates from a specific food. In other words, to determine the GI of 50 g of carbohydrates from carrots, subjects had to eat 500 g of carrots (about 5 average carrots), an amount rarely eaten at one sitting.
2. The GI doesn’t take the proportion of carbohydrates in a food into account.
Given the high GI of carrots, you might believe you should avoid them. However, carbohydrates (including fibre) represent a small proportion – about 10% – of the total weight of a carrot.
3. The GI does not take nutritional quality of a food into account.
If you rely entirely on the GI, some high fat foods would appear to be better choices than lower fat foods (e.g.: GI of potato chips = 75; GI of a baked potato = 93)!
Low GI foods are not always the healthiest choice.
4. The GI is an imprecise calculation.
Not only does the methodology and choice of control food often vary from study to study, leading to great variations in the GI value for the same food, but its calculation does not take into account a host of factors that can distort the results:
- The variety in a single food (thin linguini has a GI of 87; wide linguini, 68).
- The degree of ripeness of a food (the riper a fruit, the higher its GI).
- The way a food is processed (cooking food longer or mashing it into a purée increases its GI, whereas adding lemon or vinegar can lower it).
- The amount of fat, protein and fibre ingested at the same time (spaghetti on its own will not have the same GI as when it is combined with sauce and cheese).
- The individual him/herself (glycemic response depends on age, gender, amount of physical activity and insulin resistance).
An alternative: the Glycemic Load
Unlike the GI, the glycemic load (GL) takes into account the amount eaten and the proportion of carbohydrates contained in a food.
The GL is calculated by multiplying the GI of a food by the amount of carbohydrates in a given serving size, divided by 100.
For example:
| ½ cup of : | Carbohydrates (g) | Glycemic Index | Glycemic Load |
|---|---|---|---|
| Cooked carrots | 7 | 92 | 6 |
| Cooked white rice | 23 | 56 | 13 |
| Conclusion: | Would lead you to believe that carrots will raise your glycemia more than rice. | Rice will raise your glycemia more than carrots. |
The glycemic load helps you realize that it is both the quality and the quantity of the foods you eat that affect your glycemia. There is no reason to eliminate carrots from your diet because of its high GI.
Remember: the best way to determine your own glycemic response to a food or a meal is to check your blood glucose (sugar) levels before and two hours after eating.
Key points
Numerous factors affect GI values and complicate its use. However, although the glycemic index and glycemic load do not replace basic advice for a healthy and balanced diet, they can be useful tools to guide you in your food choices, in conjunction with your meal plan and counting carbohydrates.
Nutritional recommendations advising people with diabetes to increase their intake of fibre, to distribute their intake of carbohydrates throughout the day and to avoid concentrated sugars are still accurate. If you opt for foods that are high in fibre, know that they generally have a lower GI.