Is it 100% effective?

Yes and no. The glycaemic index of a food isn't fixed. It can vary depending on the way it's prepared, cooked, heated, reheated and chilled. For example, mashed potatoes have a higher glycaemic index than boiled potatoes, and the same applies to cooked carrots which have a higher GI than raw ones.

It also varies according to the other foods you eat, i.e. all the ingredients in a particular dish or everything that's eaten during a meal. For example, the glycaemic index of glucose is reduced if it's eaten with fat.

It also depends on each individual's activity levels, muscular mass and how their pancreas functions.

Additionally, it's important not to just rely on the index but on the overall characteristics of a food: certain foods have a low GI (chocolate bars, crisps, biscuits etc) but are high in fat, and you shouldn't have too much of them; others may have a high glycaemic index but contain only a tiny amount of carbohydrate (such as carrots).

Our conclusion: make use of the GI by all means, but know how to use it wisely as part of a balanced diet.