Counting carbs is not just a matter of adding two and two together. Other factors also have to be taken into consideration, such as how fast the carbohydrate is digested, and how much it causes your blood sugar levels to rise.

The glycemic index is a system that takes these factors into account when describing or classifying a carbohydrate food. A food with a low-GI rating will cause a small, slow rise in blood sugar levels, while a high GI food will cause a fast and dramatic spike. The GI rating of a food is based on glucose – the fastest releasing carbohydrate – having a rating of 100 . A food that releases glucose at half the rate of pure glucose has a GI of 50; a food with a quarter the rate of glucose release has a GI of 25, and so on.

High GI foods, such as white bread, potatoes, white rice, and honey have a GI of 70 or more. Intermediate GI foods, such as ice cream, sugar, and orange juice have a GI between 56 and 69. Low GI foods, including mixed-grain breads, legumes, milk and yogurt, and most fruits have a GI of 55 or less.

The glycemic load (GL) goes a step further than the GI by taking into account the amount of carbohydrate in a food. A weak point of the GI is that it fails to do this.

For example, carrots have a high GI of 47, but you have to eat a pound and a half of them for there to be a steep rise in blood sugar. Even a rabbit won’t eat that many carrots in one sitting! Because carrots have a high GI number, it seems like they are a food to avoid, whereas in fact they are full of excellent nutrients and, when eaten in normal proportions, are unlikely to cause a dramatic influx of blood sugar levels. The GL provides a more practical way of evaluating the effect of carbohydrates on blood sugar by combining both quantity and quality of carbohydrate into one number. According to the GL system, therefore, carrots are given a relatively low rating of 3.

A GL of 20 or more is high, a GL of 11-19 is medium, and a GL of 10 or less is low. Almost all foods with a low GI will also have a low GL, but foods with an intermediate or high GI often have a low GL.

GL is calculated by dividing the GI of a food by 100 and then multiplying by the food’s available carbohydrate (i.e. not including fiber) in grams. For example, the GI of an apple is 38 and its carbohydrate content is 16. Therefore: 0.38 X 16 = 6.08. So an apple has a GL of around 6.