The Glycemic Index (GI) is a way of rating carbohydrate foods according to how quickly the carbohydrates are broken down into glucose, and thus how quickly that glucose enters the bloodstream. The reasoning behind this is that carbohydrates that enter the bloodstream quickly raise blood sugar levels rapidly, causing a spike in energy, that is followed by a drop after the effects of insulin are triggered.
Insulin is one of the hormones that help regulate blood sugar levels, and tries to keep them stable. When too much glucose enters the bloodstream at a time, the body reacts by releasing insulin to remove some of that glucose back out of the blood and into our cells. Its simply a way of keeping the balance. However the effect is that when all the glucose we just ate, in the form of carbohydrates, is removed from the bloodstream, we feel tired and hungry, often craving more carbohydrates. Thus a cycle is created, where we eat more than really necessary.
Carbohydrate containing foods are rated from a scale of 1 to 100. A score of 100 is the highest, and this is akin to eating glucose in its pure form. A score of 55 or lower means a food is classified as having a low glycemic index. Thus, it theoretically breaks down in the body more slowly.
I write theoretically, because the picture is a little more complicated than that. For example, fat lowers the GI of foods. Potato crisps have a lower GI than do oven roasted jacket potatoes. However, that does not mean that potato crisps are a better choice, in terms of nutritional and fat content. So, it’s important to look at the whole equation when considering meal choices and the glycemic index.
Some suggestions for using the glycemic index in terms of one’s diet is to balance a high glycemic index food in a meal with a low one. And try to make more low or mid range GI choices than high.
Other factors that can affect a food’s glycemic index, beyond its GI rating, are the amount of food eaten. Chocolate has a low GI, but it is 30% fat. And any excess nutrients, whether they are fat, protein or carbohydrates, will be stored in the body as fat. So eating too much of low GI foods that are high in calories is not going to help with weight loss.
Another issue when considering using the GI of food, is that the time of day we eat a food may also impact its effect on blood sugar levels. This is because the GI rating given to a food is based on fasting. So, for example, we fast at night – meaning we are asleep and thus not eating for a period of hours. A food thus eaten in the morning may more accurately reflect the GI given to it than at other times of the day, when we haven’t been fasting.
Yet the GI does have benefits. One study on obese young adults found that a low glycemic index diet was associated with a reduction in the risk factors associated with heart disease, when compared to similar children with a low fat diet. Both groups lost weight, and kept it off, which is good news for dieters! And the researchers suggested that a low glycemic index diet may not lower metabolism as much compared to low fat diets. This is important for dieters as it means they would feel less cold, tired and hungry, and as a result, would find it easier to stick with the changes made during the diet period.
The GI should not be used in isolation. Both common sense and other food guidelines, like avoiding excess fat and salt,and making sure foods are full of vitamins, minerals and antioxidants, should still be used with the glycemic index.
1. Australian Healthy Food, November 2005