In my last article I discussed the different types of carbohydrates and the way that your body uses them. At the end of that article I mentioned that you should choose slow digesting, nutritionally dense carbohydrates where possible. However, sometimes it can be quite difficult to determine which carbohydrates these are. That is why today I am going to be writing about one of the methods you can use to choose the best carbohydrates – the glycemic index (GI).
WHAT IS THE GLYCEMIC INDEX?
The glycemic index is a system which ranks foods based on how they affect blood glucose levels. It is based on research where a selection of healthy individuals were given a variety of carbohydrates to eat. Their blood glucose levels were then measured every 15 minutes to see how fast and high they increased. Finally, these changes in blood glucose levels were given a GI of between 1 and 100 with 1 being the lowest, slowest increase and 100 being the highest, fastest increase. Based on this research the following conclusions were made:
– Low GI Foods = Foods with a GI below 55.
– Medium GI Foods = Foods with a GI between 55 and 70.
– High GI Foods = Foods with a GI above 70.
WHAT IS THE GLYCEMIC INDEX FOR EACH FOOD TYPE?
The glycemic index varies greatly by food type. I have provided a summary of some of the main food groups below but you can find a full list by clicking here.
BREADS:- The GI of breads varies greatly based on the ingredients used and can range from 42 to 73. Higher fibre breads generally having a lower ranking.
DAIRY:- Dairy products generally have a low GI because of their high fat content. For example, milk has an average ranking of 27 whilst yoghurt averages at around 18.
FRUITS:- The GI of fruits is linked to its carbohydrate content with certain fruits being highly concentrated and others being less so. For example, apples have an average ranking of 38 which is relatively low and reflects their high fibre and water content. Contrastingly, raisins which are highly concentrated and contain a lot of sugars have an average ranking of 64.
NUTS:- Nuts have a very low GI due to their high fat content. For example, cashew nuts have an average ranking of 22 and peanuts have an average ranking of 14.
PASTA:- The GI of pasta can range from 26 to 92 depending upon its fibre content, its shape and how quickly it is cooked. Longer cooking times increase pasta’s GI with harder pasta having a low ranking and softer pasta having a high ranking.
SUGARS:- Sugars normally have a high GI because they are generally digested and absorbed into the bloodstream very quickly. For example, glucose is ranked 100 whilst sucrose is ranked 68.
VEGETABLES:- Most none starchy vegetables cause little or no increase in blood sugar due to their high fibre and extremely low carbohydrate content. For example, broccoli has an average ranking of 0. However, vegetables that are richer in carbohydrates have a much higher GI with potatoes having an average ranking of 80.
WHAT ARE THE BENEFITS OF THE GLYCEMIC INDEX?
The idea behind the glycemic index is that you should choose foods with a low GI where possible. This helps keep your blood glucose levels stable and reduces large blood glucose surges which have a number of complications including poor appetite control and organ damage. Eating foods with a low GI is also thought to protect you from type 2 diabetes and heart disease.
WHAT ARE THE PROBLEMS WITH THE GLYCEMIC INDEX?
1) LIMITED SELECTION OF FOODS TESTED:- As I said at the beginning of this article GI values are based on research where healthy individuals were given a selection of carbohydrates to eat and then their blood glucose response was measured. However, it is not practical to test certain foods and because of this they cannot be given a GI. According to NutritionData.com only 5% of the foods in their database have GI values.
2) THE GLYCEMIC INDEX CAN BE DIFFERENT FOR THE SAME FOODS:- Certain foods can have multiple GIs with cooking times, the method of preparation, ripeness, individual differences in blood glucose responses and even the research source all having an effect. For example, unripe bananas have an average ranking of 30 whilst overripe bananas have an average ranking of 52. Hard pasta which has been cooked quickly has a relatively low GI ranking whilst slower cooked soft pasta has a much higher ranking. Peaches have a different GI depending upon the research source you choose with one study suggesting an average ranking of 28 and another study suggesting an average ranking of 56. Furthermore, everyone’s blood glucose response to food is different so a slice of bread may have a different GI for you than it does for other people.
3) THE GLYCEMIC INDEX ASSUMES THAT WE ONLY EAT ONE FOOD AT A TIME:- GI rankings are specific to each food type. Whilst they can be used when you eat one food individually there is no accurate way to calculate the GI ranking of a meal that contains a combination of foods. For example, we know potatoes have an average GI ranking of 80 but if we add some cheese (which has a much lower GI because of the high fat content) the ranking of this meal changes. If we add some other foods to the meal it changes again. The only way to accurately determine the GI of a meal is to test our own individual blood glucose response to it.
4) THE GLYCEMIC INDEX DOES NOT TAKE SERVING SIZE INTO ACCOUNT:- GI rankings are based on a standard 50g serving of carbohydrates. They do not consider that you may only consume low amounts of high GI foods (which will result in a smaller blood glucose increase) or large amounts of low GI foods (which will result in a greater blood glucose increase).
However, this problem can be addressed with the glycemic load (GL) which takes serving size into account. To calculate the glycemic load of a food you take its glycemic index, divide it by 100 and then multiply it by the net carbohydrates in the serving (total carbohydrates minus dietary fibre). So for example if you ate a 100g unripe banana which had a GI of 30 and contained 15g of dietary fibre the glycemic load would be 25.5 = (30/100 * (100-15)).
Like the GI, the GL breaks foods down into three bands:
– Low GL Foods = Foods with a GL below 11.
– Medium GL Foods = Foods with a GL between 11 and 19.
– High GL Foods = Foods with a GL above 19.
GLYCEMIC INDEX SUMMARY
The glycemic index is a flawed tool but it does have some value when choosing your carbohydrates. Selecting low GI carbohydrates can help stabilise your energy levels, improve your appetite control and prevent cell damage. The glycemic index also promotes some healthy food choices such as steering people away from sugars and towards high fibre carbohydrates which have a number of benefits. However, it can potentially promote the over consumption of fatty products because they generally have a low GI. Furthermore, there are a number of factors that can change the GI of foods making it a difficult tool to base your diet around.
Overall, the GI is definitely a good starting point when it comes to choosing your carbohydrates. Whilst it is limited and has a number of problems it can be a useful indicator of both good and bad carbohydrate choices.
What do you guys think? Is the GI a useful tool despite its flaws? Are there any better methods you know of for choosing your carbohydrates? Leave a comment and let me know your thoughts.
Sources:
Glycemic Index (NutritionData.com)
Glycemic Index List (About.com)
What is the Glycemic Index (About.com)
Understanding the Glycemic Index (How Stuff Works)