The Glycemic Index- How Important Is It ?

Copyright (c) 2009 Stephen Smith

It was previously thought and to a large extent still is, that simple carbohydrates are absorbed and utilised quickly in the body and complex carbohydrates are absorbed and utilised slowly. This belief originated from the fact that simple carbohydrates are individual molecules or two molecules bonded together whilst complex carbohydrates are made up of many molecules bonded together.

However, in 1981, Dr David Jenkins (a professor at the University of Toronto) found that this was not actually true. At the time, Dr Jenkins was examining the best foods for diabetics and from his research he discovered that certain complex carbohydrates, like potatoes, were causing rapid rises in blood glucose, whilst some simple carbohydrates, like certain fruits, were being utlised slowly.

Since many carbohydrate-containing foods weren't acting as expected, the Glycemic Index (GI) was devised. The GI indicates how rapidly a food increases the blood glucose level. If a food causes a rapid rise in blood glucose, then it is referred to as a high glycemic index food and is given a value between 70 and 100 depending on how rapidly the blood glucose level increases.

Glucose (GI of 100) is the standard and all other carbohydrate-containing foods are compared to it. If a carbohydrate-containing food causes a moderate or slow increase in blood glucose, then it is given a number that corresponds to the rate (moderate GI: 50-69 and low GI: less than 50).

A slow passage of glucose into the bloodstream means that it is easier to maintain a stable blood glucose level, and in turn, keep insulin to a minimum. This situation is ideal for fat loss because a high level of insulin promotes fat storage, increases the conversion of carbohydrate into fat and inhibits the use of fat for fuel. As a result, it would appear that low GI carbohydrates should be emphasised in the diet. However, as we will examine in a moment, this is not the most important factor that needs to be considered.

A number of factors affect the GI (its absorption rate) of a carbohydrate-containing food. They are the following:

- Its chemical composition (Is it composed of glucose, fructose or galactose molecules?)

- The amount of fibre (particularly soluble fibre), protein or fat contained in the food

- How the food is cooked/ prepared

Composition

Carbohydrate-containing foods that are composed mainly of glucose molecules (including complex carbohydrates) and that have little fibre, protein or fat in them have a high glycemic index. For example, glucose, malt, bread, potatoes and white rice all have high glycemic indexes and cause a rapid rise in blood glucose and therefore insulin levels when they are consumed on their own. Since they promote the production of insulin, the carbohydrate in them has a greater likelihood of being converted into body fat.

Fibre and protein

Fibre, particularly soluble fibre, is a component of food that has the effect of slowing down the absorption rate of carbohydrate and therefore lowering its GI. For example, an apple has a much lower GI than apple juice simply because there is fibre in the apple and the apple juice has had its fibre component removed. Fibre has a tendency to 'dilute' the carbohydrate, which slows absorption. Protein may have a similar 'diluting' effect on carbohydrate.

Fat

Fat also lowers a food's glycemic index by slowing down its passage through the gastro-intestinal tract. This slows the absorption rate of carbohydrate. Good examples of this are chocolate and ice-cream. Both of these foods contain a considerable amount of fat along with carbohydrate and both have relatively low glycemic indexes. However, this certainly doesn't mean you should increase your intake of these foods because both are very 'calorie dense' due to the large amounts of fat and sugar contained in them. They are good examples, however, of how fat slows down the absorption rate of carbohydrate.

Cooking/ Preparation

How a carbohydrate-containing food is cooked or prepared also affects its absorption rate. Overcooking increases a carbohydrate-containing food's glycemic index because some of the bonds between the glucose molecules will be broken down, making absorption easier (less digestion is required). Similarly, preparation may also affect a carbohydrate-containing food's absorption rate. For example, mashed potatoes are absorbed faster into the bloodstream than whole potatoes. On the other hand, adding vinegar (acetic acid) to a food lowers its GI.

Now before you start thinking that your days of eating bread and potatoes are over, consider the fact that carbohydrate-rich foods with fibre, protein and/ or fat already in them are low on the glycemic index. Therefore, the glycemic index of high glycemic index carbohydrates may be lowered if fibre, protein or fat is added to a meal with them.

Therefore, if you add vegetables or salad (high fibre foods), meat (protein), oil (fat) or oil-containing foods (nuts, seeds or avocadoes) to the foods containing carbohydrate, the rate of absorption of the carbohydrate will be slowed down, limiting insulin secretion and in turn, limiting fat storage. So there is no problem in having high GI 'starchy' carbs as long as you slow down their absorption rate by adding lots of fibrous (containing fibre) vegetables or salad, protein or fat (preferably only the essential fatty acids) to the meal.

Remember though, it isn't just the rate of glucose entering the bloodstream that dictates how much will be converted into body fat but also the amount of carbohydrate consumed. Even if you have a slow influx of carbohydrate into the bloodstream by consuming low-glycemic index carbohydrate or high glycemic index carbohydrate with fibre, fat or protein, a large amount of carbohydrate in a meal will still need to be stored in the body and if the glycogen stores are full the only other storage site is in the adipocytes (fat cells).

Plus, a large meal will have a greater impact on insulin response than a small meal and since the main benefit of eating low GI foods is in limiting insulin response, it makes sense to reduce portion sizes of meals.

If you choose to emphasise low-glycemic index carbohydrate in your diet, then fine. However, don't neglect eating some complex carbohydrates as well. Complex carbohydrates provide your body with many essential nutrients. Pasta for example provides your body with 6 out of the 8 essential amino acids (in small amounts though) and some contain both B vitamins and iron.

The bottom line is this, if you are eating complete meals, which means meals with carbohydrate, protein and fat then the GI becomes irrelevant since the protein and fat as well as fibre will slow down the absorption rate of the carbohydrates, lowering the GI. As covered earlier, of far greater importance is the portion size of the entire meal and the carbohydrate-density of the food. Since insulin responses are important, portion size will have an impact on this and carbohydrate-density will affect overall calorie intake. Therefore, focusing more on these areas will offer greater benefits than simply the Glycemic Index alone.