Have you ever noticed how differently the same quantity of potatoes affects blood sugar depending on if your child eats them mashed or whole? Although the carbohydrate units are the same, the blood sugar rises much faster after consuming the mashed potato.

How is this possible? The answer to this question lies in the glycaemic index, which is calculated according to the type of effect carbohydrates have on the blood sugar levels. This method is not perfect, but can be very useful. How does it work?
 
In diabetes therapy, the glycaemic index (GI) has long been a familiar concept. But is it still used today?

The GI: What is It?
The GI categorizes foods containing carbohydrate according to how they affect the blood glucose levels. It is given as a percentage. Fifty grams of carbohydrate is the standard quantity, with dextrose being the reference food, which has a GI of 100%.

The GI of any test food can be calculated by getting subjects to consume 50 g of the very same carbohydrate and measuring the average blood glucose in the following two hours.

The higher the GI of any given food, the higher and more rapid the blood sugar will surge after it is consumed. With a low GI, the carbohydrate is absorbed more slowly and raises blood sugar only minimally.  Foods such as meat, fish, fats and oils are not considered part of the calculation, as they contain next to no carbohydrate.

Three GI Categories
GI values are usually categorized into the following three groups (as percentages):

• High (70–100): white bread, pretzels, cookies, sweets, baked potatoes
  
• Medium (55–70): rice, brown bread, bananas, table sugar, icecream
  
• Low (<55): vegetables, legumes, apples, milk, whole grain products, natural yoghurt.

GI and Insulin
The manner in which the blood sugar rises after a meal depends upon the type of food consumed.

This determines what kind of insulin and at what dosage people with diabetes need to use. Foods with a low GI require less insulin. Hence, these types of foods are preferable. Moreover, foods with low GIs tend to be less fattening and are therefore helpful for people trying to lose weight. Recent studies have shown that those who use the GI method to determine what they consume, indeed require less insulin. Also, foods with a low GI tend to be more satisfying and ward off hunger.

What Factors Influence the GI?
It is important to take into consideration that the GI is only a measurement of one particular food eaten alone. It is unsuitable for food combinations. Meals, for example, usually consist of a variety of foods. The combination of carbohydrate, fat and protein within one meal influences the GI of each food separately.

Factors that can influence the GI of the same foodstuffs are:

• Constitution: The strain, maturity and the conditions of origin vary considerably within one type of food. Different kinds of rice, for example, have very different GIs.
• Degree of processing and method of preparation is decisive—is it raw, pickled, or cooked? The manner in which a foodstuff is prepared makes a huge difference to its GI.
• Speed of gastric emptying: The faster it takes a foodstuff to pass from the stomach into the intestines, the higher its GI. This is why the manner in which it is prepared—juiced, whole, or mashed—is so decisive. Fluids, for example, are absorbed much faster than whole foods. Hence, it is better to eat a whole apple than to drink apple juice.
• Manner of consumption: How often and how fast you eat also influences the blood sugar curve.
• Fat and fibre content: The more fat and fibre a food contains, the longer it takes for the blood sugar levels to rise. It is, therefore, important to eat a lot of fibre and the right types of fat.
• The individual: The blood sugar of two individuals can react very differently to the same portion of the same food. Even the way the blood sugar reacts from day to day can differ in the same very individual.

Keeping Track of the GI
As seen from the discussion above, the GI can only be used as a guideline due to the numerous factors that can influence its value. Therefore, it is difficult to calculate the GI of every food you eat, and to use the GI values alone to predict the subsequent blood sugar reaction—especially when eating meals with various food combinations.

Don’t Ignore the GI
All that being said, don’t lose sight of the GI completely. Here are some general dietary suggestions stemming from its use:

• Eat lots of vegetables, salads, legumes and fresh fruit.
  
• Eat fibrous and wholemeal foods.
  
• Avoid eating bread, rolls, and cookies, that have been made with highly processed white flour. Rice and noodles should be wholemeal.
  
• Avoid sweets.
  
• Avoid sugary drinks.

The above points follow the guidelines of the German Association for Nutrition (DGE), who recommend eating wholemeal foods. Whoever eats according to these suggestions, will automatically avoid eating too many foods with a high GI, and will enjoy sound health, without having to use calculations.

Nevertheless, if you do want to follow the GI concept, you need to learn to exchange foodstuffs with high GIs for foodstuffs with low GIs. There are many ways to do this, as can be seen in the table.

Cathrin Guntermann
Diplom-Ökotrophologin
Kinderkrankenhaus auf der Bult, Hannover
E-Mail: adipositas(at)hka(dot)de