It compares foods gram for gram of carbohydrate. Carbohydrates that breakdown quickly during digestion have the highest glycemic indexes.
The blood glucose response is fast and high. Carbohydrates that break down slowly, releasing glucose gradually into the blood stream, have low glycemic indexes.
Foods with a high glycemic index convert into sugar very quickly, with negative physical effects. Foods with a low glycemic index turn into sugar gradually, helping maintain your body’s chemical balance. In general, foods with a low index are preferable.
Glycemic Load measures the amount of sugar a food actually releases in the body. Foods with a low glycemic load usually have a low glycemic index, yet still have a low glycemic load.
Other foods have both a high index and a high load. You should avoid high load foods as a regular part of your meal plan.
When you choose carbohydrate foods, check both their glycemic index and glycemic load. Detailed tables with this information are widely available. Use the chart below to get started.
High Glycemic Index
- Fruits and Vegetables
- Cranberry juice
- Orange juice*
- Bread (white)
- Refined cereal
- Tortilla (flour)
Medium Glycemic Index
- Fruits and Vegetables Starches
- French Fries
- Pita Bread
Low Glycemic Index
Fruits and Vegetables
- Brussels sprout*
- Green Bean*
- Green pepper*
* * Low glycemic load foods.
Simply eating more fruits and vegetables is not the answer – they must be the right fruits and vegetables.
Starchy vegetables such as peas or lentils (200 to 250 calories per cup) are healthy, but they contain more calories than you may want. If you need to eat more to satisfy your hunger, add low glycemic load vegetables.
For example, spinach and asparagus are better choices than higher calorie corn and peas. A cup of spinach topped with 1/2 cup of tomato sauce has only about 90 calories, but it gives you nutrients from two colour groups.
Why Not Brown and Beige?
When considering which foods to enjoy sparingly, also use colour as a guideline. Many brown and beige carbohydrates, like pasta, beans and potatoes, while healthy, also tend to be high in calories.
How much carbohydrate you eat is important
The amount of the carbohydrate-containing food you eat will also affect your blood glucose levels. For example, even though pasta has a low GI.
It is not advisable for people with diabetes or impaired glucose tolerance to have a large serve.
This is because the total amount of carbohydrate, and therefore the kilojoules, will be too high.
The glycaemic load (GL) is a concept that builds on GI, as it takes into account both the GI of the food and the amount of carbohydrate in a portion.
GL is based on the idea that a high GI food consumed in small quantities would give the same effect on blood glucose levels as larger quantities of a low GI food.
GL is easily calculated by multiplying the GI by the amount of carbohydrates (in grams) in a serving of food.
GI and weight loss
When high and low GI diets are compared head-to-head, however, scientific evidence has shown that there is little additional benefit for weight loss of a low GI diet over a similar diet of nutrient composition that is high GI.
While GI can be a useful guide in planning a diet and controlling blood sugar levels, it should not be the only consideration.
Both the serving size of foods and the nutritional quality of the diet are just as important to consider.
GI and exercise
Eating low GI foods two hours before endurance events, such as long-distance running, may improve exercise capacity.
It is thought that the meal will have left your stomach before you start the event, but remains in your small intestine releasing energy for a few hours afterwards.
On the other hand, high GI foods are recommended during the first 24 hours of recovery after an event to rapidly replenish muscle fuel stores (glycogen).
Factors that affect the GI of a food
Factors such as the size, texture, viscosity (internal friction or ‘thickness’) and ripeness of a food affect its GI. For instance, an unripe banana may have a GI of 30, while a ripe banana has a GI of 51. Both ripe and unripe bananas have a low GI.
Fat, protein, soluble fibre, fructose (a carbohydrate found in fruit) and lactose (the carbohydrate in milk) also generally lower a food’s glycaemic response. Fat and acid foods (like vinegar, lemon juice or acidic fruit) slow the rate at which the stomach empties and slow the rate of digestion, resulting in a lower GI.
Other factors present in food, such as phytates (used to store phosphorus in plants) in wholegrain breads and cereals, may also delay a food’s absorption and thus lower the GI.
Cooking and processing can also affect the GI – food that is broken down into fine or smaller particles will be more easily absorbed and so has a higher GI.
Foods that have been cooked and allowed to cool (potatoes, for example) can have a lower GI when eaten cold than when hot.
GI symbol on packaged foods
Food Standards Australia New Zealand allows companies to make nutrient content claims regarding the GI of a food.
The GI symbol, G – Glycemic index tested, indicates the GI rating of packaged food products in supermarkets. It ranks food products based on the speed at which they break down from carbohydrate to sugar in the bloodstream.
However, this labelling is not compulsory for food companies to follow.
The GI symbol only appears on food products that meet certain nutrient criteria for that food category. High and intermediate GI soft drinks, cordials, syrups, confectionery and sugars are excluded.
Tips for healthy eating
Some practical suggestions include:
- Use a breakfast cereal based on oats, barley or bran.
- Use grainy breads or breads with soy.
- Enjoy a range of fruit and vegetables.
- Eat plenty of salad vegetables with vinaigrette dressing.
- Eat a variety of carbohydrate-containing foods. If the main sources of carbohydrates in your diet are bread and potatoes, then try lentils, legumes, pasta, basmati rice and pita breads.
- Focus more on the serving size of foods, rather than just their GI rating.