Q. Do I have to give up high-GI foods altogether?
Of course not! While many people may benefit from eating carbohydrates with a low GI value at each meal, this doesn't mean consuming them to the exclusion of all other carbohydrates. All foods fit into a healthy diet, in moderation. And some high-GI foods make a valuable contribution to your diet. Mashed potatoes and white bread, for example, contribute energy and important nutrients, such as vitamin C, potassium, iron and a number of B vitamins.
Furthermore, when you eat a combination of low- and high-GI foods at a meal, such as peanut butter on whole wheat toast, or rice and lentils, the final GI value of the meal is medium. Base your food choices on the overall nutritional content in order to reap the nutritional benefits of many different foods, and try to eat a variety of foods from all four food groups in Canada's Food Guide to Healthy Eating every day.
Remember, the GI shouldn't be your only criterion for making healthy food choices. Healthy eating also means eating healthy portion sizes at regular times throughout the day, limiting sugars and sweets, eating foods that are high in fibre, and limiting salt, alcohol and caffeine. You also need to make sure you get enough healthy monounsaturated and polyunsaturated fats and reduce the amount of saturated and trans fats in your diet.
Q. Does the GI increase with serving size?
No. The GI value of a carbohydrate-containing food remains the same even if you increase the amount you consume. On the other hand, if you eat more than one serving of the food, your blood sugar level will reach a higher peak and take longer to return to normal than it would if you ate a normal-size serving.
Q. What are some of the factors that influence GI values?
Type of starch. There are two types of starch in foods: amylose and amylopectin. Amylose has a lower GI rating because the molecules form tight clumps and are harder to digest. Amylopectin has a higher GI rating because the molecules are more open and are easier to digest. The more amylose starch a food contains, the lower the GI value of that food. For example, pasta, parboiled rice and many varieties of beans are higher in amylose starch and therefore have a low GI.
Cooking or processing starch. When a food is highly processed or cooked for a long time, the structure of the starch changes and the granules become swollen (gelatinized), softening the food and making it quicker to digest. The more quickly a food is digested, the higher the GI value. Less-gelatinized starch is digested more slowly, resulting in a lower GI. For example, al dente pasta has a lower GI than overcooked pasta.
Acids. Acids in foods slow down stomach emptying, thereby slowing the rate at which the food is broken down and lowering its GI value. Vinegar, lemon juice, lime juice, salad dressings, grapefruit, oranges, pickled vegetables and sourdough bread are good examples of acidic foods. Adding acidic foods to high-GI foods will result in an overall lower GI value.
Dietary fibre. Soluble fibre, such as that found in large-flake rolled oats, beans and apples, slows down digestion and lowers a food's GI.
Protein and fat. These slow down the rate of stomach emptying and carbohydrate digestion, which lowers the GI of the food. Some high-fat foods have a low GI and may seem like a good choice, but if the fat is saturated or trans fat, it may increase your risk of heart disease. The best advice is to look for foods that have a low GI and are low in saturated and trans fats. For example, chocolate cake with chocolate frosting has a lower GI than angel food cake, but is much higher in fat, thanks to the buttery frosting.
Sugar. Sugar helps prevent the swelling (gelatinization) of starch granules. Less-gelatinized starch is digested more slowly, resulting in a lower GI. This helps explain why some cookies and frosted breakfast cereals have low GI values despite their high sugar content -- and not necessarily high nutritional values.
