Glucose, commonly known as blood sugar is a monosaccharide, which means it is chemically the simplest form of sugar. Other monosaccharides include fructose and galactose. We then have disaccharides including lactose, which are two sugar molecules, and polysaccharides which are three or more sugar molecules. Both of which break down into monosaccharides during metabolism.
When we eat food, it goes down into our stomach, where many digestive enzymes start to break it down. After a while, the partially broken-down food travels to the small intestine where it is further broken down. One of the molecules carbohydrates are broken down into is glucose. From the small intestine, glucose is absorbed into the bloodstream. As glucose is absorbed into the bloodstream, blood glucose levels rise. The glycaemic response refers to how quickly this process of digestion, absorption, and raising of blood sugar levels occurs.
Low and High GI
Foods that undergo the above process quicker and hence cause a rapid rise in blood sugar levels are deemed a high glycaemic index (GI) food, and those that undergo this process slower and cause a more progressive increase in blood sugar levels, are deemed a lower GI food. The glycaemic index is a scale ranging from 0-100 with pure glucose sitting at 100. On this scale, a food is considered high GI if it has a GI of 75 or above, medium if a GI of 56-69 and low if 55 or less.
Low GI foods demand the body’s digestive enzymes to work harder for a longer duration in order to break down the food into glucose and then allow the glucose to reach the bloodstream. Because these foods provide a more sustained trickle of glucose into the blood, they typically keep one fuller for longer.
The reason this process takes longer for some foods over others e.g. multigrain bread over white bread as explained by The Glycaemic Index Foundation is that they might have more fibre which resists the action of digestive enzymes, or during food production, they are not as processed as other carbohydrate-containing foods. As a result of the food not having undergone much food processing and refining prior to us eating it, the body needs to work harder and for longer to break down all components of the food including the carbohydrate, to transform it into glucose.
Examples of High and Low GI Foods
High GI foods include white pasta, white bread, grapes, watermelon, cakes, biscuits, lollies, soft drinks and fruit juice.
Low GI foods include multigrain and brown bread, brown pasta, corn, berries, rolled oats, sweet potato and chickpeas.
Fun fact #1: Cooking methods can also affect the GI of a food. For example, pasta that is cooked el dente has a low GI whereas pasta that is overcooked has a higher GI.
Fun fact #2: The more fat and protein the meal contains, the lower the GI of the meal. This is because both fat and protein delay gastric emptying, so the contents of the stomach (including the carbohydrate portion of the meal) are metabolised slower. For example, spaghetti Bolognese made with well-cooked white pasta is actually a low GI meal because the fat and protein released from the meat come into play and affect the glycaemic response.
The above image illustrates the differing glycaemic responses of high and low GI foods. As depicted, when glucose from a high GI food is absorbed into the bloodstream, it produces a rapid rise in blood glucose levels, peaks, and then falls soon after. Glucose from low GI foods arrives in the bloodstream more progressively and is, therefore, a more prolonged release of energy. It does not have as high of a peak and it drops less drastically.
What Is the Glycaemic Load and How Is It Worked Out?
Glycaemic load is worked out by combining both the quality and quantity of the carbohydrate-containing food and dividing this value by 100. The quality is the GI number and the quantity being how many grams of carbohydrate the food contains. According to the Glycaemic Index Foundation, an apple has a GI of 38 and contains 13g of carbohydrate. So its glycaemic load would be 38 x 13 ÷ 100 which equals 4.94. However, the closest rounded number, in this case, 5, is the assumed value.
Glycaemic Load Values
The Glycaemic Index Foundation states that a food with a glycaemic load of 10 or less is considered low, 11-19 is considered medium, and 20 or more is considered high. They believe that for optimal health, one should aim to have a daily glycaemic load of under 100.
The Interplay Between Glycaemic Index and Glycaemic Load
However, just considering one’s daily glycaemic load as a measure of health is not advised because it does not take into consideration one’s overall nutritional intake. Yes, you may be eating mostly complex, high fibre carbohydrate foods which is a good thing, however, you may also be consuming some discretionary foods which contain unhealthy fats, and you may not be eating enough protein or vegetables which provide you with your micronutrients. Whilst the glycaemic load is an informative tool, the glycaemic index of foods and their overall nutritional profile should be a top priority.
A particular food may have a low glycaemic load, but it could have a very high glycaemic index. For example, watermelon has a high GI of 80 however due to the typical serving size that individuals consume, its glycaemic load is only 5.
Harvard Medical School constructed a useful table detailing common foods and their pertaining glycaemic indexes and glycaemic loads per serve.
What If I Have Diabetes?
This concept is particularly important for people with diabetes to be aware of. Using the watermelon example, although the glycaemic load is low, watermelon is still a high GI food. Because people with diabetes may count their carbohydrate intake at meals and snacks and match it to their insulin intakes, it is important for them to understand the glycaemic response of high and low GI foods.
What If I Do Not Have Diabetes?
Following a low GI diet is a healthy way of eating for anybody, not only those with diabetes. It can assists in weight loss but more importantly in healthy weight maintenance as one is not left feeling hungry soon after a meal, as is usually the case after consuming higher GI foods. This in turn often leads to reduced total calorie intake.
Lauri is a student dietitian anticipating graduation in September of this year. Lauri is passionate about health and fitness and strives to embody the values of: everything in moderation, positive body image, lifestyle eating not dieting, as well as eradicating the war on carbs.