Understanding the glycaemic index and how to use it
In recent years, GI — or Glycemic Index — has become a commonly used term. You hear about it on the radio, read about it in magazines and see snippets on TV. Many people are using the GI in their day-to-day life to try to lose weight, manage their diabetes, reduce the effects of polycystic ovary syndrome (PCOS) or simply have better energy throughout day.
The GI is a way of measuring the effect that foods containing carbohydrate have upon our blood sugar levels and therefore how our body is supplied with energy. Unfortunately, the GI alone doesn’t give a full picture of the overall effect of a foodstuff upon blood sugar levels because it doesn’t take into account the amount of carbohydrate in a meal — which is where the GL, or Glycemic Load, comes in.
The GL is a slightly newer scale than the GI and it provides a way of measuring the effect on blood sugar levels produced by a normal serving of the food.
It’s all about carbs
Carbohydrates are present in most of the foods we eat. Vegetables, fruit, dairy, nuts, seeds, lentils, chick peas, soy products, olives, avocado, pasta, bread, rice, couscous, cakes, biscuits, lollies, apple pie — all of these foods contain carbohydrates. In fact, the only foods that don’t contain carbohydrates are meat, fish and oils. Carbs also come in a variety of forms, most commonly sugars, fibres and starches.
Carbohydrates are important to us, as they are the primary source of energy for our bodies. They provide essential fuel to our brains and are the main source of fuel for our muscles. We use carbohydrate to fuel the functioning, movement and activity of our bodies. Every time a muscle contracts, every time your heart beats, every time you speak or smile or think about something, you’re using carbohydrates. So we all need them and we need them every day.
While this food group is an important part of a healthy diet, many people eat too much carbohydrate and this excess energy (kilojoules) can be converted to fat in the body. It’s also true that some carbohydrates are better for you than others.
Eating a diet too high in carbs and eating too many of the wrong kind can increase your risk of putting on weight, of becoming obese or developing diabetes.
Carbohydrates are the most widely consumed substance in the world, second only to water. Most cultures have carbohydrates as their staple foods, whether they be rice, corn in the form of polenta or maize, pasta, breads, potatoes or couscous. These are the foundation foods of the majority of the world’s population.
The basic building block of carbohydrate is a sugar molecule, which is a simple combination of carbon, hydrogen and oxygen atoms. Starches and fibres are made up of chains of sugar molecules, bound together in different ways and different combinations, with some carbohydrates containing hundreds of sugar molecules.
For us to make use of carbohydrate from the foods we eat, it first has to be broken down into a form that can be absorbed and used by our cells. This process of digestion and absorption takes the long chains of carbohydrate and breaks them down into glucose, a single sugar molecule small enough to cross into the bloodstream and be utilised by the body.
This digestive process starts in the mouth, where enzymes in saliva start breaking down starch molecules. Digestion and absorption are continued in the small intestines, where enzymes produced by the pancreas work to break down the remaining carbohydrate, again into simple glucose molecules.
The speed at which this occurs depends on the nature of the carbohydrate itself: the structure of the molecule, whether it’s bound up with fibre, how it exists in the foodstuff. This means that some carbohydrates break down really quickly and easily whereas others take a longer time, are much more difficult to break down and release their glucose molecules more slowly.
The role of insulin
Once the carbohydrate has been broken down and the glucose from it absorbed into the blood stream, your pancreas is stimulated to produce the hormone insulin. Insulin helps the glucose move from the blood stream into individual cells, where it is put to use in powering your body.
The amount of insulin released by your pancreas is dependent upon the amount of glucose in the blood stream. If there’s a lot of glucose in the blood stream, a lot of insulin has to be released by the pancreas to deal with it, whereas only small amounts of glucose means that only small amounts of insulin are needed.
The problems with too much insulin
There are problems with churning out too much insulin. Firstly, over long periods of time, this overstimulation can exhaust the poor pancreas, limiting its ability to function and leading to insulin resistance and type 2 diabetes. Even in the short term, high levels of insulin are problematic, affecting weight control, PCOS and increasing your risk of cardiovascular disease.
To prevent all the problems associated with excess insulin and prevent your pancreas from being overstimulated, it’s important to eat carbohydrates that are digested and absorbed slowly. As these foods break down slowly, the release of glucose into the blood occurs over a longer period of time and smaller quantities of insulin are needed. These foods also provide more sustained energy.
In the 1980s the Glycemic Index or GI was developed as a way of measuring the effect carbohydrates have on blood sugar levels. It’s a classification system, by which all foods containing carbohydrate are given a number according to their effect on blood sugar levels.
Foods with a low GI (below 55) have a lesser effect on blood sugar levels than foods with a high GI (above 70). The glucose from foods with a lower GI is released slowly and steadily over a longer period of time, providing us with a slow and steady supply of energy and ensuring that a lower amount of insulin is required, whereas foods with a high GI release their glucose very quickly.
The table below shows the GI of some common foods:
High GI (over 70)
white bread – 70
potatoes, boiled -88
jelly beans -78
jasmine rice -87
pumpkin – 75
Medium GI (56-69)
basmati rice – 58
beetroot, tinned -64
breakfast wheat biscuit cereal –68
Low GI (under 55)
Pasta – 38
grainy bread – 49
sweet potato – 44
carrots – 49
lentils & chick peas -29
skim milk – 32
apples and pears – 38
Limitations of the GI
While the GI is useful in assessing the effect a carbohydrate will have upon blood sugar levels, its limitation is that it doesn’t tell us anything about the amount of carbohydrate in that food. Carbohydrates are found in all sorts of different foods and, while the GI varies between these different foods, so too does the amount of carbohydrate contained in those foods.
For example, both potato and pumpkin contain carbohydrate and have a similar GI value (85 and 75 respectively). According to the GI, therefore, these two foods should have a similar effect on blood sugar levels and the release of insulin. However, potatoes and pumpkin contain different amounts of carbohydrate, which means that despite their similar GI they will have a different effect on blood sugar levels.
Another example is watermelon, which also has a high glycemic index (72). However, a slice of watermelon contains only a very small amount of carbohydrate — about 5 per cent — and eating that slice of watermelon isn’t going to have a large effect on blood sugar levels. Again, using the GI value alone as a gauge of which foods to eat would lead to you avoiding the wonderful juicy goodness of watermelon.
The GI of a food is based on how much and how rapidly a standard amount (usually 25 or 50g) of the carbohydrate from that food increases blood sugar levels over the course of two hours. This isn’t always an accurate reflection of how people eat that food in their everyday life, because different foods have different carbohydrate densities.
Referring back to the potato and pumpkin example, cooked potatoes have a GI of 85 and are about 12 per cent carbohydrate. In order to eat 25g of potato carbohydrate, you need therefore to consume just over 200g, the equivalent of two small potatoes, which is a realistic serving. However, pumpkin, which also has a high GI, has a lower carbohydrate content, about 5 per cent. To eat 25g of pumpkin carbohydrate you would need to eat 500g of actual pumpkin, roughly 2 cups. This is quite a lot of pumpkin and more than most people would consume as part of one meal.
So, without knowing both the GI and also the amount of carbohydrate in a foodstuff, you can’t really judge the effect that food will have on blood sugar levels. From the example above it can be seen that a serving of potato will have more impact on blood glucose levels than a serving of pumpkin. While the GI of potatoes is a good guide to its effect on blood glucose levels, the GI of pumpkin isn’t because, while the GI of pumpkin is high, it’s less carbohydrate dense.
The Glycemic Load, or GL, was developed to answer just this problem. The GL takes into account both the GI of a food and its carbohydrate content, thereby giving a fuller picture of the effect of that food on blood glucose levels.
The GL tends to be higher for those foods which provide the most carbohydrate, especially the ones we eat in larger amounts. A food’s glycemic load is calculated by multiplying its glycemic index by the amount of carbohydrate it contains (and dividing by 100). While the GI of watermelon may be 72, its GL is 4 because of its low carbohydrate content; whereas potato has a GI of 85 and a GL of 26, reflecting both its high GI and comparative carbohydrate density.
The GL is an improvement on the GI because it provides an estimation of both quality and quantity of carbohydrate. A list of low, medium and high GL foods is below:
Low GL (up to about 10)
High-fibre fruits and vegetables, excluding potatoes
High-fibre/bran cereals (30-40g)
Legumes ¾ cup cooked (chick peas, kidney beans, black beans, lentils)
Dried apricots (about 10)
Low fat yoghurt (200g)
Medium GL (about 10-20)
Pearled barley, 1 cup cooked
Brown rice, ¾ cup cooked
Oats, 1 cup cooked
Bulgar wheat, ¾ cup cooked
Rice cakes, 3 cakes
Whole grain breads, 1 slice
Whole-grain pasta, 1¼ cup cooked
No-sugar-added fruit juices, 1 cup/250ml
High GL (above 20)
Refined breakfast cereals, 30g
Sugar-sweetened soft drinks, 375ml
Jelly beans, 30 small
Couscous, 1 cup cooked
White basmati rice, 1 cup cooked
White-flour pasta, 1¼ cup cooked
At the moment there is no indication on food labels whether foods have a low, medium or high GL. However, there are a number of books, including pocket shopping guides, which you can use to look up the GL of the foods you’re eating. The University of Sydney also has a searchable online database that gives the GI and GL levels of a wide range of foods (address at the end of this article).
Limitations of the GL
The GL is a really useful tool for assessing carbohydrates and the effect they are going to have upon blood glucose levels and the corresponding amount of insulin released by your pancreas. It’s a more comprehensive tool than just using the GI by itself, as it takes into account the carbohydrate density of the food.
However, the GL, like the GI, cannot be used in isolation and it’s not the only tool you need to assess the healthiness (or otherwise) of a foodstuff. While the GL gives an indication of the value of the carbohydrate in a particular food, it is part of but not the whole picture. The GL tells you nothing about protein or fats and zilch about vitamins and minerals — all important nutrients in our daily diet.
It’s getting complicated!
All this GI/GL stuff can seem very complicated. There’s high GI, low GL, different numbers, values, ranges of what’s high and what’s low — and now I’m telling you the GL is limited. It’s tempting just to give up and carry on eating the way you are at the moment.
But there are some simple guidelines that will work for most people, ensuring you’re eating low GL, regulating your insulin blood glucose levels and also eating well. If your current diet doesn’t look like this, try to change it over gradually to these guidelines:
- Try to eat 5 servings of vegies and 2 of fruit every day
- Fruit and vegetables are important because they provide us with much needed fibre, vitamins, minerals and antioxidants and most fruit and vegetables (except potatoes) have a low GL. We need 5 + 2 to make sure we’re getting all the micronutrients we need. Add salad to your sandwich at lunchtime, have some steamed vegies or a salad in the evening and snack on vegie sticks and hummous during the day
- Have some protein every day
- Eat lean or "trim" meats, include fish 3-4 times per week, and try to use more lentils and legumes (chick peas, red kidney beans, white beans etc)
- Use wholegrain/sourdough breads, basmati rice, pasta and low-GI cereals. These will ensure that you’re eating a low-GI and low-GL diet. Try not to make these the focus of your meals; instead, use them to fill up once you’ve had all your vegies and some protein. Use one of the shopping guides or the online database at the University of Sydney (see resources at the end) to check the GL of your breakfast cereals and bread
- Limit your saturated fat intake
- Choose lean cuts of meat, reduce your dairy intake and use low-fat dairy options where possible
- Have monounsaturated and polyunsaturated fats. These help protect us from cardiovascular and other degenerative diseases
- Use olive oil and avocado, have nuts and seeds for snacks and eat fish 3-4 times per week.
Regardless of what you’ve read or heard about the dangers of carbohydrates, they are an important part of a healthy diet. Carbohydrates provide the body with the fuel it needs for physical activity, brain function and also the general working of our body. The best sources of carbohydrates — fruits, vegetables, and whole grains — also provide essential vitamins and minerals, fibre and a wide range of important antioxidants.
While the GI is important for assessing the quality of a carbohydrate, the GL, by taking into account the carbohydrate density of a food, enables us to more usefully evaluate carbohydrates as we eat them in our every day lives. Follow the dietary guidelines above to make sure you’re eating a low-GL, healthy diet that ensures your body is running at its peak capacity.
“Glycemic Load, Diet & Health”, Harvard Women’s Health Watch, June 2001. Available at http://www.50plus.org/Libraryitems/2_5_glycemicload.html Brand-Miller, J., Foster-Powell, K., Colagiuri, S., The New Glucose Revolution, Hodder, Sydney 2002
Brand-Miller, J., Farid, N.R., Marsh, K., The New Glucose Revolution: Managing PCOS, Hodder, Sydney, 2004
The University of Sydney’s GI website, including an online searchable database of the GI and GL values of different foods: http://www.glycemicindex.com/
Harvard School of Public Health nutrition website: http://www.hsph.harvard.edu/nutritionsource/carbohydrates.html
Kathryn Elliott is a nutritionist and herbalist with a busy practice in Sydney’s CBD. Her clients have health issues ranging from menstrual problems through to chronic fatigue, insomnia and stress management. When she’s not seeing clients or cooking, Kathryn is also a verging-on-obsessive blogger and her writings, recipes and opinions can be found at www.kathrynelliott.com.au/blog.