Food

Glycaemic index

Updated 01 Dec 2007
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Introduction

The glycaemic index is hard enough to spell, let alone understand. But don't be put off. It's a tool that can help you develop a healthier approach to eating.

According to the glycaemic index, Christmas pudding appears to have a better rating than watermelon. So is this the impossible dream made real - diet advice that turns festive food into healthy eating? If only.

We explain this complicated nutrition theory.

What is the glycaemic index?

The glycaemic index (GI) is a measure of how quickly, and by how much, the carbohydrates in a food raise your blood glucose levels. This is important because, for most of us, carbohydrates like breads, cereals, fruit and vegetables are the basis of our food intake.

It used to be thought that the type of carbohydrate (simple or complex) determined the value of a food. The GI shows that the way the carbohydrate works in your body is more important.

Most foods with a low GI rating fill you up quickly and provide sustained energy. That's usually a good thing.

High GI foods are not likely to be so filling, so you eat more, and they give you a quick, short-lived energy hit. This is less desirable if you are watching your weight, because you can eat too much and still feel hungry again quite soon.

Low GI foods are also preferable for people with diabetes, because the slower absorption of carbohydrates avoids any sudden rise in blood glucose levels.

But sometimes you need that quick hit, and at these times high GI foods are preferable.

The nutrition advice that follows from this is not always simple. Among the complicating factors:

  • Your blood glucose response will vary, depending on your energy needs at any given time. It also varies from person to person.
  • High-fat foods seem to buck the trend. They tend to have a low GI rating, but may not fill you up quickly.
  • The GI is not the only measure of a food's value.
  • Glucose response to any food varies depending on how you prepare it and what you eat it with.

So how do you make sense of it all, and what is the value of the GI anyway?

The GI's strengths

The GI concept is backed by both the World Health Organisation and the Food and Agricultural Organisation because it helps prevent coronary heart disease, diabetes and obesity.

Sustained energy levels

Low GI foods help keep your blood glucose levels constant. This keeps your energy levels up and avoids the feeling of sluggishness you might get after a quickly digested high GI meal.

Conversely, blood glucose levels drop after a bout of strenuous exercise. High GI foods will allow you to replenish your glucose stores and get you back on an even keel.

Diabetes management

Keeping blood glucose levels constant is especially important for people with diabetes. Understanding the GI values of foods can help explain changing levels, and may help match certain foods with medication.

But, when a person with diabetes has low blood glucose levels, which may result in a "hypo", a high GI food will bring them back to normal.

Sorting those carbohydrates

Until very recently, the main message about carbohydrates was that they should form the basis of our diet.

This isn't wrong in itself. But critics argue that it is wrong to treat all carbohydrates in the same way, and that it is misleading to say just that "simple" carbohydrates (sugars) are less valuable than "complex" carbohydrates (starches).

Potatoes are perhaps the touchstone for the new thinking. They are mostly starch, which gets broken down very quickly into blood glucose. In fact, a potato (especially if it's hot and mashed), can give your body a faster hit than a bowl of sugar.

Any decent nutritional theory has to be able to explain this - and what we should do about it. The GI does both these things. (The good news: you don't have to give up potatoes! See The GI of common foods.)

Weight loss

When it comes right down to it, if you eat more food energy than your body needs, you will get fat.

Therefore, reducing your overall food energy intake (while getting more exercise) is generally regarded as the central strategy in a good weight control programme. Eating low GI foods can help with this, because that should mean you're not likely to eat so much.

Some studies have also suggested that eating a low GI diet can lower blood cholesterol levels, lower the risk of heart disease and lower the risk of diabetes.

But that's not the whole story.

As Professor Jim Mann, head of human nutrition at Otago University, says, "The GI concept is a valuable tool but using it isn't as simple as just choosing low GI foods."

The GI's weaknesses

Weight gain!

The GI's role in controlling weight is far from assured. The reason is that fat tends to reduce the GI of a food. However, fat provides twice as many kilojoules as carbohydrate, so it's easy to overeat.

In addition, sugar (sucrose) has a medium GI. It's also easy to overeat because high-sugar foods provide a concentrated source of energy.

It's clear that not all low or medium GI foods are useful for weight loss.

The likely reason for this has to do with fibre. Many foods with a low GI rating are high in fibre, which is known to fill you up quickly. Most high-fat and high-sugar foods, however, have very little fibre.

A low GI rating is therefore useful if you're trying to lose weight, provided it indicates a low fat, relatively low sugar and high-fibre count. You need to look past any "low GI" marketing label and check the details on the nutrition information panel.

Vitamins and minerals

The nutrients you get from vegetables and fruit are essential to good health, but are not considered in the GI rating. However, many low GI foods are unrefined so are a good source of vitamins and minerals.

Protein

Protein in food tends to lower the GI, but it doesn't follow that the more meat you eat, the healthier you will be. Protein should be eaten in moderation.

The GI of common foods

Potatoes
Potatoes have a high GI. But they are also cheap, plentiful, tasty and contain various nutrients your body needs. You don't have to give up potatoes. Instead, you can:

  • Eat them in a meal with other food, especially high-fibre foods, lean protein and small amounts of unsaturated fats. All will lower the GI.
  • Eat new potatoes. The more immature the starch, the lower the GI.
  • Eat them cooked and cold, as in a potato salad. The starch will have changed into a form that has a lower GI.
  • Keep the salt to a minimum.

Chips have a lower GI than baked potatoes because of the fat they've been cooked in. But don't be misled - fat provides excess kilojoules, and if it's saturated fat, which many fast food outlets and supermarket brands still use, those chips will not be good for you.

Bread

White bread has a high GI; most wholegrain is low. But white bread is rather like potatoes - the overall value is influenced by how you serve the food.

If the only sandwiches your kids will eat are made with white bread, give them a piece of high-fibre fruit as well. Kiwifruit, oranges, apples and pears all qualify.

Rice and pasta
They're not the same. The GI of most types of pasta is low, but with rice it varies.

The colour (white or brown) isn't so important, but the length of the grain is: long grain (like Basmati) has a lower GI than short grain.

Pasta differs from rice because it has less "amylopectin", a starch that is quickly digested, and more "amylose", which digests more slowly.

Chocolate and ice cream

And many other kinds of sweets and Christmas treats for that matter, may have a low GI.

But don't be fooled. Chocolate is 30 percent fat and over 50 percent sugar, and some ice creams are little better.

They're not good for your waistline or your teeth!

What's more, treat foods like this have few other nutritional qualities.

If it's time for a treat and you don't fancy an apple, the GI is not a useful guide. Look for a food where the energy content isn't too great.

Watermelon
Watermelon has a high GI rating. But this doesn't mean you should avoid it.

GI is measured using a quantity of the food large enough to contain 50g of carbohydrate. Because there is relatively little carbohydrate in watermelon, subjects need to eat an enormous quantity to do the measurement.

No one would eat this much watermelon normally.

How to influence the GI

You can change the GI rating of your food, or choose low GI versions of foods that may seem similar in other ways. Here are some of the variations:

Processing

This usually raises a food's GI. White bread has a higher GI than wholegrain.

Fibre

Soluble fibre lowers the GI. Foods like oats, legumes and fruit slow the digestion of starches and absorption of glucose into the blood. Porridge has a lower GI than Weet-Bix, which in turn is lower than Rice Bubbles.

Ripeness

Ripening raises the GI. A greenish banana will have a lower GI than a very yellow one.

Sugar

Surprisingly, table sugar has a medium GI. This is because it is made up of glucose (the highest possible GI) and fructose (low GI). Fructose is slowly converted to glucose, which lowers the GI compared to glucose and other foods like potatoes and white bread.

The trouble with sugar is not its GI rating. Sugar contains no other nutrients, it can cause tooth decay, and it often comes in high-energy foods.

Salt

This may raise the GI, because it speeds up the digestion.

Acidity

Lemon juice and vinegar lower the GI of a meal. You don't have to squeeze them over potato - just squeeze lemon over the veges or put vinegar or lemon juice on a salad, and when they get to your stomach they will slow down the absorption of starches in the potato anyway.

Food combinations

Fat and protein in foods lower the GI, by slowing digestion.

Individual variation

There is individual variation in the way people respond to a food (except when the GI is very low). This means a sandwich may sustain you for longer than it would your workmate.

Individuals also have varying blood glucose levels. You might find that sandwich is more sustaining today than another just like it tomorrow.

Food quantities

Don't forget this one. If you eat a lot of a particular low GI food, you will still raise your blood glucose levels significantly.

The GI is valuable, but a calculation called the Glycaemic Load (GL) is even more so. It takes the GI of a food and multiplies it by the grams of carbohydrate in a serving. This concept has not been sufficiently developed to be used routinely.

Two diets compared

Lily is a fairly active 20-year-old. She keeps junk food to a minimum and reckons her diet is pretty reasonable. But she eats a lot of high GI foods. As a result, Lily starts her day with lots of energy but feels sluggish towards the end.

Mia, Lily's friend, is also a fairly active 20-year-old. She eats a similar amount of energy (kilojoules) and gets it from much the same types of food. But Mia seems to be able to keep going from morning to night. The reason: her typical daily intake includes more low GI foods.

The foods that make the difference are marked in bold in Lily's list.

Note: we haven't marked the high GI post-exercise snacks. After exercising a high GI snack may be important to replenish your blood glucose levels.

Lily Mia
Breakfast Glass of orange juice, cornflakes and canned peaches with low-fat milk, one piece of white toast with honey and a scraping of table spread. Glass of orange juice, porridge with raisins and a sprinkling of brown sugar topped with low-fat milk, one piece of soy and linseed toast with honey and a scraping of table spread.
Snack Two Arrowroot biscuits. Two Ryvita or other grainy crackers with Marmite.
Lunch White bread roll with tuna and salad, banana, can of Coke. Wholegrain sandwich with tuna and salad, apple, mineral water.
Snack (pre-exercise) Rice Bubbles with low-fat milk. Wholemeal pita pocket with hummus.
Snack (post-exercise) One piece of white bread with honey. Bowl of cornflakes or one piece of white bread with honey (deliberate high GI food).
Dinner Grilled chicken breast, baked potato with sour cream, Greek salad (no dressing). Grilled chicken breast, Basmati rice salad, Greek salad with oil and vinegar dressing.
Later Two pikelets with jam. Low-fat fruit yoghurt, orange.

Testing and labelling GI

The GI ranks food on a scale from zero to 100, based on how quickly it raises your blood glucose levels. A low value is less than 55, medium is 56 to 69, and high is greater than or equal to 70 and above.

The GI rating of a food is assessed in laboratory trials involving real people (usually in a batch of 10) and real food. First, the subjects must fast overnight, before being fed 50 grams of pure glucose.

Blood samples are taken over a two-hour period to measure what happens to their blood glucose levels. The response is given a rating of 100.

Then portions of the trial food, containing 50g of carbohydrate, are fed to the subjects. If the food is sugar, which is all carbohydrate, it weighs just 50g. For other foods the portion sizes will be bigger.

Blood samples are taken and the blood glucose responses measured against the glucose standard. If a subject's response is exactly half that of glucose, the GI is said to be 50. The average GI of all subjects becomes the published GI.

What's in a logo?

There's a GI logo you may start noticing on some foods here, developed by the University of Sydney, Diabetes Australia and other groups. Companies apply to use the mark and pay for a licence, much like the Heart Foundation tick.

We advise caution. First, the GI mark doesn't mean low GI. Any food (whether it gets a low, medium or high GI rating) can be eligible for the mark, provided it has had its GI accurately tested, contains at least 10g of carbohydrate, and meets certain nutritional criteria for energy, total and saturated fat, fibre and sodium.

If you see the GI logo, check the number on the back.

Second, the guidelines do not limit the amount of refined sugar in a food, so it's possible a pavlova (provided it meets the other nutrition criteria) could display the logo. Low GI perhaps, but definitely no better than a treat food!

Third, some foods without the logo will have a lower GI than others that have it.

Our advice

Check out the example of our two friends, Lily and Mia. As their diets show, you can use the GI in quite a simple way to improve what you eat. Get it right, and you may well find you won't need or want to eat so much, and your energy levels will remain topped up throughout the day.

  • The GI helps with this by identifying the relative values of carbohydrate-rich foods like breads, cereals and vegetables. Unless you have special dietary needs, aim for a low GI diet based on high-fibre foods: wholegrains, legumes, and many vegetables and fruit.
  • Don't forget other nutritional issues. Include some lean protein and low-fat dairy products. Limit your intake of saturated and trans fats, added sugar, salt and alcohol. If you're worried about your size, cut back on your food energy intake overall, and increase the amount of exercise you do.

More help

Report by Belinda Allan.