Glycaemic Index

CHOICE investigates whether our national waistline can benefit from new research on the glycaemic index.
 
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  • Updated:2 Jun 2009
 

01 .What is GI?

Low GI food

In brief

  • Low-GI diets have been shown to help people with diabetes and pre-diabetes, but they may also help those who need to lose weight.
  • Low-GI foods aren’t necessarily healthy in themselves, so look for healthy foods that also happen to be low GI.

The Glycaemic Index (GI) is a measure of how the carbohydrate in a food affects your blood glucose levels as it’s digested, absorbed and metabolised. If this happens quickly, there’s a rush of glucose into the bloodstream followed by a quick fall – the food is higher in the GI scale. If it gives a slower and gentler rise and fall in blood glucose, the GI is lower.

It’s a way of ranking carbohydrates and their effect on blood sugar, by comparing their effect against pure glucose. It’s this slower rise and fall in blood sugar that has led to claims low-GI foods give you longer-lasting energy – they keep blood sugar at a moderate level for a longer time than the sharp peak and fall of high-GI foods. Don’t confuse this with your personal energy levels or your get-up-and-go: the two things aren’t the same.

When CHOICE last visited this topic in 2004, a healthy, low-GI diet was thought to be most helpful for people with diabetes, but evidence has since emerged to suggest a much wider group of people may benefit, including those who need to lose weight. Not only are Australians getting fatter, but more importantly, it’s estimated as many as one in every four Australian adults has diabetes or pre-diabetes – both of which can be managed with a low-GI diet.

Please note: this information was current as of June 2009 but is still a useful guide today.


Who should be eating low GI?

  • Diabetes People with diabetes cannot control their blood sugar levels well. A healthy, low-GI diet has been shown to keep blood sugar levels under control.
  • Pre-diabetes This is where your blood sugar is higher than normal, but not high enough to count as diabetes. Since it’s only diagnosed by a glucose tolerance blood test, most people don’t realise they have pre-diabetes unless it develops into diabetes.
  • Low-GI healthy eating may improve blood sugar control and reduce the risk of developing the disease.
  • Overweight While not all experts are convinced, there’s evidence that lowering the overall GI of your diet by selecting low-GI foods may help you lose weight – see Can a Low GI Diet Fight Flab?.
  • Cholesterol and heart disease This is also debated, but there’s some good evidence low-GI diets may help improve heart disease risk factors.
  • Hypoglycaemia Many people may think they suffer from low blood sugar, but experts believe hypoglycaemia isn’t as common as people assume. Low-GI foods give a less dramatic and more sustained blood sugar release, so some people may find them useful to get them through between meals.

How to follow a healthy low-GI diet

The basics of healthy eating invariably include plenty of fruit, vegetables and wholegrain cereals; lean meats; fish a few times a week; watching the bad fats (saturated and trans) while getting in some of the good ones (nuts, seeds, olives, avocado); consuming low- or reduced-fat dairy foods; and going easy on salty and sugary foods.

So while GI isn’t the only factor to worry about, for many people it’s useful to finetune your healthy diet by swapping high-GI for low-GI carbohydrates. Choose a lower GI rice variety (such as basmati) or go for pasta instead; and swap white or wholemeal bread for a grainy seed bread. Of course, some foods – such as chocolate bars, ice-cream and cake – are not healthy choices, no matter how low their GI. Equally, you shouldn’t necessarily avoid otherwise healthy foods such as watermelon because of its high GI. As always, moderation is the key.

 
 

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02.Can a low GI diet fight flab?

 

A 2007 review of the scientific evidence on GI and obesity – a Cochrane review, widely considered the gold standard – found overweight and obese adults on low-GI diets lost on average 1kg more than those on conventional weight-loss diets. Not only did they lose more weight, they also lost more body fat and had greater improvements in their cholesterol levels as a result.

Two of the studies examined didn’t control how much the dieters ate, but advised them to choose low-GI carbohydrate foods and include protein and healthy fats at each meal and snack. They were allowed to eat and snack when they were hungry – restricting kilojoule intake was not a primary concern.

The results from these studies led the authors of the Cochrane review to conclude low-GI diets work as well as or better than conventional diets even when the amount of food eaten is not restricted. Low-GI foods seem to fill you up more and satisfy you for longer.

Not all experts agree

However, while many high-profile scientists are enthusiastic about the potential for low-GI diets in weight loss, not all experts agree. Some believe the evidence still has a considerable way to go before it’s proven, citing other studies that found little or no difference in weight loss between low-GI and traditional low-fat diets.

There have been some large studies completed since the Cochrane review that didn’t find an effect, sparking further debate. And some studies suggest that any effect may be significant only in certain types of people. As ever, there are no simple answers and more research is needed to identify the real benefits, and what happens long term.

Are high GI carbs addictive?


Many people were surprised early this year to see front-page headlines in major newspapers suggesting high-GI foods may be addictive. The theory is that the intense glucose rush given by a high-GI, high-carb food may stimulate the same parts of the brain in the same way as addictive drugs. It’s an interesting hypothesis, but for now that’s all it is – a hypothesis, with some plausible explanations. More research is needed before we get too excited about it, and some of this research is about to get under way at the University of Sydney.

Low GI sugar

CSR recently introduced LoGICane – a low-GI sugar that hit the streets with much fanfare. Its GI, at 50, is in the low range, compared with regular sugar’s medium GI of 65. Low GI sounds a lot better than medium, but the difference is not huge; using less sugar overall is another strategy.

Even though the new sugar contains beneficial phytochemicals, usually removed from regular sugar, it still has the same number of kilojoules. And once you use it to cook a cake or dessert, there’s no guarantee the end result will also be low GI.

LoGICane may be a useful lower-GI swap for regular sugar if you’re trying to reduce your diet’s overall GI, but it’s not a health food. And like all sugar, it should be used in moderation.

03.How GI is measured

 

You cannot tell what a food’s GI is likely to be just by looking at it or knowing its nutritional value. So while there are some general rules of thumb, you’ll need to use a table of high, medium and low GI values.

The gold standard for measuring GI is to use real people: a group of volunteers are each given 50g of glucose then the effect on their blood sugar over the next two hours is measured. The GI result for glucose is given a value of 100. The same people are then given a serving of the test food, also containing 50g of carbohydrate. The effect on their blood sugar this time is compared with the first test, and the GI is calculated from the average of all the volunteers. This is a complex process and there are only a few Australian laboratories that can do this type of testing.

GI labelling

GI claims appear on all sorts of carbohydrate foods; some labels simply say low GI, while others give a GI test figure. There’s no food standard that specifically covers the GI labelling of foods, so you have to take the manufacturer on trust.

One way to be sure of a product’s GI is to look for the GI symbol, below. Foods with this symbol must also list the exact GI figure, near the nutrition panel, along with the words “low” or “medium”. To display this label the GI will have been reliably measured and must also meet other nutrition criteria, depending on the type of food – the food must be a healthy choice in its category.

The GI symbol is a not-for-profit program founded by the Juvenile Diabetes Research Foundation, the University of Sydney and Diabetes Australia. Companies pay a licensing fee to be part of the program and use the logo.

The University of Sydney recently check-tested a wide range of foods that have a GI claim on the label but don’t carry the GI symbol. It found eight of the 10 products it tested using the gold-standard method didn’t have the GI rating claimed.

04.GI values in staple foods

 

Potatoes

  • Potatoes aren’t all created equal. Most are high GI, with some such as desiree and mashed pontiac right up near the top of the GI scale. But other varieties such as nadine, as well as new potatoes boiled in their jackets, are almost down in the medium range.
  • Nicola potatoes, now fairly widely available, are a good alternative with a GI of only 58 (medium range).

Pasta

When starchy foods are cooked, the starch grains absorb water – the more they absorb, the easier the body finds it to get to the starch, and the higher the GI. So while most pasta falls into the low GI category, al dente pasta is even lower.

Rice

Different varieties of rice contain different types of starch, which is why jasmine rice is very high GI, while basmati and other rices high in amylose starch are medium.

Breads

Breads are mostly high. In both regular white and wholemeal breads, the grains are ground and the outer coating of the grain is broken, so digestive enzymes can get to the starch easily. Very grainy multigrain bread and sourdough are lower – it’s the acidity of traditional sourdough bread that results in lower GI. You can also buy specially developed low-GI white breads.

Vegetables

Most vegetables don’t contain much carbohydrate, so their GI is negligible. The exceptions are starchy vegetables such as potatoes, carrots, parsnips, sweet potato, pumpkin and corn. The old furphy that carrots (GI 42, not 92 as previously thought) have a higher GI than a Mars Bar (62) is demonstrably wrong.

Sugar

At 65, sugar is actually only medium GI. This is because each molecule of table sugar (sucrose) is made up of one molecule of glucose (which contributes to the GI) and one of fructose. The fructose heads straight to your liver for burning and doesn’t change your blood sugar level. Even though many sugary foods have a moderate or even low GI, it doesn’t mean they are the best diet choices – sugar adds kilojoules but no valuable nutrients.

Fruit

Fresh fruit is generally low GI, although a few such as cherries, apricots, pineapple and rockmelon are medium range. The notable exception is watermelon, which has a high value of 76. Dried fruits are mainly low or medium.

Roll over the "Low", "Medium" and "High" tabs along the bottom of the graphic below to see the GI for your favourite foods.