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The glycemic index, or GI, is an inherent property of carbohydrate-containing foods and beverages. It is a relative ranking from 0–100, describing the rate and extent to which available carbohydrate (total carbohydrate minus dietary fibre) is digested, absorbed and metabolised into glucose and released into the blood.

Glycemic index testing follows an international standard. Volunteers who have fasted overnight are given 50g of glucose in water as a reference food, and researchers measure their blood samples at eight intervals over a two-hour period. The data enables the researchers to plot a curve, where area under the curve (AUC) is set at 100. On a separate day, the volunteers fast overnight again, and are then given the test food containing 50g of available carbohydrate. The AUC of the test food is expressed as a ratio of that of the glucose reference and is used to calculate the GI value which is a percentage.

GI test curve
Low GI foods (55 and under) are characterised by a slower and lower rise in blood glucose levels. High GI foods (70 and over) are characterised by a faster and higher rise and fall in blood glucose levels.

But speed of digestion is only one part of the story. Quantity counts. How high blood glucose actually rises and how long it remains high after we eat a meal containing carbohydrate foods depends on both the amount of carbohydrate in a food or drink as well as its glycemic index. Researchers from Harvard University and the University of Toronto came up with a term to describe this “speed/quantity” combo: glycemic load (GL). It is calculated by multiplying the GI of a food by the available carbohydrate content (carbohydrates minus fiber) in the serving (expressed in grams), divided by 100 (because GI is a percentage). (GL = GI/100 x available carbs per serving.)

For example, a typical medium-size apple has a glycemic index of 38 and contains 15 grams of available carbohydrate. Therefore, its glycemic load is 38÷ 100 × 15 = 6. If you are hungry, and the apples are particularly crispy, juicy, and delicious, and you eat two, the overall glycemic load of this snack is 12. If you have three, it’s 18.

One unit of glycemic load is equivalent to 1 gram of pure glucose. High GL foods and beverages have a GL value of 20 and above; medium GL foods and beverages have a GL value between 11 and 19; and low GL foods and beverages have a GL value of 10 and under.

What does this all mean for our health and wellbeing? The higher the glycemic load of a food or meal, the more insulin your pancreas needs to produce to drive the glucose into your cells. When we are young, our pancreas is able to produce enough insulin to cover the requirements of high-glycemic load foods and meals, but as we get older, it may no longer be able to cope with higher insulin requirements. This is when type 2 diabetes and other “lifestyle” diseases can start to develop.

In population studies, the GL simultaneously assesses the effect of available carbohydrate and GI on the risk of type 2 diabetes. GI and GL are calculated for the whole diet and adjusted for energy (kilojoules or calories) to help reduce confounding (e.g., the larger a person the more energy they require to maintain their body weight). Around the world, the average adult consumes around 2000 calories (8400 kilojoules) each day, so this is commonly used as the standard comparator.

The most recent systematic review and meta-analysis of population studies found that people consuming a diet with an average GI of 76 have an 87% higher risk of developing diabetes than people consuming a diet with an average dietary GI of 48. The simplest way to reduce average dietary GI is to replace high GI foods and beverages with lower GI alternatives within a particular food group.

Swap it
Similarly, the recent systematic review and meta-analysis found people consuming a diet with a GL of 257g per 2000 calories have an 89% higher risk of developing diabetes than people consuming a diet with a GL of 73g per 2000 calories.

Because GL is the product of GI and available carbohydrate, you can reduce it by either consuming a low GI diet (e.g., aim for an average GI less than 48), or by consuming less available carbohydrate (e.g., aim for between 130–230 grams per day if you consume 2000 calories (8400 kilojoules) per day), or both.

For example, for people who typically consume a relatively high carbohydrate diet (e.g., greater than 230g per day) with a high average GI (greater than 70), reduce average daily available carbohydrate intake to 165g and GI to 45 for an average daily GL of 74g per 2000 calories, keeping type 2 diabetes risk to a minimum. An example meal plan could look something like this:

Low GL meal plan

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Dr Alan Barclay
Alan Barclay, PhD is a consultant dietitian and chef (Cert III). He worked for Diabetes Australia (NSW) from 1998–2014 . He is author/co-author of more than 30 scientific publications, and author/co-author of  The good Carbs Cookbook (Murdoch Books), Reversing Diabetes (Murdoch Books), The Low GI Diet: Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment, New York).

Contact: You can follow him on Twitter or check out his website.