People have long known that, compared to the other major nutrients protein and fat, the amount and type of carbohydrate that we eat or drink has the most pronounced effect on blood glucose levels in people with diabetes.
Before the glycemic index (GI) was conceived, people with diabetes predominantly counted carbohydrate using carbohydrate exchanges (15 grams of carbohydrate per serve) or portions (10 g of carbohydrate per serve). The carb-counting systems were originally designed to help people with diabetes limit the amount of carbohydrate they consumed throughout the day in order to manage their diabetes. Later, with the increasing availability of home blood glucose monitors, and fast acting insulins, amongst other developments, they were used to match the amount of carbohydrate consumed at each meal with the amount of insulin administered. There is now compelling evidence from 7 randomised controlled trials that counting carbohydrates helps people with type 1 diabetes manage their glycated haemoglobin (HbA1c) levels, reducing their risk of developing common diabetes-related complications (1).
However, around 50 years ago, it was becoming increasingly clear that carbohydrate exchange / portion lists did not accurately reflect the physiological effect of foods and drinks on blood glucose levels. This research ultimately led to the development of the GI in the early 1980’s (2). By comparing 25 g or 50 g available carbohydrate portions of a variety of foods and drinks, it became clear that the process of digestion, absorption and/or metabolism affected blood glucose levels independent of the amount of carbohydrate. A relatively large number of randomised controlled trials (29) have been conducted since the GI was conceived, and systematic reviews and meta-analyses of these provide compelling evidence that consuming low GI foods/drinks instead of higher GI varieties significantly improves HbA1c levels (3).
While both the amount and type of carbohydrate independently affect blood glucose levels, it should come as no surprise that combining both measures together provides an even better estimate of how a food or drink will affect blood glucose levels. This is of course where the glycemic load (GL) comes in. The GL is calculated by multiplying the GI of a food by its available carbohydrate content in a serving (expressed in grams), divided by 100 (because GI is a percentage):
GL (grams) = GI ÷ 100 x available carbs (grams) per serving
One unit of glycemic load is equivalent to 1 gram of pure glucose. Research has found that the GL accounts for 85–94% of the variability in blood glucose in single foods and around 90% of the variability in mixed meals. GL is also the most powerful predictor of insulin levels in the blood, after consuming a meal or drink. It is therefore superior to using either carbohydrate counting techniques, or GI, by themselves.
Given its powerful effect on blood glucose and insulin levels, it is probably not surprising that habitual diets with a high average dietary GL (more than 95 g GL/2000 calories (8400 kilojoules)) are strongly associated with the risk of developing type 2 diabetes in observational studies (4).
Therefore, consuming a moderate glycemic load diet is a useful way of managing diabetes if you already have it, or reducing your risk if you don’t.
- Bell and colleagues. Efficacy of carbohydrate counting in type 1 diabetes: a systematic review and meta-analysis. Lancet Diabetes Endocrinol. 2014.
- Jenkins DJ, and colleagues. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr. 1981.
- Chiavaroli L, and colleagues. Effect of low glycaemic index or load dietary patterns on glycaemic control and cardiometabolic risk factors in diabetes: systematic review and meta-analysis of randomised controlled trials. BMJ. 2021.
- Livesey G, and colleagues. Dietary Glycemic Index and Load and the Risk of Type 2 Diabetes: A Systematic Review and Updated Meta-Analyses of Prospective Cohort Studies. Nutrients. 2019.
Dr Alan Barclay, PhD, is a consultant dietitian and chef with a particular interest in carbohydrates and diabetes. He is author of Reversing Diabetes (Murdoch Books), and co-author of 30-plus scientific publications, The Good Carbs Cookbook (Murdoch Books), Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment Publishing).