Dr Alan Barclay
Glycemic index, glycemic load and risk of type 2 diabetes.
How high your blood glucose level rises and how long it remains elevated when you eat a food or meal containing carbohydrate depends on both the glycemic index (GI) of the carbohydrate and the total amount of carbohydrate in the food or meal. We use the term “glycemic load” or GL to describe this. You calculate it by multiplying the GI of a food by its available carbohydrate content (carbohydrate minus fiber in the USA) in the serving (in grams), divided by 100 (because GI is a percentage). Or, if you prefer: GL = GI/100 x available carbs per serving.
Here’s why the overall GL matters. One unit of GL is equivalent to 1 gram of pure glucose. So, the higher the GL of a food or meal, the more insulin your pancreas needs to produce to drive the glucose out of your bloodstream and into your cells. Consuming constantly high GI and GL foods and meals effectively speeds up the rate at which your pancreas wears out, and when it reaches the tipping point, type 2 diabetes develops.
Thanks to observational studies, we have a good idea of the level of risk of eating high GI/GL foods and meals. Research findings from Harvard University’s Nurses Health Studies 1 and 2 and the Male Health Professional Study were in fact the first studies to demonstrate a link between dietary GI and GL and the risk of developing type 2 diabetes. When results from these Harvard studies are combined with the results of similar studies from around the globe using meta-analysis techniques, we see that high GI diets increase the risk of developing type 2 diabetes by 19% and high GL diets by 13%. This risk is on top of other risk factors you may have such as age, ethnic background, smoking, alcohol consumption, body mass index and total calorie (kilojoule) consumption.
To reduce your risk, the evidence suggests you have a choice. You can lower either the GI, or the GL of the diet, to decrease it. This is important, because there are many ways of lowering the GL of the diet:
- Consuming less carbohydrate
- Consuming lower GI foods
- Or both, less carbohydrate AND lower GI foods
This flexibility is important. Not everyone needs or wants to consume less carbohydrate.
In Australia, for example, people consume on average only 45% of calories from carbohydrate (or 220 grams carbohydrate a day on a 2000 calorie diet). This is right at the bottom end of the recommended range of carbohydrate consumption (45–65%). In addition, total carbohydrate intake is decreasing, yet rates of obesity and diabetes continue to rise. So the take-home message for most Australians is that rather than consume less carbohydrate, the average person needs to be more concerned about eating better quality (low GI) carbohydrate.
In the United States, however, carbohydrate consumption is 48.7% and is increasing (as both a percentage of calories and in total amount), so Americans may wish to decrease both the total amount of carbohydrate they consume and improve its quality (low GI) in order to lower the GL of their diets.
The flexibility of reducing the GI or GL of your diet to decrease your risk of developing type 2 diabetes has important cultural considerations. A Mediterranean diet which tends to be lower in carbohydrate than typical Western diets is a healthy way of eating and is associated with decreased risk of a range of chronic diseases. But Asian diets which are higher in carbohydrate such as a traditional Japanese diet are also associated with a reduced risk of chronic diseases. What these very different dietary patterns share is eating lots of minimally processed cereals, plus plenty of legumes, seafood, vegetables, and fruit. This is why both dietary patterns have a relatively low glycemic load, despite their very different total carbohydrate content.
The bottom line: You can either eat a low GI or low GL diet as both are associated with a significantly decreased risk of developing type 2 diabetes. This choice gives you the flexibility to include your personal and cultural food preferences – a vital ingredient if you are going to improve and maintain healthy eating habits for life.