The glycaemic index (GI) is a measure of how quickly the carbohydrates in a particular food enter the blood stream after it is eaten. The GI concept can be extended from single foods to meals or even the entire eating pattern of an individual or group. The glycaemic load (GL) accounts for the amount of carbohydrate as well as its GI (GL= GI x grams of carbohydrate).
Foods are classified as low, medium or high GI according to a somewhat arbitrary scale:
- GI = 100 for pure glucose. All other foods are compared to this standard.
- GI over 70 is HIGH
- GI 55 – 70 is INTERMEDIATE
- GI less than 55 is LOW
These measurements are all done using a single serve of a single food containing a standard quantity of carbohydrate in a small group of subjects. Results can vary wildly between laboratories. Whole plant foods vary according to season, variety and ripeness. Results vary between subjects and on different occasions for the same subject. Despite these limitations, foods can be classified according to their GI and the concept has been shown to have useful applications.
A great many health benefits are claimed for low GI diets. Most of them not well supported by research. The health benefits of low GI diets may be restricted to particular groups of subjects – in particular, those who are obese and/or diabetic. An individual who is lean, active and has a low fat diet, all of which improve the capacity of the body to deal with carbohydrates, may not benefit greatly from replacing whole high GI carbohydrates with whole low GI carbohydrates.
When using GI to assist in food choices it is important to consider the energy density of the food item. An item such a carrot has a high GI but is so low in calories that a GI serve is a much larger quantity of carrots than you are likely to eat in one sitting. A small quantity of carrots will not have much impact on your low GI diet.
The higher [animal] protein, low GI diet is gaining popularity among Australian health professionals. It may compare favourably with the typical Australian diet and a diet rich in sugars and other highly refined carbohydrates. We would welcome research directly comparing the higher protein diet with a low fat whole foods plant-based diet.
Beware of highly processed foods and low carbohydrate/ high fat foods which misuse the GI health claim for marketing. The overall nutritional value of foods should be considered first, and the GI only used to assist in selecting between similar items. A whole foods plant-based diet has a lower GI than the typical Australian diet.
Medical conditions that may be improved with a low GI diet include:
- Type 2 diabetes, type 1 diabetes and pre-diabetes (impaired glucose tolerance)
- High triglyceride levels (and the associated low HDL levels)
Reducing the overall GI of your diet is not difficult. You can find a few healthy very low GI foods that you enjoy and use more of them in your meals and cooking.
Examples of very low GI whole plant foods:
- Lentils and beans
- Oats and barley
- Apples and pears
The physical form of whole plant foods influences GI. Chopping, grinding and blending foods tends to make the GI higher. For example, quick oats have a slightly higher GI than ordinary rolled oats. Some foods have a lower GI because they are lumpy and claggy. For example pasta has a lower GI than flour.
Knowledge of GI values can also be used to replace foods with similar items of lower GI. The following are some examples of low GI food exchanges:
Breakfast cereal (breakfast cereals) rolled oats
Bread wholemeal bread generally heavier more grainy breads
Potato potato sweet potato
Rice medium grain long grain
Fruit watermelon apple
- Dr. David Jenkins discusses the glycemic index of foods – (9 min. video) Jenkins was the first to define and explore the concept of glycemic index back in the 1980s. He explains that GI is simply one of the indicators of a food and should not be the only indicator used regarding the health of a food.
- Matthan, N. R., Ausman, L. M., Meng, H., Tighiouart, H., & Lichtenstein, A. H. (2016). Estimating the reliability of glycemic index values and potential sources of methodological and biological variability. American Journal of Clinical Nutrition, 104(4), 1004-1013.
- Search for the Glycemic Index (University of Sydney)
- Glycemic Index – Not Ready for Prime Time – Dr John McDougall
- Do you recommend that diabetics avoid white potatoes?– by David Richards, D.C., M.D.