Food manufacturers are starting to list the glycaemic index (GI) values of foods on their labels and most health-conscious consumers are starting to get familiar with this concept.
However, the concept of the GL, or glycaemic load, is less well known. Since we're going to be hearing a lot more about the GL in future and as food manufacturers have also started to list the GL on food labels, it's important to know what these two terms mean.
Just to recap: the GI is a value that expresses to what extent a portion of a given food will increase the blood-sugar levels of an individual as compared to an equivalent portion of white bread or sugar (the standard).
Foods with a low GI will not cause exaggerated increases in blood sugar and insulin levels and not contribute to insulin resistance or diabetes, and will be less fattening if eaten in moderation. Examples of low-GI foods are low-GI bread (100% rye bread, seedbread and the whole range of new low-GI health breads), brown rice, sorghum, pasta made of durum wheat, all lean meats, fish, eggs, low-fat milk and dairy products, deciduous and citrus fruits, and healthy oils and margarine.
Foods with a high GI do cause insulin and blood-glucose levels to shoot up rapidly and can contribute to insulin resistance, diabetes, obesity and a variety of other degenerative diseases. Examples of high-GI foods are white, brown and wholewheat bread, refined starches, mealiemeal porridge, potatoes, sweets, tropical fruit and sugar.
But in some cases high-GI foods can be used successfully – for example, when athletes require a rapid release of energy or when they're carbo-loading before an event. High-GI foods can also be combined with low-GI foods as part of a healthy diet.
The GL of a food or a meal is defined as “the mathematical product of the GI of a food/meal and its carbohydrate content” (Venter, 2005). The GL is usually expressed as a percentage.
Foods with a GL below 10 are regarded as “low” and those with a GL above 20 as “high”(Nutrition Data, 2008).
Standard uncooked oats have a GI of 87 (high) and a 30g portion contains 18.5g of carbohydrate, so the GL can be calculated as follows:
87 x 18.5/100 = 16.1 GL (high)
All-Bran has a GI of 60 (intermediate) and a 30g portion contains 17.3g of carbohydrate, so the GL will equal:
60 x 17.3 = 10.4 GL (just above the low cut-off point)
If we add 100ml of skim milk to these breakfast cereals (GL of 100ml skim milk = 46 x 4.8/100 = 2.2 GL), then the GL of these breakfasts will equal:
Oats + skim milk = 16.1 + 2.2 = 18.3 (high)
All Bran + skim milk = 10.4 + 2.2 = 12.6 (intermediate)
So the GL confirms what the GI tells us.
Drawbacks of the GL
Although the GL can be used to calculate more or less how much a serving of food will increase blood-sugar levels, it has certain drawbacks.
To use the GL properly, we need to know the GI of the food in question and as the GI can only be determined by doing extensive tests with different human subjects, this is a rather expensive and time-consuming process. There are also not many laboratories in South Africa that test the GI of foods, so we still don’t have values for quite a number of our common foods.
Food manufacturers are also constantly producing new foods at such a fast rate that GI testing can’t keep up with this flood of new products. According to Nutrition Data (2008), “Each year, tens of thousands of new packaged food items are added to the grocery shelves, but only a few hundred foods are tested for GI.”
We also don’t know the GI of compound recipes or home-cooked meal combinations.
At the moment, the use of the GL is limited to statements on labels of foods that have been analysed by their manufacturers. If you see a GL value on a label that is lower than 10, you know that food is a low-GI food and you can use it to control your blood-sugar and insulin levels.
Just keep in mind that the GL applies to a specific portion size (for example two slices of bread or 30g of breakfast cereal). If you eat double the portion, then the GL will also double. This means that you cannot eat uncontrolled amounts of low-GI foods and think that this will not cause weight gain.
Eating unlimited amounts of low-GI foods
A common error that many people make is to think that because a food has a low GI, it's okay to eat as much as they like. This pushes up their energy intake, which means that they don’t lose weight.
If a food has a low GI or low GL, it doesn't mean that you now have carte blanche to eat as much of the food as you like. Moderation and sticking to portion sizes is still important.
Slabber (2005) warns: “Health professionals should strictly avoid suggesting to overweight clients and diabetic patients that low-GI carbohydrates may be eaten in unlimited quantities without the overt risk of increasing obesity and/or hyperglycaemia (high blood-sugar levels).”
Keep these rules in mind:
- Don't overindulge in low-GI or low-GL foods.
- Portions sizes are always important.
- The GI or GL of a food is NOT the only reason why a food should be included in a diet or not.
(Dr I.V. van Heerden, DietDoc, September 2008)
(Nutrition Data, 2008. http://www.nutritiondata.com/help/estimated-glycemic-load; Slabber, M (2008). Complexities of consumer understanding of the glycaemic index concept & practical guidelines for incorporation in diets. SA Journal of Clinical Nutrition, Vol 18(3):252-257; Venter, CS (2005). Editorial: The glycaemic index - scientific evidence on the practical use. SA Journal of Clinical Nutrition, Vol 18(3):211-212.)