The Glycaemic Index - Margaret Moss takes issue

Margaret Moss suggests that too much reliance on the Glycaemic Index can be misleading.

The glycaemic index is claimed to be a way of ranking carbohydrate foods according to the extent they raise blood sugar levels. This is not true. It ranks them, according to how they raise blood glucose levels. It ignores fructose in fruit/table sugar, galactose in milk, and other sugars.

Galactose is more dangerous for the arteries than fructose, which in turn is more dangerous than glucose. Fructose is thought to contribute greatly to the upsurge in Type 2 diabetes. Galactose is a major cause of coronary heart disease.

Moreover, the glycaemic index does not measure how high a food raises the blood glucose level. To obtain the glycaemic index, 8 to 12 people are given a carbohydrate food containing 50g of carbohydrate. The blood glucose level is measured every 15 minutes for 3 hours. A graph is drawn. The area under the graph is calculated. It is divided by the area under the graph for the same person eating 50g of pure glucose, and multiplied by 100. The result is averaged for the 8 to 12 people.

The trouble is that a food that caused a high glucose peak, followed by a rapid drop might have the same glycaemic index as a food that caused a gentle rise, and slower drop. Soft drinks and mashed potato have a similar glycaemic index, but soft drinks are quickly absorbed, and soon leave the consumer hungry.

The glycaemic index does not measure the rate of absorption. Moreover, few people would have a meal of nothing but potato. The
carbohydrate would be absorbed more slowly, if there were butter in the potato, and it was eaten with broad beans and a chop.

You can see that the processed food industry is likely to love a concept that promotes soft drinks and yogurt, and demotes sources of vitamin C and other nutrients, like potatoes. For example, they can say, eat Mars Bar (GI of 68) instead of carrot (92), or eat ice-cream (36) instead of banana (62) or new potato (70)!

Some people see the glycaemic load (GL) as more useful.
The glycaemic load is the glycaemic index, times the number of grams of carbohydrate per serving, divided by 100.
The idea is that you should look at the overall effect of the diet on blood sugar, rather than looking at each individual food as good or bad. This is an improved concept, but still ignores sugars other than glucose.

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First Published in 2006

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