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.
For more information about Margaret and her work check www.nutritionandallergyclinic.co.uk
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First Published in 2006
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