People with prediabetes who eat the exact same foods can have very different blood sugar levels.
These findings, which were presented at the online last week, are the latest to suggest that the glycaemic index (GI) is an unreliable predictor of how foods affect blood sugar levels.
The idea behind GI is simple: foods are scored based on how quickly they increase glucose levels in the blood. Those above 70 are high GI foods, meaning they rapidly raise blood sugar levels. Those at or below 55 are low GI foods, meaning they increase blood sugar more slowly.
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The trouble is, mounting evidence indicates that no two people metabolise food the same. For example, of 63 people found that the GI of white bread ranged from 35 to 103. When the same experiments were repeated, even within individuals, blood sugar responses varied by about 20 per cent.
鈥淭his effectively places white bread in all three GI categories [low, medium, high], indicating that the GI methodology is not reproducible even under highly standardised conditions,鈥� the lead author of that study, , from Tufts University in Massachusetts, said at the conference.
There is now good evidence that a host of things 鈥� such as or 鈥� can affect blood sugar levels.
In the new study, at Texas Woman鈥檚 University and her colleagues focused on people with prediabetes, a condition where blood sugar levels are higher than normal, but not high enough to be considered type 2 diabetes. For one week, they gave eight participants an identical diet high in resistant starch, a kind of starch found in oats and beans that can help regulate blood sugar levels.
The researchers measured participants鈥� blood sugar levels at the start and end of the week, in both instances after they drank a glass of chocolate milk.
Not only did they find wide variation in participants鈥� glucose responses at the start, but also at the end 鈥� even after a week on exactly the same diet, says Patterson. The findings, which are still under review, reinforce the idea that there is no consistent blood sugar response to specific foods.
鈥淭he utility of the glycaemic index is very limited,鈥� says Patterson. 鈥淧eople think, well if I consume this food that is low GI it鈥檚 going to be healthier for me, when in essence, you don鈥檛 know how you鈥檙e going to respond to that food.鈥�
So, is it time to scrap it altogether? Not quite, says at the University of Toronto, Canada, who created the GI more than 40 years ago. 鈥淭he fact that many things alter glucose response to food and people have noticed these differences is great,鈥� he says. But 鈥渋t must be remembered GI is only one attribute of food鈥�.
Instead of worrying about GI, Patterson and Matthan recommend a diverse, healthy diet: eat whole grains, vegetables and fish, and limit red meat and sugary drinks.
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