Why are many high-fibre foods high GI?
Dietary fibre can be divided into soluble and insoluble types. Soluble fibre is often viscous (thick and jellylike) in solution and remains viscous even in the small intestine. It slows down digestion, making it harder for enzymes to digest the food. Foods with more soluble fibre, like apples, oats and legumes, are low GI as a result. Insoluble fibre, on the other hand, is not viscous and doesn’t slow digestion, especially if it’s finely milled. This is why wholemeal bread and white bread have similar GIs, and why brown pasta and brown rice have values similar to those of their white counterparts.
Does retrograded starch ever revert back to regular starch?
Yes, retrograded starch will revert back to normal starch if it’s reheated – perhaps not all of it, but much of it. That’s the basis of making stale bread into ‘fresh’ bread by heating it up in the oven. So twice-cooked potatoes will remain high GI. Even cold potatoes have a high GI because only 10 per cent of the starch is retrograded. Some critics debate about the fact that cooking makes a difference to the GI of potatoes, but the effect is relatively small and the overall message is that potatoes usually have a high GI. Having said that, we are discovering that some varieties, such as Nicola, have a lower GI – in the high 50s. These are usually called waxy potatoes and they are recommended for making a potato salad because they keep their firm shape.
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