Please explain the numbers in the glycaemic index

We are now being bombarded almost daily with glycaemic food lists, thanks to the work of Dr Michel Montignac and Dr Robert Atkins. But can you tell me the quantity of food the figures given represent? Wholemeal bread has a GI of 50. Is this per slice? Mashed potatoes have a GI of 90. Is this per serving? Yoghurt is quoted as ‘35 full-milk’, but many are made with skimmed milk.- C. Shepherd, Exeter, Devon

WDDTY replies: Unlike calorie-counting, the GI index isn’t a quantity, but a rank score in a range both up and down from 100 (for pure glucose sugar). So, whether you eat one or two slices, or the whole loaf of bread, for example, its GI index score will remain the same.

A food’s ability to trigger insulin production remains the same whatever the amount consumed, an assumption that appears to be backed up by Montignac’s diet, which allows adherents to consume any quantity of specific foods so long as they have a GI score of 50 or less. Yoghurt, whether made from full-fat or skimmed milk, is within the desirable range so either one, from Montignac’s glycaemic-index point of view, is acceptable.