The GI diet is based on a distinction between “good” and “bad” carbohydrates. Good carbohydrates are those that are low in sugar, or which release their sugar slowly, providing energy throughout the day. By comparison, high GI foods are rapidly digested, causing a surge of blood glucose and insulin. The excess insulin mops up the glucose and transfers it to the body’s fat stores — this can lead to weight gain, and ultimately diabetes and heart disease. The GI diet rates foods between 1 and 100; anything over 70 is to be avoided and you are advised to eat foods below 50.

But according to a more sophisticated measure, some apparently low GI foods might not be so good after all, and might even encourage weight gain. Meanwhile, some “bad” (ie, high) GI foods, could actually be beneficial. Watermelon, for instance, comes in at 72 on the GI rating, which puts it up with other “bad” fast-releasing foods such as white bread (70) and white rice (72), which should be avoided. But if you look at watermelon’s glycaemic load (GL) rating, it is a different story. Under the GL system the “avoid” foods score 16 and over, while the “eat all you want” are 10 and below. Watermelon’s GL is a very desirable 4, while white rice and white bread are still high at around 29.

The principles of GI and GL are essentially the same, explains Professor Jennie Brand Miller, of the University of Sydney. “If your glucose and insulin levels keep being pushed up day after day, your body will keep using glucose for fuel and storing fat. Lower insulin levels mean more fat stores are used for energy.”

Both GI and GL have been used for years by academic nutritionists trying to influence blood sugar levels in diabetics. But while GI will give you a pretty good indication about which foods to go for and which to avoid, the GL rating is more accurate.

You can see why by comparing carrots and chocolate. On the GI scale they both come in at a “just healthy” 48. However, GL rates them quite differently — carrots score a good 3.9 and chocolate a nearly bad 14. This is because although both contain carbohydrates that are digested fairly quickly, you’d have to eat more carrot — about two of them — to provide the same amount of fast-releasing carbs as you would get in two squares of chocolate. It is this difference that GL factors in. GI measures the quality of the carbohydrate in a food, explains Patrick Holford, a clinical nutritionist and author. “But you need to know how much of that type of carbohydrate there is in the food. The GL index comes from combining the quality with the quantity in each food. It’s the most accurate way of telling how much weight you will gain or lose if you choose certain sorts of food.”

The number of anomalies thrown up by the two types of rating is small, but the distinction between GI and GL means you could have been overdoing “bad” carbs at the expense of the “good”. And the list of what’s in and what’s out in the GL calculus may take some getting used to. Healthy pineapple, for instance, an acceptable 46 on the GI scale, is a dodgy 16 on the GL rating, while good old wholemeal bread is neck and neck with high-fibre white at 9 GL (their GIs are 64 and 70 respectively).

But there may be something even more important than weight loss going on here. By keeping your glucose and insulin levels steady, low GL/GI may have a protective effect on the heart. “It’s early days yet,” says Dr Anthony Leeds, senior lecturer in nutrition at King’s College London, “but I suspect that any beneficial effect of the GI/GL diet has on the heart comes because of the way it controls insulin levels, which in turn affect blood vessel walls.”

Leeds is one of the authors of a study which found that patients put on a low GI diet before heart surgery had greater insulin sensitivity More support from this idea comes from a recent study which compared a GL diet with a low-fat one. While both sets of patients lost the same amount of weight, those eating low GL not only felt less hungry but also had fewer harmful fats in their bloodstream and lower blood pressure.

RATINGS COMPARED

THE GI diet rates foods between 1 and 100; anything over 70 is to be avoided and you’re advised to eat below 50. GL, by contrast, sets 10 and below as recommended and over 16 as “avoid”.

Because GL takes account of the amount of an actual serving it reveals that some low GI foods are actually blood glucose boosters while others rating as high GI are actually fine.

The “bad” foods that are now “good” under the GL system:

Pumpkin (75 GI; 4.3GL)
Swede (72GI ; 7GL)
Broad beans (79GI; 4.1GL)
Watermelon (72 GI; 4.GL)

The “good” foods that are now “bad” under the GL system:

Sweet potato (44GI ; 26 GL)
White pasta (47GI; 21GL)
Basmati rice (58GI; 22GL)
Semolina (55GI: 28GL)
Banana cake (47GI; 18GL)
Chocolate (48GI; 14GL)

www.glycemicindex.com provides both GI and GL ratings, as does www.glycemicdietsw.com/

The Holford Diet by Patrick Holford, Piatkus (£7.99).

The New Glucose Revolution by Anthony Leeds and Professor Jennie Brand Miller (Hodder Mobius, £8.99).

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