What is the low-GI diet?

GI stands for glycaemic index. This is a scale of 1 to 100 that describes the speed at which you digest carbohydrates — in everything from bread and pasta to cakes and biscuits, jellybeans, sugar, honey and even fruit. Most carbohydrate must be digested and broken down into its simplest form, glucose, before it can pass through the walls of the intestine and get into your bloodstream to provide energy. If a carbohydrate is digested rapidly and creates a large increase in glucose levels in your blood, it is said to have a high glycaemic index, or high GI. If, on the other hand, a high-carbohydrate food takes a long time to be digested, before releasing a slow, gradual trickle of glucose into your blood, it is said to have a low glycaemic index (a low GI). The foundation of a low-GI diet is eating carbohydrates with a low GI.

Why does this help weight loss?

When you eat carbohydrates with a high GI, it sends your blood-glucose level soaring. Because high levels of glucose are dangerous (just like high blood pressure), your body instantly recognises the problem and releases a hormone called insulin, whose job it is to remove the excess glucose and restore levels to normal. This rapid removal of excess glucose has two effects. First, the glucose needs to be taken away and stored somewhere. It ends up in your muscles and liver, and, once they are full, it is converted into fat and dumped in your fat cells. Insulin opens the gates of these fat cells to allow this excess in. Second, the sudden drop in blood glucose (also known as blood sugar) leaves us craving more high-GI food for a further glucose hit. If you find that you are hungry again soon after eating a bowl of sugary breakfast cereal, or that one biscuit is never enough, you are on the blood-glucose rollercoaster. Your glucose levels soar after eating these foods, subsequently dipping sharply and sparking the need for another sugar rush. However, when you eat a low-GI carbohydrate, your blood-glucose level rises gently, which, in turn, leads to only a small rise in insulin. Paradoxically, while a big surge of insulin soon leaves you feeling hungry, a small rise in insulin will help to keep you feeling full. It also leads to the release of a fat-burning hormone called glucagon. In a nutshell, high-GI foods cause you to store fat and make you hungry: this triggers overeating. Low-GI foods help burn fat and keep you feeling fuller for longer: this helps you to eat less, so the pounds drop away effortlessly.

How can I tell the GI of a food?

It isn’t always obvious. However, there are some basic rules to give you an idea of whether a food is low or high GI.

Is it processed? The truer to their natural state the carbohydrates you eat, the more likely they are to be lower GI. The more extensively processed a food, the easier it is to digest, as the processing has already done a lot of the hard work that would have been left to your intestines.

For example, the flaked oats in muesli are still whole (they have not been cooked, nor had any sugar added), whereas in a breakfast cereal such as cornflakes, the corn has been ground, mixed with sugar and baked. Because they require little effort to digest, the cornflakes cause your blood-glucose levels to rise soon after eating, while the oats, in their more natural state, take longer to break down. This is why having muesli for breakfast will keep you feeling full all morning; after cornflakes, you’ll be ferreting around in the biscuit tin in a couple of hours.

Does a grain or pulse still have its outer coating? If the outer husk of a plant is still in place, it will have a lower GI. For example, multigrain bread contains whole grains with their fibrous outer husks intact. This makes them harder to digest than white bread, which has had the outer husk removed before milling.

Are the starch grains tightly packed, or open and fluffy? When starch grains are tightly packed together — as in pitta bread, tortillas or pasta, for instance — they are harder to digest than when they are open and fluffy, in, say, a french stick or a baked potato.

So can I eat as many low-GI carbs as I like and still lose weight?

No. Low-GI carbs help you to lose weight because they are
naturally filling and help you avoid the sugar rollercoaster that drives most people to overeat. But if you eat three bowls of porridge with multigrain toast for breakfast, lots of pitta for lunch and a huge serving of pasta for dinner, it will be hard to lose weight.

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