Glycemic Index (GI) and why it is important to what you eat

The glycaemic index (GI) was originally designed for people with diabetes to help keep their blood sugar levels under control. But whether you are diabetic or not the GI is a useful tool for all of us when we are planning healthy meals and making food choices.

Glucose is the primary source of energy required by every cell in the body. The GI ranks carbohydrate foods based on the rate at which they are broken down into glucose. When glucose levels in the blood start to rise, the pancreas releases a hormone called insulin which promotes the take up of glucose by the cells and as a consequence brings blood sugar levels back into a more manageable range.

How does it work?

The GI is a measure of the rate at which our bodies break down the carbs in our food to energy, in the form of glucose. The speed at which this digestion occurs, and the amount it raises glucose levels in the blood is measured by a score on the GI scale. Glucose, is the reference point for all other foods and carries a score of 100. Foods with low GI ratings such as lentils, beans, wholegrains, nuts and seeds release their energy more slowly and help prevent sugar highs.

What makes a food low rather than high GI depends on the proportion of a type of starch, amylose, to another, amylopectin. Foods with a greater proportion of amylose such as lentils have lower GIs than those with more amylopectin, like potatoes, which have a high GI.

If you typically eat a lot of high GI foods such as white bread, processed breakfast cereals, cakes and biscuits you will have a lot of readily available energy in your blood, and your body will use this energy rather than turning to your fat stores.

How low-GI foods can speed up fat burning.

A sharp increase in glucose in the blood triggers the pancreas to release a rush of the hormone insulin, which removes any excess glucose. Insulin removes the surplus glucose from the blood and lowers the speed at which the body burns fat. A large surge in insulin, caused by eating high-GI foods, will start reactions in the body that leave you feeling lethargic, hungry and craving more sugar.

Eating low-GI carbohydrate foods causes a steady rise in the level of glucose in the blood, which in turn leads to a small and gentle rise in insulin. Small increases in insulin keep you feeling full and energised for hours after eating and also encourage the body to burn fat.

  • Foods with a GI of 70 or more are typically called 'high-GI foods' as they trigger a rapid rise in blood sugar.

  • Foods with a GI of 55-69 are 'medium-GI foods' as they trigger a moderate increase.

  • Foods with a GI below 55 are 'low-GI foods' because they have a minor impact on blood sugar.

Basic principles

Low-GI foods provide natural, slowly released energy. Generally, the less processed a carbohydrate, the more likely it is to have a low-GI score. Foods that are white, including processed foods made with white flour and white sugar, tend to have a high-GI. High fibre foods take longer to digest and therefore produce a slower rise in blood sugar levels. Fibre also keeps you feeling fuller for longer, which helps prevent overeating. Most vegetables, wholegrains, legumes, nuts, seeds and fruits are rich in fibre when you eat them whole.

How GI scores vary

A food's GI is not fixed. It will vary depending on a number of things - how the food has been prepared, whether it has been cooked, how hydrated it is and in the case of fresh produce like fruit, how ripe it is. An average serving of raw carrot, for example, has a GI of 16 but once peeled, diced and boiled this rises to 49. Your glycaemic response to a food depends on the other foods you eat with it

What you choose to eat with your carb-rich food will also impact the overall GI of your meal, so by combining a high GI food like potatoes with some protein like chicken will mean you lower the overall GI of your meal. When a meal includes proteins and fat the impact of the carbohydrate foods will be minimized. This is because by combining foods in a single meal the overall impact is to slow down the rate at which your body releases sugar from any single ingredient.

Follow these guidelines to keep meals and snacks low GI:

  • Choose brown (wholegrain) versions of foods like bread, pasta, rice and crackers.

  • Always combine protein like fish, chicken and dairy foods with carbs like bread, potatoes and pasta – for example when snacking combine a handful of nuts (protein) with a piece of fruit (carbs).

  • Use new potatoes instead of old and boil in their skins rather than mashing, baking or chipping.

  • Thicken sauces using a little tahini or nut butter rather than high GI cornflour.

  • Choose amylose-rich basmati rice instead of other varieties of white rice.

  • Avoid 'instant' or 'easy cook' foods which tend to be more highly processed.

  • Snack on unsalted nuts, seeds or oatcakes rather than sweet treats and biscuits.

What are the benefits of eating this way?

A positive side effect is that you may lose weight following a low GI eating regime that’s because these sorts of foods tend to keep you feeling fuller for longer. Although it’s worth remembering that low GI does not mean low fat, so you may need to watch the fat content of your meals.

A low GI eating plan can also be helpful if you’re worried about your risk of type II diabetes and heart disease that’s because a low GI diet improves blood sugar and insulin control and helps manage cholesterol levels.

The effect of stabilising blood sugar levels should also mean you feel improvements in energy, mood and concentration levels.

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