What is the glycaemic index?
Originally designed for people with diabetes, the glycaemic index (GI) helps manage blood sugar levels. It is a useful tool both for those with diabetes as well as for those of us who simply want help with planning meals and making healthier food choices.
Carbohydrates are found in bread, cereals, rice, pasta, fruit, vegetables and dairy foods and are an important part of a healthy, balanced diet. The GI ranks these foods from 0-100 based on the rate at which the energy they provide is broken down by the body into glucose.
How do low-GI diets 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.
Glucose is the primary source of energy required by every cell of our body. 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 to a more manageable range.
Foods with low-GI ratings such as lentils, beans, wholegrains, nuts and seeds release their energy more slowly and help prevent sugar highs. 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, your body will have to work hard to manage this and, if you are unable to use it as energy, the body will store it for later use.
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.
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A sharp increase in blood sugar (glucose) triggers the pancreas to release more insulin to remove the excess glucose from the blood. At the same time it lowers the speed at which the body burns fat. Conversely, focusing on 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 may also encourage the body to burn fat.
What are low-GI foods?
Foods can be categorised as high, medium or low as follows:
• High GI – food with a GI of 70 or more
• Medium GI – foods with a GI of 56-69
• Low GI – foods with a GI of 55 or lower
What makes a food ‘low’ rather than ‘high’ GI?
The GI of a food is influenced by a number of factors including:
- The type of sugar it contains – e.g. fructose found in fruit has a low GI, whereas maltose found in bread has a high GI
- The structure of the starch the food contains – there are two main starches, amylose and amylopectin. Foods with a greater proportion of amylose, like lentils, have a lower GI than those with more amylopectin, like potatoes, which have a high GI
- Whether the food has been processed and the methods used. For example milling disrupts the starch molecules making them easier to digest and increasing the food’s GI score
- How you prepare the food and whether the food is eaten raw or cooked – if a food is cooked the sugars are digested and absorbed more quickly so the food in its cooked form will have a higher GI
- In the case of fresh produce like fruit, the riper the fruit the higher its GI.
Low-GI food list
• Non-starchy vegetables – asparagus, broccoli, cabbage, celeriac, lettuce, mushrooms, raw carrots, turnips
• Fruits – apples, avocado, berries, grapefruit, plums, peaches, tomatoes
• Beans & pulses –butter beans, kidney beans, pinto beans, chickpeas, lentils, peanuts
• Grains – barley, rye, wild rice and pseudo-cereals including quinoa as well as wholegrain breakfast cereals.
• Nuts – almonds, hazelnuts, pecans, walnuts and their oils
High-GI food list
• Starchy vegetables – potato (baked, mashed), frozen or canned corn, parsnips
• Fruit – dried dates, pineapple, overly ripe banana
• Grains - rice, instant cereal, tapioca and breakfast cereals such as cornflakes and rice snaps.
• Any foods that are white, including processed foods made from white flour and white sugar, including bread.
How do I follow a low-GI diet?
In order to keep meals and snacks low GI follow these tips:
• Select predominantly low-GI foods – these are typically brown (wholegrain) versions of carb foods like pasta, rice and crackers. Generally, the less processed a food, the more likely it is to have a low-GI score
• Avoid 'instant' or 'easy cook' foods, because these tend to be more highly processed
• Choose amylose-rich basmati rice instead of other varieties of white rice
• 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
• Snack on unsalted nuts, seeds or oatcakes rather than sweet treats and biscuits
• Always combine protein like fish, chicken and dairy with carb-rich foods like bread, potatoes and pasta.
Is a low-GI diet good for you?
Following a low-GI diet helps you manage your appetite and as a result may help manage weight. One reason for this is that 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 may also be helpful if you’re worried about your risk of type 2 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.
What’s the evidence for a low-GI diet?
Studies of low-GI diets have shown varied results, however, in general, they suggest a low-GI diet may be helpful for:
• Managing weight
• Lowering blood pressure
• Lowering total cholesterol levels
• Helping to manage blood sugar levels, especially in those with diabetes
• Lowering the risks associated with diabetes and cardiovascular disease.
Some experts believe the benefit of the diet may be linked to the nutritional quality, including higher fibre content, of the diet, rather than the specific GI value of the food items eaten.
Do low-GI diets work for weight loss?
There is evidence that following a low-GI diet, that accomplishes a significant reduction in GI, may be moderately effective in lowering body weight. However, compliance with the diet may be an issue in the longer term. Another drawback is that the GI score of a food does not consider the amount of the food eaten. This is important because the rate at which foods raise your blood sugar levels depends on the type of carb the food contains, its nutrient composition and the amount you eat. The glycaemic load (GL) was designed to resolve this – it is a measure of the GI of a food combined with the quantity (grams per serving) of the food consumed.
A nutritionist’s view:
Following a low-GI diet has a number of benefits including its ability to help manage blood sugar levels, aid weight management and lower the risk of chronic health conditions like heart disease and type 2 diabetes. From a practical perspective there is no counting calories or tracking the macronutrients in your diet. You simply swap high-GI foods for low-GI ones. That said, a healthy, varied and balanced diet should comprise a variety of whole, unprocessed foods, regardless of their GI score. Furthermore, the GI of a food doesn’t reflect all of a food’s health qualities or issues. For example, a low-GI food can still be high in fat or have high salt levels, but the low-GI diet doesn’t cater for this. The concept of low GI eating also doesn’t account for the amount of carbs eaten and doesn’t consider the impact of the other foods included in a meal. Finally, the scores given on GI lists do tend to vary; this is especially relevant for cereal foods including wholegrain and wholemeal versions. One reason for this may be manufacturing differences and, although data has improved over recent years, there is still room for improvement.
Does a low-GI diet work?
If you choose to follow a low GI diet, do so by including naturally occurring, minimally processed foods from each of the food groups – cereals, fruit and vegetables (including beans and pulses), as well as nuts, seeds and dairy foods. The foods you choose should also be lower in saturated fat and moderate in salt levels. Don’t forget the impact of preparation and cooking on GI as well as the influence of the other foods that make up your meal.
Think bowls of steamed greens and pulses, ratatouille or veggie soups. And for dessert, as tropical fruits have a moderately high-GI score, try citrus, stone fruits and apples, pears, berries and rhubarb.
Kerry Torrens is a qualified nutritionist (MBANT) with a post graduate diploma in personalised nutrition & nutritional therapy. She is a member of the British Association for Nutrition and Lifestyle Medicine (BANT) and a member of the Guild of Food Writers. Over the last 15 years she has been a contributing author to a number of nutritional and cookery publications including BBC Good Food.
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