Glycaemic index & Glycaemic load
This fact sheet aims to provide information on GI & GL and what they may mean for you.
What is GI (Glycaemic Index)?
GI is related to carbohydrates, which supply energy to our bodies. Whenever you consume something starchy or sugary, glucose in the blood increases. Sugars (both natural and free) are more readily absorbed than other carbohydrates and are often shown to have a high GI.
GI is a measure based on blood glucose levels over a period of around 2 hours after consumption of a food containing carbohydrate. The GI measure wasdeveloped in 1981 for diabetic patients to help them avoid foods providing rapid increases in blood sugars after a meal. High GI foods are those with a GI of 70 or above and low GI foods, 55 or less. In general, the lower a food’s GI, the slower blood sugars rise after consuming that food. The more processed a food is, the higher its GI is likely to be, and the more fibre, protein or fat in a food, the lower its GI is likely to be.
High GI foods are not necessarily “bad”foods and low GI foods are not necessarily “good” foods; for example, potatoes have a high GI, but they also have many essential nutrients and contain fibre, especially when eaten with the skin (eating with skin reduces the GI), whereas ice-cream has a medium GI, but is higher in fat and free sugars. GI is only one measure for foods, it does not give a complete indication of how high blood sugar could rise when a food is consumed, because it does not consider the amount of carbohydrate consumed. To identify a food’s full effect on blood sugar levels, we need to calculate the Glycaemic Load (GL).
What is GL (Glycaemic Load)?
Glycaemic load (GL) is the GI of a food multiplied by the amount of carbohydrate in the portion of the food being consumed: GL = GI x carbohydrate in portion size (g). For example, Watermelon has a high GI (72), but a 100g serving of watermelon has little carbohydrate, so its GL is only 5.GI & GL are useful measures for diabetics but are only part of the picture; identifying the total amount of carbohydrate in the diet and staying a healthy weight are also important for blood sugar and overall health.Consideration should also be given to fats and overall energy (calories) consumed to maintain an overall healthy diet.
Some foods will have labels to identify low GI foods however, GL is usually not indicated, and many foods do not have a GI label at all. Many packaged foods have “traffic light” labels to help consumers identify the amount of energy, fat (total and saturated), sugars and salt (per 100g of product) in the food. If a “portion” is bigger than 100g for a food (or 150ml for a drink), there are additional criteria: if more than 30% of the recommended reference intake is in one portion, it will automatically be high/red.
Check before you buy! Remember to look at all highlighted nutrients; buying a product green for one nutrient but red for the rest, will not be a healthy choice. When trying to identify low GI foods, the “sugars” part of the traffic light label can help – it is more likely that the GI is low if the sugars section is green for a healthier choice.
There are books and websites that provide GI values for many foods however, GI compares the glycaemic effect of an amount of food which has 50g carbohydrate, but we eat differing amounts of different foods which contain different amounts of carbohydrate; this makes identifying the foods producing the lowest sugar spikes more difficult. GI values given are for foods eaten on their own however, meals usually have a combination of foods which all affect the overall GI & GL of the meal e.g. potatoes are likely to be eaten with meat and vegetables.
What affects GI?
- Cooking methods (adding any heat to grains will increase GI as the grains are broken down).
- Ripeness and processing of fruit and vegetables (ripe fruits have more sugar than unripe ones; also processing releases free sugars, so the GI will go up).
- Wholegrains and high-fibre foods slow carbohydrate absorption (not the same with “wholemeal”, as the wholegrain has been ground; some mixed grain breads, that include wholegrains, have a lower GI than wholemeal bread).
- Fat lowers the GI of a food e.g. in chocolate and potato crisps.
- Protein lowers the GI of a food. Most dairy products have a low GI due to their high protein content; also because they contain fat.
Potential effects of low GI?
Choosing low GI foods as part of a balanced diet, can help to minimise sugar spikes in the blood; this can help reduce risks of complications from diabetes in the long term e.g. heart disease and kidney problems. There is also some evidence that preventing intense sugar spikes in the blood could help control appetite and sugar cravings.
Using GI as a method for making food choices is not straightforward; while incorporating low GI foods into your diet may be beneficial, it is still best to focus on consuming an overall healthy and balanced diet based on the Eatwell Guide. If you are diabetic, seek advice from your GP or diabetes counsellor.
FoodSwitch UK is a free app which allows you to scan food and drink barcodes to immediately see whether they are high, medium or low in total fat, saturated fat, sugars and salt. FoodSwitch UK also looks for similar, but healthier, alternative products; this provides an easy means of making healthier food choices. Find out more.