Author Nina Evans, Researcher & Trainer in Public Health/28 February 2019
After consuming a meal, the available carbohydrates are absorbed into the blood stream, increasing blood glucose levels. However, not all carbohydrates are equal in the way they are digested, absorbed and metabolised and because of this, they impact blood glucose levels differently. Depending on their effect on post-prandial blood glucose, carbohydrate-containing food can be ranked by using the Glycaemic Index (GI) or Glycaemic Load (GL).
What’s Glycaemic Index?
Glycaemic Index is a ranking system that was invented in the early 1980s in Canada to use as a supplementary tool alongside carbohydrate exchanges (an amount of food that contains approximately 15g carbohydrate). It was developed to help improve blood glucose management.
GI assigns a numerical score (on a scale of 0 to 100) to food containing carbohydrate, based on how quickly the food affects blood glucose levels. It is calculated by using an international standard method; feeding 10+ healthy people a portion of food (either test or reference) containing 50g of available carbohydrate and measuring the effect on blood glucose levels over the next two hours. The GI is then calculated as the incremental area under the curve (AUC) after the test food is eaten, divided by the AUC after the reference food (usually pure glucose), multiplied by 100. This gives the test food a numerical score which is presented as a percentage of the response to the same amount of carbohydrate from a reference food.
If carbohydrate foods cause a sharp increase and rapid decline in postprandial blood glucose, they have a high glycaemic index, classified as 70 or over. Examples of high-GI (quick releasing) carbohydrates are bread (white/brown), Cornflakes™, rice (white/brown) and potatoes (baked, mashed, chips).
Medium GI foods are those which have an index score of 55-69. Examples are sugar, jam, Shredded Wheat™, scones, and crisps. Slow releasing carbohydrates are muesli, porridge, yoghurt, pulses, and dried fruit as they have a glycaemic index score of less than 55. As a general rule, carbohydrate food that contains a high amount of fibre, fat and/or protein are more likely to have a lower GI.
GI identifies how quickly a carbohydrate food impacts blood glucose levels but it doesn’t take into account the quantity of food consumed. This is where the glycaemic load comes in.
What is Glycaemic Load?
The GL considers the GI and how much food is in a serving, it can be calculated using direct or indirect methods. The indirect method of determining GL is to multiple the GI of the food by the weight of available carbohydrate/given amount of food. GL can also be measured directly using a standard glucose curve.
Glycaemic load is also classified as low, medium or high. A low glycaemic load is less than ten; medium is between ten and 20; high is over 20.
The glycaemic load was developed with the intention to give a more accurate picture of a food’s ‘real-life’ impact on glycaemic response in comparison to GI.
Are they both useful?
Glycaemic index has been used as an additional resource to guide food choice, principally used by people who have been diagnosed with diabetes to assist with blood glucose management. This is because low GI foods are less likely to cause a rapid increase in blood glucose levels and are therefore a better option at keeping blood glucose levels stable. A low GI diet has also grown in popularity as being an effective way to help lose weight by controlling appetite, resulting in people feeling fuller for longer.
However, the glycaemic index ranking system hasn’t come about without criticisms. Doubt has been cast upon its validity due to factors which can modify index values such as food variety, cooking method, processing and ripeness (of fruit/certain vegetables). Additional factors such as individual characteristics cannot be considered using GI, e.g. metabolism or eating style (e.g. degree to which we masticate).
One of the main criticisms of following a low GI diet is the implication that all food with a low GI is healthy. For example, chocolate is a low-GI food, this (unfortunately) doesn’t mean it is a healthy or a great choice in terms of nutritional value. Another criticism is that GI rankings for mixed meals are usually based upon a calculation of singular food items. We know most foods are not eaten in isolation, therefore it may be inappropriate to calculate the average GI of a meal based on individual food items.
Despite the limitations, making swaps from higher to lower GI alternatives may be beneficial for those diagnosed with diabetes, conscious of losing weight or wanting to improve certain health markers. Similarly with glycaemic load, whilst glycaemic load may not be appropriate for everyone (especially due to the amount of calculation involved), it can still be a useful tool for people with diabetes to assess the type and serving size of carbohydrate-containing food that might be more suitable for maintaining blood glucose control. Even though GI/GL may be a useful resource, neither ranking system should be relied upon as the only resource when deciding food choice.
Overall, using either the GI or GL ranking system may have some beneficial effect on overall health and glycaemic control. However, the amount of carbohydrate consumed is the most important factor, especially when considering the influence on post-prandial blood glucose levels, rather than GI or GL alone.
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