Glycaemic Index (GI) and Glycaemic Load (GL)

Both GI and GL refer to the speed and extent to which carbohydrate[1] from foods is converted into blood glucose[2] by the body. The key difference between GI and GL is that GL takes into account the typical amount of a food consumed in a meal, while GI does not.

What is Glycaemic Index (GI)?

- GI rates the speed and extent to which a food containing 50g carbohydrate is broken down into blood glucose against a 50g dose of pure glucose.

- i.e. 50g of pure glucose is benchmarked at 100 and other foods containing 50g carbohydrates are divided into low GI foods (= 55), medium GI foods (56-69) and high GI foods (= 70), by comparison.

- Examples of common low, high and medium GI foods are shown in table 1.

Table 1. Common foods and their GI values*

Low GI (= 55)	Med GI (56-69)	High GI (="70)
Lentils (30)	Apricot (57)	Watermelon (72)
Skim milk (32)	Pita bread (57)	Bagel (72)
Apple (32)	Basmati rice (58)	Brown rice (76)
Rolled oats porridge (42)	Banana (58)	Water crackers (78)
Apple muffin (44)	Shortbread (64)	Jelly beans (78)
Spaghetti (44)	Raisins (64)	Cornflakes (81)
Low-fat yoghurt (47)	Pineapple (66)	Baked potato (85)
Baked beans (48)	Sucrose (std. sugar) (68)	Parsnip (87)
Whole grain breads (49)	Wholemeal bread (69)	Rice bubbles (87)

* note that while one GI value is given to each food in this table, this is an average figure from a number of published studies. The GI of any particular food can vary within and between individuals. Often the degree of variation is very large - especially for foods in the medium and high category.

What is Glycaemic Load (GL?)

- GL is is the most up-to-date, accurate and easy-to-use measure for managing blood glucose levels. It is not as universally accepted as GI yet however, due to it being slightly more complicated to understand.

- GL is the percentage of the food’s available carbohydrate (CHO) content per portion, multiplied by its GI value[3] .

- eg, GL for an apple = CHO content of apple X GI of apple /100 = 13.9 X 32 /100= 4

- There are three categories of GL per serve - low (0-10), medium (11-19) and high (20 or more).

- Examples of common low, high and medium GL foods are shown in table 2. Note - these are the same foods as listed in table 1, but are not all in the same category as they were for GI (the shading indicates their GI ranking as per table 1).

Table 2. Common foods and their GL values*

Low GL (0-10)	Med GL (11-19)	High GL (20+)
Apricot (3)	Parsnips (12)	Spaghetti (21)
Apple (4)	Apple muffin (13)	Cornflakes (21)
Skim milk (4)	Water crackers (14)	Basmati rice (22)
Lentils (5)	Banana (18)	Rice bubbles (22)
Whole grain breads (6)		Jelly beans (22)
Pineapple (6)		Bagel (25)
Watermelon (7)		Baked Potato (26)
Baked Beans (7)		Raisins (28)
Sucrose (std. sugar) (7)		Brown rice (29)
Wholemeal bread (8)
Rolled oats porridge (9)
Low-fat yoghurt (10)
Pita bread (10)
Shortbread (10)

* note that while one GL value is given to each food in this table, this is a figure derived from average GI figures published in a number of studies. It is important to remember that the GI of any particular food can vary within and between individuals. Often the degree of variation is very large - especially for foods in the medium and high category.

Benefits of GL over GI

- GL takes into account the amount of carbohydrate in a typical food, as well as the GI of the food. In this way it measures both the quality and the quantity of carbohydrates within a certain food.

- GL therefore more accurately reflects the impact on blood glucose of a typical serving portion of any food. GLs can also be easily calculated for less typical portion sizes, provided the GI of the food is known.

- GI is only useful to compare foods that have similar amounts of carbohydrates in them. Therefore, if you are comparing the effect of a white bread with a wholegrain bread for example, GI can be a useful measure because the one with the higher GI will have a higher blood glucose response. However, when comparing foods with different levels of carbohydrate, it can be misleading. For example both apricots and bananas have medium GI levels, but because a banana has six times the amount of carbohydrate in an apricot, it will have six times the blood glucose response. GL accounts for these differences as a banana has a GL of 18 and an apricot has a GL of 3.

- Because GI is a ratio, it remains the same for a given food, no matter how much of it you eat. So, for example, the GI remains the same whether you eat one muesli bar or three. Obviously if you eat three times the amount, it will have three times the blood glucose response. This is reflected in the GL of the food, where if you eat three muesli bars instead of one, the GL triples accordingly.

Individual food vs meal

- using GL it is easy to calculate how a whole meal or snack will influence blood glucose levels, by adding up the GL of each food portion

- it is not possible to do this with GI because people generally don’t consume foods that contain exactly 50g of carbohydrate at a time

- therefore GL can be used as a guide to choosing a variety of food combinations to ensure that blood glucose levels remain in the healthy range for a longer time.

- It is very important for blood glucose levels to remain in the healthy range for as long as possible

- people with diabetes in particular need to monitor how different foods and meals effect their blood glucose levels

- very active people who take part in endurance sports also need to ensure that their blood glucose levels can sustain them during prolonged sporting events.

Examples of practical application of GL over GI

- the GI of watermelon is high (GI = 72), but its GL is low (GL = 7), because the quantity of carbohydrate in a serving of watermelon (150 g or a 5 mm thick slice) is minimal, as it contains a lot of water.

- Pineapple and raisinsboth have medium GIs of 59 and 64 respectively. However, an average serve of raisins will raise blood glucose levels four times higher than an average serve of pineapple because raisins contain significantly more carbohydrate per serve. Therefore a serve of raisins has a GL of 28 and a serve of pineapple has a GL of 7.

Sugar and GI/GL

Understanding of GI and GL shows us that it is possible to include sugar in a meal or snack and still have a relatively low impact on blood glucose levels, provided the bulk of the food consumed contains lower GI carbohydrate foods (vegetables, fruit, low GI starches, legumes and/or dairy). Thereby you can include sensible amounts of sugar in your diet without it significantly increasing the GL of certain meals or snacks. People have the power to lower the GL of their diet and still include sugar together with lower GI foods in a meal or snack.

- White sugar (sucrose) has a medium GI value of 68 but because it is usually consumed in small quantities it has a low GL value of 7. When sugar is included in a mixed meal or snack containing other foods, it can have an insignificant effect on blood glucose levels compared with the impact of other foods in the meal.

- eg, low fat yoghurt containing fruit and sugar has a GL of 10. Including yoghurt in a lunch consisting of a portion of meat filled ravioli and an apple would add to a combined GL total of 29 for the whole meal.

By comparison, including yoghurt in a lunch consisting of a portion of stir-fried vegetables, chicken and white rice and a banana would add to a combined GL total of 77 for the whole meal.

Who should use the GI/GL concept?

- Traditionally the GI and GL concept of eating has been used by people with diabetes who need to control their blood sugar levels closely. More recently however, this way of eating has become more popular with other groups such as the weight conscious and athletes.

- The GI/GL concept is therefore relevant to a wide variety of people.

GL, satiety[4] and weight management

- Low GL carbohydrate-containing meals help the average person to fell fuller for longer, so they can be an effective form of appetite control to limit snacking between meals.

- eg, a high GL breakfast of cornflakes and milk is likely to leave you feeling peckish at morning tea time. A low-medium GL breakfast of porridge made with rolled oats, a little brown sugar and milk, while containing the same level of energy, is more likely to keep you feeling full until lunch time due to the slower release of glucose in the bloodstream.

GL and sports

Sportspeople find that including combinations of foods from the moderate and high GI and GL group at certain times during and after events or training can help them to maintain better energy levels.

Pitfalls of GI and GL

Many nutritionists and health professionals are still uncertain about the value of GI and GL as a “magic bullet” approach to managing diabetes and weight loss. The reasons for such uncertainty are:

- the GI value of different foods provided here and in published papers are just average figures. Any one individual may have quite a different blood glucose response to a certain food (especially those with higher GI values) than another individual. The same people are also known to have different blood glucose responses to the same food at different times. Because the GI is required for calculation of the GL, the same is true for GL.

- the GI and GL of foods do not in any way reflect their energy content. It is only a measure of the speed and extent by which carbohydrates in a food are converted into blood glucose. Therefore low carbohydrate, high fat and protein foods are not relevant for the GI scale. High fat foods in particular, should only be consumed in moderation due to their high energy content. The fact that they have a low GI is irrelevant to this point.

- the GI and especially the GL concepts are difficult to understand, and health professionals find it difficult to explain the detail of this concept to their clients in a simple way, other than providing lists of low GI/GL foods and asking people to include one of these foods per meal.

Q&A's

17. “If wholegrains are supposed to have a lower GI, why is brown rice in the highest category for both GI and GL?”

1. Different varieties of rice have different impacts on blood glucose levels. It just so happens that the type of brown rice most commonly consumed in Australia and New Zealand has a more marked effect on average than other rice varieties (eg, basmati). This does not mean however, that all types of brown rice will have the same effect. The figures quoted here relate specifically to Pelde brown rice grown in Australia, but other published figures for brown rice grown in India and North America have much lower GI levels (50-66) and GL levels (16-21).
   
   

17. “Is it good to follow a diet made up of solely low GI foods?” or “If I eat a low GI diet will it be healthy?”

A. No. Moderate and high GI foods are fine to consume in moderation, but you should try to include a low GI/GL food each meal - particularly if you are trying to lose weight or if you have diabetes. You also need to consume a wide variety of foods from all four food groups that contain some fat and protein. High fat foods in particular, should only be consumed in moderation due to their high energy content. The fact that they have a low GI is irrelevant to this point.

Useful weblinks on GI/GL

http://www.ajcn.org/cgi/content/full/76/1/5 a published paper listing GI and GL values for common foods. (Foster-Powell K, Holt SHA, Brand-Miller JC. International table of glycaemic index and glycaemic load values: 2002. Am J Clin Nutr 2002; 76: 5-56.)

http://www.health.harvard.edu/hhp/article/content.do?name=WN0104d contains GL values for many common foods

http://www.glycemicindex.com/ good information about both GI and GL, dietary recommendations, the GI symbol etc.

http://www.diabetes.org.nz/food/gindex.html is a New Zealand site containing information on the GI.

[1] Carbohydrate is a type of nutrient in food that our body uses as fuel. The other main nutrients of this type from foods are protein and fat. Carbohydrate from food is broken down in the intestine and converted into glucose (small carbohydrate molecules).

[2] Blood glucose (or blood sugar, or glycaemic) is formed by the breaking down of carbohydrates in food. The blood transports glucose molecules to individual cells to use or store for fuel.

The normal range of blood glucose levels is between 4-7 mmol/L. People with diabetes and pre-diabetes may often experience blood sugar levels outside of this range. Your blood sugar should not be too high or too low, so it is important to maintain blood glucose at a healthy level. Low blood glucose levels results in lack of energy, disorientation and even unconsciousness, however this usually only occurs in people with diabetes.

While it is normal to have higher blood glucose levels after a meal, continuously high levels puts extra strain on the body and is linked to adverse health status such as the development of diabetes and impaired glucose tolerance.

[3] GI and GL values for many foods are available for free at http://www.ajcn.org/cgi/content/full/76/1/5 .

[4] Satiety is defined as the feeling of fullness or satisfaction after eating or drinking.