Glycaemic Index

Following training and competition, an athlete's glycogen stores are depleted. To replenish them, the athlete needs to consider how carbohydrate is converted into blood glucose and transported to the muscles. The rapid replenishment of glycogen stores is vital for the track athlete who has many races in a meeting. The food Glycaemic Index (GI) indicates the rise in blood glucose levels is characterised by the food's Glycaemic Index (GI), and the faster and higher the blood glucose rises, the higher the GI. Studies have shown that consuming high GI carbohydrates (approximately 1grm per kg body) within 2 hours after exercise speeds up the replenishment of glycogen stores and therefore speeds up recovery time.

The six elements to determine the GI of a food

Does it contain carbohydrates?

Pure protein foods, such as meat, fish, poultry and eggs, and pure fats, such as oils, butter and margarine, contain no carbohydrates. As a result, the effect they have on glucose production is negligible. These foods are therefore low Gl.

How much starch does it contain, and in what form?

The easiest ingredient for our body to convert into glucose is starch. When foods are raw, this starch is generally found in hard, compact particles that the body finds hard to break down. However, if something disturbs these starch particles (e.g. milling into flour), the body finds it much easier to digest, turning into glucose faster.

How much fibre does it contain?

Fibre slows the time it takes the body to break down food. One reason beans and pulses (wrapped in a fibrous shell) have such a low Gl.

What kind of sugar does it contain?

There are four main types of sugar, and they raise blood sugar levels at different rates. Foods with high glucose concentration (such as sports drinks) need no conversion, so they raise blood sugar rapidly. However, fructose (the sugar in fruit) converts slowly, as does lactose, the primary sugar in dairy products. This gives the majority of foods containing either fructose or lactose a low Gl. The fourth sugar, sucrose, has a medium Gl.

Impact on Glucose

Fat slows the speed at which food leaves the stomach and reaches the liver, slowing glucose production.

How acidic is it?

Foods can contain acid ingredients - citrus fruits like oranges or lemons are a good example. The tang they create on your tongue comes from the citric acid they have. Other acidic ingredients include lactic acid in milk products and added ingredients, such as vinegar, in pickled products. Like fat, acidity slows a food's progress through the system and, therefore, the rate it converts into glucose.

GI rating for some common carbohydrates

A Glycaemic Index of less than 55 is considered low, 56 to 69 Medium and greater than 70 is high. Values will vary depending on brand, variety, ripeness, preparation etc. The following table contains the GI for a selection of foods (Hamilton 2005)^[1].

All-Bran 43
Apple 37
Apple juice (clear) 44
Apricot (dried) 30
Apricot (jam) 55
Apricot (tinned) 64
Baked beans (tinned) 46
Banana (ripe) 58
Banana (unripe) 30
Beetroot 64
Butter beans 31
Carrots 51
Cashews 22
Cherries 22
Chickpeas 33
Chocolate 49
Cornflakes 81
Croissant 69
Dark rye bread 76
Dates (dried) 72
Digestive biscuit 60
Doughnut 76
French baguette 68
Fructose 46
Glucose 100
Grapefruit 25
Grapes 48
Hazelnuts 33
Ice cream 61
Jellybeans 80
Kidney beans 28
Kiwifruit 53
Lentils 28
Mango 56
Mars bar 65
Milk (full fat) 27
Milk (skimmed) 32
Mixed grain 49
Muesli 58
Oat bran 50
Orange 44

Orange juice 55
Parsnips 68
Pineapple 66
Peach 42
Peanut butter 29
Peanuts 22
Pear 36
Peas 48
Pineapple juice 46
Pinto beans 40
Pitta bread 58
Plums 32
Popcorn 55
Porridge 46
Potato (boiled or mashed ) 74
Potato (jacket baked) 72
Potato crisps 54
Potato: new 62
Puffed Wheat 80
Raisins 64
Rice Crisps 83
Rich Tea biscuits 57
Rye bread 65
Shredded Wheat 70
Sourdough 57
Soya beans 20
Spaghetti (white) 43
Spaghetti (wholemeal) 39
Special K 54
Split peas 32
Strawberry 32
Sultanas 57
Swede 72
Sweet corn 55
Sweet potato 54
Table sugar 65
Tomato juice 38
White bread 70
Wholemeal bread 69
yoghurt (low-fat, sweetened) 33
yoghurt (low-fat, unsweetened) 14

Glycaemic Load

While GI is a very beneficial concept, it cannot be taken as the sole predictor of the effects of eating a particular type of carbohydrate. That is because blood glucose response is also determined by the amount of food consumed. A more reliable rating system is the 'glycaemic load' (GL), which considers both the quality (the GI value) of a given carbohydrate and the amount consumed, more accurately predicting its effects on blood sugar.

The glycaemic load, in units, of a portion of carbohydrate is expressed as:

GI rating x grams of carbohydrate in portion size / 100.

Note that each unit of GL produces the same effect on blood sugar as eating 1g of pure glucose.

A 120g banana contains around 24g of carbohydrates, with a GI value of 58.
The GL is: (58 x 24) / 100 = 13.92 units.
120g of chocolate provides 75g of carbohydrate, a GI value of 49
The GL is: (75 x 49) / 100 = 36.75 units.

By totalling the GL units for foods you eat during the day, you can arrive at an overall GL for the day. A Glycaemic Load of fewer than 80 units is considered Low, 80 to 120 units are Medium and greater than 120 units are High.

References

HAMILTON, A. (2005) The glycaemic index: how athletes can make it work for them. Peak Performance, 217, p. 1-4

Page Reference

If you quote information from this page in your work, then the reference for this page is:

MACKENZIE, B. (2001) Glycaemic Index [WWW] Available from: https://www.brianmac.co.uk/gindex.htm [Accessed