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Glycaemic Index

Following training and competition, an athlete's glycogen stores are depleted. In order to replenish them the athlete needs to consider the speed at which carbohydrate is converted into blood glucose and transported to the muscles. The rapid replenishment of glycogen stores is important for the track athlete who has a number of races in a meeting. The rise in blood glucose levels is indicated by foods 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 carbohydrate?

Pure protein foods such as meat, fish, poultry and eggs, and pure fats such as oils, butter and margarine, contain no carbohydrate. 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 them and they turn into glucose faster.

How much fibre does it contain?

Fibre slows the time it takes the body to break down a food. This is one reason why beans and pulses (which are 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 a high concentration of glucose (such as sports drinks) need no conversion, so they raise blood sugar rapidly. Fructose (the sugar in fruit), however, converts slowly; as does lactose which is the main 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.

Does it contain fat?

As well as having no effect on glucose itself, fat slows the speed at which food leaves the stomach and reaches the liver, slowing glucose production. This is the reason why potato crisps have a lower Gl than most other types of potato.

How acidic it?

Foods can contain acid ingredients - citrus fruits like oranges or lemons are a good example of this. The tang they create on your tongue comes from the citric acid they contain. Other acidic ingredients include lactic acid in milk products, and added ingredients, such as vinegars, in pickled products, just like fat, acidity slows a food's progress through the system, and therefore slows the rate at which 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
  • Jelly beans 80
  • Kidney beans 28
  • Kiwi fruit 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
  • yogurt (low-fat, sweetened) 33
  • yogurt (low-fat, unsweetened) 14

Glycaemic Load

While GI is a very useful 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 eaten. A more reliable rating system is the 'glycaemic load' (GL), which takes account of both the quality (GI value) of a given carbohydrate and the amount consumed, so 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 carbohydrate, which has a GI value of 58.
    The GL is: (58 x 24) / 100 = 13.92 units.
  • 120g of chocolate provides 75g of carbohydrate, which has a GI value of 49
    The GL is: (75 x 49) / 100 = 36.75 units.

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


References

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

Related References

The following references provide additional information on this topic:

  • FOSTER-POWELL, K. et al. (2002) International table of glycaemic index and glycaemic load values: 2002. The American journal of clinical nutrition, 76 (1), p. 55-56
  • BJORCK, I. et al. (2000) Low glycaemic-index foods. British Journal of Nutrition, 83 (S1), p. S149-S155

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

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