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
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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) .
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:
Note that each unit of GL produces the same effect on blood sugar as eating 1g of pure glucose.
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.
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