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