BEAT THE BONK by Jenny Brown

Carbohydrate is the fuel preferred by your muscles.  As JENNY BROWN explains, just like petrol there are grades of carbohydrate fuel – it depends on the GI rating.

 This article is reprinted courtesy Bicycling Australia magazine March 2002, Lake Wangary Publishing Co Pty Ltd.  Bicycling Australia is available at most newsagents and good bike shops and is published 6 times a year.

 Thanks to Jenny Brown for allowing us to reprint this article in the RBCC Web Site. Jenny is an Accredited Practicing Dietitian and Sports Dietitian with a private sports nutrition practice located at Winning Edge Nutrition, Busselton, Western Australia.  Her qualifications include a Bachelor Applied Science, Nutrition and Food Science and Postgraduate Diploma in Dietetics.

 Ride long enough and eventually you will bonk or go hunger flat – it’s when you have run out of fuel.  In January/February edition of Bicycling Australia, I discussed how to keep your muscles well fuelled with carbohydrates (carbs) to help you to train and compete longer and harder.  I gave you the tools to work out just how many carbs you need each day and where you could find them in foods.  The key point was making sure there is enough carbohydrate available at the right time to fuel your sport.

There is another factor in the carbohydrate story that has created a lot of interest amongst athletes and sports scientists alike in recent years.  It’s called the Glycaemic Index or GI.  The GI is a ranking of how quickly a carbohydrate food is digested and sent into the bloodstream as glucose (which is your muscles’ preferred fuel).

 Researchers have found that carbohydrates from some foods are quickly converted into blood glucose and transported to the muscles, while others are converted to blood glucose much more slowly. 

To test the GI of a food, researchers across the world have used real people and real foods to measure the rate at which blood glucose levels rise after a particular food is eaten, and how long it stays elevated.  This level is then compared to how much the blood glucose level rises after eating straight glucose, which has been given an arbitrary number of 100.  For example baked beans have a GI of 48.  If you ate enough beans to supply 50g of carbohydrate you would receive the same amount of carbohydrate as 50g of pure glucose, but spread over a much longer period of time.  They give a lower and more sustained supply of blood glucose than straight glucose.

A GI of less than 55 is considered low, between 55 and 70 is intermediate and above 70 is high.  Not all foods have been tested for GI as it is a time consuming and expensive process.  Different brands or foods will have differing GI’s.  However the number of foods that have been tested is gradually growing.  See the table in this article for examples of foods in each of the different categories.

What Determines GI.

GI research has given some surprising results.  For example potato and bread have higher GI’s than white sugar!  It is not possible to predict the GI of a food based on its chemical structure, but we do know some factors that effect GI.  Some examples are:

	The form of the food.  For example the size of the particles after they have been milled or processed.  Smaller particles take less time to be digested and generally result in a higher GI.
	The degree of processing or cooking – both of these increase GI.
	The way in which starch is structured.  The two forms of starch are called amylose and amylopectin.  Amylopectin has a number of chains of glucose so that the body’s digestive enzymes can break it down more quickly, whereas amylose is one long chain of glucose and it takes longer to be broken down.  Starches that have more amylose than amylocpectin will take longer to digest and send glucose into the blood stream more slowly.  This explains, for example, why the different varieties of rice have different GI’s.
	The interaction between carbohydrate and other food components, for example fat slows down digestion and generally lowers GI.
	The ripeness of the food.  For example riper bananas have a higher GI.

Of interest to people trying to control their weight, is that low GI foods are generally more satisfying and help you feel fuller for longer.  So you take longer to get hungry and eat again.  Compare how you feel after eating a bowl of Rice Bubbles (GI 83) and a bowl of porridge (GI 42).  They both supply the same amount of carbohydrate, but porridge ‘sticks to your ribs’ longer.

Fatty foods have only a weak effect on satisfying the appetite, relative to the number of kilojoules they provide.  Carbohydrate foods generally make you feel fuller and are less fattening!

Click here to find out how  to use the GI