Randwick Botany Cycling Club

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Beat the Bonk (Glycaemic Index explained)

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!

How to use the GI

So what does this mean for he cyclist who is aiming to keep their muscles well fuelled for enhanced performance?  This is an area that is creating a lot of interest in sports nutrition and most would agree that there is still a lot of work to be done.  However, based on current research, sports scientists have come to the following general conclusions on how GI could effect the athlete:

Before and event or training session

It is thought that a slow release, low GI carbohydrate meal before an exercise session of long duration may improve performance.  This is because the fuel is being released some time after the carbohydrate has been eaten.  This is most likely to be useful where a sustained release of fuel can’t be provided by intake during the session itself.

If you want to try having a low GI meal before an event to see if it works for you, you may find that you are more comfortable during the session if you choose foods that are not very fibrous, bulky of gas producing.  Examples of these include pasta with a low fat sauce, some varieties of rice (eg. basmati) with stewed apple, porridge with skim milk, Sustagen Sport or tinned spaghetti on grain or barley bread.

We know though that the key factor is that you get enough carbohydrate pre-exercise.  Other factors to consider in addition to the GI will be taste preference, nutritional value, cost, practicality and gastric comfort.

During an event or training session

Carbohydrate taken during a long ride makes more glucose available to the muscles and brain so that you can exercise longer and with a clearer head.  Carbohydrates that are easily digested, absorbed and released into the blood stream would seem to be a good choice.  This means food with an intermediate to high GI.  Most sport drinks and popular exercise snacks fit into this category.  The exception is fructose (the carbohydrate in fruits), which are easily absorbed, but takes some time to be converted into glucose.  Hence it is used sparingly in sports drinks and would not supply your muscles quickly if you were to choose fruit juice to drink during a long ride.  Low GI foods are not usually taken during sport, except in ultra endurance events.

During the recovery phase after an event or training session

There is good evidence that intermediate to high GI foods accelerate the rate at which glycogen is replenished, compared to low GI foods.  (Carbohydrate is stored in the muscle and liver as glycogen, which is lots of glucose units stored together.)  this is particularly important if you have another event or training session in the same day.

In addition to sports drinks and some of the snack type foods already mentioned, you may like to try such things as potatoes, high GI cereals, jam sandwiches o creamed rice.  To kick start the recovery process, you need to have at least one gram of carbohydrate per kilogram of body weight within 30 minutes of finishing your ride.  Low GI foods such as lentils and legumes are probably less suitable for speedy glycogen replenishment and should not be the main source of carbs in recovery meals.

If you have 24 hours or more before you have to get on the bike again, it is more important that you eat enough carbs rather than being unduly concerned about the GI of your meals.

Different GI Combinations

There is another factor that needs to be considered in any discussion of the GI factor.  What you eat before and after exercise is likely to be a meal, not a single food.  The GI of the whole meal is the approximate average of the GI of the individual ingredients.  For example if you has baked beans (low GI) and white toast (high GI) you would have an intermediate GI meal.  The proportion of carbs from food will effect the overall GI.  This is however only and estimate, as we know, since other components in the meal can alter the GI, as can the way in which the meal is cooked and processed.

In Summary

The GI is a measure of how quickly carbohydrate foods are digested, absorbed and supply glucose to the bloodstream and working muscles.  It is not a measure of the nutritional quality of a carbohydrate rich food, but a factor that is useful to consider when choosing foods to fuel your sport.  It’s probably more important that cyclists eat enough great tasting carbs each day rather than focusing solely on the GI of foods and meals.

The GI is an area of active research.  Sports scientists may have to refine their advice once more studies are done.  If you would like to do more reading on the subject of the GI and how it may impact on your cycling performance, the following books are comprehensive without being difficult to read:

The GI Factor.  Brand-Miller et al, 3rd edition, Hodder and Stoughton, 1998.

The GI Factor – Pocket Guide to Sports Nutrition.  O’Conner et al, Hodder and Stoughton, 1997.

Examples of the Glycaemic Index of Carbohydrate Rich Foods

HIGH GI >70

Glucose                                    100

Honey                                       87

Cornflakes                                 84

Puffed Wheat                             80

Rice Bubbles                             83

Sultana Bran                             73

Weetbix                                    70

Breakfast bars                           78

Bakes Potatoes (avg)                 88

Instant mashed potatoes            83

Sports drink                               95

Lucozade                                  95

Jelly beans                                80

Bread – white or wholemeal        70

Watermelon                               72

Rice, calrose (cooked)                87

Rice instant                               91

Race, Jasmine (cooked)             109

Morning Coffee Biscuits              79

Rice cakes                                82

Water crackers                          78

Saos                                         70

 

INTERMEDIATE GI (55 – 70)

One minute oats                        66

Just Right cereal                        60

Muesli, untoasted                      56

Nutrigrain                                  66

Sustain                                     68

Basmati rice (cooked)                58

Doongara rice (cooked)               59

Sweet potato                             54

Raisins                                     64

Sultanas                                   56

Muesli bars (avg)                        61

Soft drink – Coca Cola                63

Soft Drink – Faint                       68

Apricots (fresh)                          57

Banana (ripe)                             55

 

 

INTERMEDIATE GI (55 – 70) Cont.Paw Paw                                   58

Pineapple (fresh)                        66

Rockmelon                                65

Ice Cream (full fat)                      61

White sugar                               65

 

LOW GI (<55)

Porridge                                    42

Muesli toasted                           43

All Bran                                     42

Guardian                                   42

Orange Juice                             53

Pineapple Juice (unsweetened)   46

Sustagen Sport                          43

Apricots, dried                           43

Apple                                        36

Banana (unripe)                         30

Cherries                                    22

Orange                                      43

Pear                                         36

Peach                                       28

Bread – rye                               50

Bread – mixed grain                   45

Bread – white fruit loaf                47

Pasta – fettuccini (cooked)         32

Pasta – spaghetti (cooked)         41

Two-minute noodles                   46

Baked Beans                             48

Chick peas                                33

Lentils                                       28

Split peas                                  32

Mile, full cream                          27

Milk, skim                                 32

Milk, flavoured                            34

Yoghurt, low fat fruit                   33

Yoghurt, diet                              14

Custard                                     43

So Good, soy drink                    31

Fructose                                   23

 

 

 

Updated: December 15, 2013 — 5:21 pm
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