What is carbohydrate?
- Carbohydrate provides the most readily available superior source of fuel or energy for muscles to use in moderate to high intensity exercise.
- Even when the body starts to use fat as an energy source in endurance exercise carbohydrate must still be present.
- Carbohydrate is broken down by the digestive system and carried around the body as blood glucose.
- The glucose is stored in muscle and the liver as glycogen to be used by the body during activity.
- Glycogen in the liver is released into the blood to maintain normal blood glucose levels.
- Blood glucose is used by muscles and the brain. The brain relies on blood glucose to function properly.
- Muscles use stored glycogen as their primary energy source with blood glucose as an additional source.
- When muscle glycogen becomes low, fatigue occurs in the muscles being used. Muscle glycogen depletion occurs after 2-3 hours of continuous training at low intensity.
- Symptoms of hypoglycaemia include poor coordination, light headedness, inability to concentrate and weakness.
What is bonking?
When blood glucose becomes low (hypoglycaemia), marathon runners complain of “bonking or ‘hitting the wall”; they no longer have enough glucose in their blood for their brain to function properly.
Did you know that the term “bonking” comes from the sport of cycling? When blood glucose levels got too low, riders fell off their bikes and “bonked” their heads on the ground!
To read more about the science behind bonking click here
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The Glycaemic Index
The glycaemic index (GI) ranks carbohydrate foods according to how quickly they release glucose into the bloodstream. Foods are ranked on a scale of 0-100. Glucose has a ranking or 100 as it enters the blood quickly. Dried peas and beans release glucose into the blood slowly and have a low glycaemic index ranking. Other foods fall in between these extremes. The glycaemic index can be used to help athletes choose foods that will be of most benefit at specific times of their training and performance:
- Foods with a glycaemic index ranking of 71-100 are called high GI foods. They increase blood glucose levels quickly, providing a fast source of blood glucose for muscles to take up and use. These foods are good to eat during and after exercise.
- Intermediate glycaemic index foods have a ranking or 55-70 and provide a moderate release of glucose into the blood. These foods are also good during and after exercise.
- Low glycaemic index foods are ranked 0-54 and release glucose into the blood slowly over a longer period of time. These foods should be included in main meals in the marathon training diet and may be useful before exercise.
Examples of foods that have a low, intermediate or high glycaemic index
|mixed grain bread||muesli bars||bagel|
|oat bran bread||crumpet||white bread|
|fruit bread||pita bread||calrose rice|
|porridge/oatmeal||brown rice||baked/mashed potato|
|bran fibre cereal||beetroot||pumpkin|
|long grain white rice||new potatoes||swede|
|pasta||ripe banana||broad beans|
|instant noodles||kiwifruit||sports drinks|
|peas||pineapple||jelly beans & similar sweets|
Have you heard about simple and complex carbohydrate?
Using the terms simple and complex carbohydrate is an old way of grouping carbohydrates according to their pre-digestion chemical structure.
Examples of simple carbohydrate include fruit juice, dried fruit, sports drinks, milk, sugar, jam, honey, soft drinks and cordials.
Examples of complex carbohydrate include pasta, rice, breads, breakfast cereals, and some fruits and vegetables.
It was thought that due to their chemical structure simple sugars caused a rapid rise in blood glucose while complex carbohydrates caused a more gradual release of glucose into the blood. Recent research has shown that the rise in blood sugar after eating food can also be affected by the type of sugar or starch in the food, the processing or cooking of the food, fat and fibre content and ripeness of the food. It is more useful to rank carbohydrate foods according to the glycaemic index, which takes these factors into account.
Why do you need carbohydrate?
Carbohydrate is the superior energy source for marathon runners. Carbohydrate is stored in limited amounts in your muscles and liver. The amount of carbohydrate-containing foods you eat can influence the amount of glycogen stored in your body. If you start training with high amounts of glycogen stored in your muscles and liver you will be able to train harder and for longer. This applies whether doing intense training for a short time or lower intensity training for a longer time. Consuming carbohydrate during exercise will provide blood glucose as an energy source. Carbohydrate foods after exercise will maximize recovery and bring glycogen stores back to high levels for your next training session. You need carbohydrate to:
- Store optimal levels of muscle and liver glycogen before exercise, providing energy and delaying fatigue.
- Keep your blood glucose levels topped up during exercise to further delay fatigue.
- Replace used up muscle and liver glycogen after exercise so your body can recover faster.
How much carbohydrate do you need?
The results of scientific studies suggest that athletes should get 55-70% of their energy from carbohydrate (7-10g per kg of body weight per day). A diet high in carbohydrates will allow you to get more out of your marathon training and lead to better results on competition day.
Timing when you eat
When you are training you go through a continual 24 hour process of using your glycogen stores during training and then replenishing glycogen after training. Therefore your total daily intake of carbohydrate needs to be adequate. Planning the timing of your meals will optimize your available fuel stores before and during exercise, and will improve your bodys recovery. Planning when to eat usually involves being practical and using main meals and snacks around training to ensure you have enough to eat before and after training.
Carbohydrates should be included in all meals and snacks during the day. When you are eating a high-carbohydrate diet, snacks are an easy way to eat additional carbohydrate.
To maximize your glycogen stores it is important to eat at least 200g of carbohydrate in the four hour period before you train.
For example, if you are training at 11am you could have breakfast at 7am and a mid-morning snack at about 9am to provide 200g of carbohydrate.
A 200g breakfast meal and morning snack could consist of 1 cup of cereal with trim milk and a medium banana (50g), 2 slices of toast bread with 2 tablespoons of jam (60g) and 1 glass of fruit juice (20g). For a morning snack you could have a pottle of yoghurt (20g) and a cereal bar (40g).
You need to be practical when planning your pre-training meals. For example if you are planning on a marathon training run at 6am you may not feel like eating much. Have a high carbohydrate meal the night before and aim for a minimum of 50g carbohydrate before early morning training.
For example, you could have a smoothie or a banana and a glass of fruit juice.
How much you eat will also depend upon how long the training session is going to be. If you are planning on a long training run make sure you have at least 200g of carbohydrate in your pre-exercise meal. If you are going for a half hour run you will be fine with a light snack.
For example, a piece of fruit or a slice of toast.
Training is a good chance to experiment with different foods at different times to determine what suits you.
In the past athletes have worried about eating before exercise because it was thought that blood glucose went up and then came down lower than resting levels, reducing performance. Recent research has shown that this does not usually impair performance. In fact, eating before training or competition has a positive effect on performance.
When training for over an hour and a half you should plan to consume some fluids and carbohydrate. This may not always be practical when you are going for an hour-and-a-half training run. If you do not eat or drink during training you must pay special attention to you pre and post-exercise carbohydrate intake.
When practicing your during-exercise carbohydrate intake for a competition situation upi should aim to consume 0.5 1 gram of carbohydrate per minute, or 30 60 grams of carbohydrate each hour to maintain adequate blood glucose.
For example, 500ml of sports drink with 7% carbohydrate will provide 35g of carbohydrate. This is enough to maintain your blood glucose levels.
Sports drinks and foods with a moderate to high glycaemic index will provide glucose quickly to blood for muscles and the brain to use as energy.
To cope with daily intensive marathon training you need to plan your recovery nutrition. After a marathon training session you muscle and liver glycogen stores are depleted. You need to replace these stores as quickly as possible so that you have energy for your next training session. This is especially important for marathon runners who have more than one training session each day.
Glycogen stores will be replaced most efficiently if you have approximately 1 gram of carbohydrate per kg of bodyweight within the first 30-60 minutes after your marathon training. This should be followed up with a high-carbohydrate meal and regular carbohydrate meals and snacks for the rest of the day.
For example, a 75kg woman needs to eat 75g of carbohydrate in the first 30-60 minutes after her marathon training session. A smoothie made with 400ml of trim milk, a medium banana and 200ml of fruit-flavored yoghurt, as well as a handful of raisins will provide 75g of carbohydrate. She can follow this up with a high carbohydrate meal and snacks.
You need to be able to plan your post marathon training carbohydrate so it is practical and fits in with your main meals and snacks. Although high glycaemic foods will supply glucose to the blood and muscles quickly, it is the total amount of carbohydrate that has the greatest effect on recovery.