The International Marathon of Mauritius is now only 8 weeks away and the training ?long runs? are getting long. The Novice Program long run is 16 miles this week, and the Intermediate Program is 18 miles. These long runs are the cornerstone of marathon training. An understanding of the long run physiology and experimenting will help formulate your marathon plan.

Your fuel is provided in two main forms: carbohydrate and fat. Protein doesn?t provide a significant energy source.

Carbohydrates are readily broken down into glucose, the body?s principal energy source. Glucose needs less oxygen to burn than fat-related energy sources; therefore carbohydrates provide the most efficient fuel. Glucose is stored in the liver and muscles as glycogen or can be used immediately. During exercise, muscle glycogen is converted back to glucose and used as fuel by muscle fibres. The liver also converts its glycogen back into glucose and maintains the blood glucose (blood-sugar) level. Muscles also draw on blood glucose in addition to their own glycogen stores. Blood glucose is also the brain?s only source of energy.

Fat is the body?s most concentrated source of energy, providing more than twice as much energy as carbohydrate. During exercise the body?s stored fat is broken down into fatty acids and transported by the blood to muscles for fuel. However, this process occurs relatively slowly compared to carbohydrate mobilisation. Fat is also stored in muscle fibres where it can be more easily accessed during exercise.

Your body uses a mixture of fuels during every activity that you do. The intensity and duration determine what proportion of fuels the body uses. At rest and during low-intensity exercise such as walking, you use mainly fat. If the intensity of the exercise is low to moderate, your body relies on a mixture of fat and glycogen. As the pace picks up, the body burns more glycogen and less fat. This is because your body has trouble consuming enough oxygen to burn fat; it therefore changes to the more efficient glycogen. Fat mobilisation is also too slow to support faster energy demands. At very high intensity exercise levels, the body relies essentially on glucose.

The body?s glycogen storage capacity is limited, providing approximately 1,800 to 2,000 calories. Fat storage, however, can supply up to 100,000 calories: enough for several marathons. It doesn?t matter how fast you run or walk, you use approximately 100 calories per mile. If you cover 20 miles, you?ll burn around 2,000 calories. Therefore, if you are over-enthusiastic and start too quickly, a large proportion of the calories will come from the limited glycogen stores. As muscle glycogen stores deplete, blood glucose plays a larger role in meeting the body?s energy demands. If exercise continues and the liver cannot release glucose fast enough to fuel the brain and muscles, you?re in trouble.

If inadequate glycogen remains, the nervous system starts to shut down making exercise very difficult. You have ?Hit the wall?. Since the brain doesn?t function properly due to low energy, you?ll lose focus, find it difficult to concentrate, become dizzy, disorientated and experience difficulty in maintaining balance. You may not finish.

So what can be learnt?

Firstly, don?t run the long runs too fast, be sure to finish them. By slowing your pace, you?ll take in more oxygen and burn your slow-burning fat storage, conserving glycogen. If you start too quickly, you?ll use your glycogen stores too quickly.

It?s important to replenish what you are using during the run. Sports drinks typically have 6-8% carbohydrates, around 140 calories in 50cl. These also replenish electrolytes lost by sweating. Experiment with energy bars, bananas or chocolate. Now is the time to experiment with food and drinks that will get you to the finish line. Don?t wait until marathon day to try something new.

Here?s the plan:

1. Eat a carbohydrate-rich diet, rather than rich in protein or fat. Boost carbohydrate intake further for three days before each long run (known as ?carbo-loading?).

2. One hour before a long run, top-up on carbohydrates, such as cereal, yoghurt or juice.

3. Start the long run slowly to conserve glycogen stores: train your body to burn fat.

4. Use a sports drink during the run every two miles. Eat an energy bar every 45 to 60 minutes if you can.

5. For the novice, run the long run approximately one minute-per-mile slower than you normally run your shorter daily run.

6. For the intermediate runner, run the first 16 miles at 45 seconds-per-mile slower than target marathon pace. Then run the last miles at marathon pace. This will teach your body to run at target pace on tired legs.

7. Within 30 minutes after your run, have a banana or energy bar and drink half-a-litre of sports drink.

The long runs adapt the body to become more efficient at using fat during exercise. Muscles will also be stimulated to store more muscle glycogen and use it at a slower rate. These adaptations will allow you to exercise at a higher pace or for a longer duration before fatigue due to glycogen depletion.

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