Carbohydrates and fats are the two main fuels, though during prolonged intensive training bouts, proteins will play a more important role. For instance, during the last stages of a marathon, when glycogen stores are exhausted, the proteins in muscles and organs may make up 10% of the body’s fuel mixture.
During a period of semi starvation or a low carbohydrate diet, glycogen would be in short supply so more proteins would be broken down to provide the body with fuel. Up to half of the weight lost by someone following a very low calorie or low carbohydrate diet is likely to come from protein (mainly muscle loss). Some people think that if they deplete their glycogen stores by following a low carbohydrate diet, they will force their body to break down more fat and lose weight. This is not the case and this strategy will invariably lead to losing muscle and fat in equal amounts.
Carbohydrates during exercise?
After 60 minutes of moderate to high exercise, most muscle glycogen will be used up, and the body will have to rely more on blood sugar. After 2 – 3 hours the body will be relying totally on blood sugar (and fats and proteins from muscle). Therefore when exercising for more than 60 minutes, consuming carbohydrate during a workout will help to delay fatigue and allow performance at a higher level for longer.
An intake of between 30 – 60g (120 – 240 kcals) of carbs per hour is recommended, as this is the maximum amount that can be taken up by the muscles, and so consuming more will not produce beneficial results. It is also important to start consuming carbs before fatigue sets in, as it can take 30 mins for the glucose to enter the blood stream.
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