Edible energy comes in four forms: carbohydrate, protein, fat and alcohol. The calorific values vary, and fat is the most calorie dense at 9 kcals per gram. Protein and carbohydrate have 4 kcals (calories) per gram, and alcohol has 7. Not only do these nutrients have different Kcal values, they have different digestion processes.
Carbohydrates 4 kcal / g Are broken down into single sugar molecules known as monosaccharides. Glucose is the basic unit of carbohydrate, Fructose (fruit sugar) & Galactose (milk sugar) are also monosaccharides.
Fats 9 kcal / g Are broken down into glycerol and fatty acids and then either transported to the cells for use as energy, or re-combined to form triglycerides and stored in the adipose tissue.
Protein 4 kcal / g Digestion requires the breakdown of these vast and complex structures, into the single amino acids from which they were initially built.
Alcohol 7 kcal / g Is absorbed and burned directly in the liver as energy, as it cannot be stored – it is also expelled as a toxin in breath and urine.
Carbohydrate fuel is stored as glycogen in the muscles and liver. The total amount of glycogen stored in the body is up to 2 kg (approximately 80% in the muscles and 20% in the liver) and for every one part of carbohydrate, the body needs three parts water to store it as glycogen. In other words there are approximately 500g of carbohydrate held in the body of a normal person. This energy store represents around 2000 kcals. Once the glycogen stores are full, excess carbohydrate will be converted to fat, which the body has infinitely more capacity for energy storage.
Endurance athletes have higher muscle concentrations of glycogen compared with sedentary people. Increasing muscle mass will also increase storage capacity for glycogen. The purpose of liver glycogen is to maintain blood glucose levels at rest. During prolonged exercise the liver will provide residual back up, as skeletal muscle glycogen becomes exhausted. Small amounts of glucose are present in the blood approximately 15g (60kcals) and in the brain 2g. These concentrations are maintained in very narrow bands both at rest and during exercise.
Fat is stored as adipose tissue located in almost all parts of the body. There are significant stores under the skin (subcutaneous) adipose tissues, and other storage sites are abdomen (central adiposity), buttocks, thighs and upper arms (peripheral adiposity). The vital organs are also surrounded in fat to protect them from impact damage. Fat storage location is determined by genetics and individual hormone balance. The male sex hormone testosterone favours central adiposity, and the female equivalent oestrogen pushes fat stores peripherally.
Central adiposity is considered more of a risk factor for coronary heart disease than peripheral storage, and it is thought that this is because fat contained in the peritoneal cavity (central obesity) is significantly more likely to be linked to metabolic disturbances such as insulin resistance and elevated plasma lipids. Higher intensity exercise appears to shunt fat out of the peritoneal cavity (organ cavity) and this is one of the health benefits of regular exercise.
Protein is not stored in the same way as carbohydrate and fat (in fact other than around 100g in the amino acid pool, protein strictly speaking cannot be stored). It forms muscle and organ tissue, so it is mainly used as a building material rather than an energy store. However, proteins can be broken down to release energy if need be (in emergencies) so muscles and organs do represent a large potential energy storage. This process is called gluconeogenisis.
What are the important fuels for exercise?
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.
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