Where Does the Body Store Its Energy?
Certain cells of the body store surplus food in the form of fat or animal starch. This is released from storage when there is not enough food in the blood to supply the energy demands of the body. Few of the things we eat can be used directly by the body cells. They must be changed chemically before they can supply the energy required. This is called digestion and is carried out in the stomach and in the small intestine, from where this digested food must be transported to the cells.
Now the food, in the form of digested sugars, proteins, and starches dissolved in water, passes into the blood system where, as blood, it can circulate through the body in less than one minute. Once the food is in the blood stream it is soon delivered to all the cells of the body by means of the red corpuscles in the blood. It is when this supply falls low that the energy stores come into action.
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.
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.
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.
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 4 kcal / g Digestion requires the breakdown of these vast and complex structures, into the single amino acids from which they were initially built.
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.
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.