THE ENERGY RESERVE FUNCTIONS OF FAT

Fat makes up between 13-30 per cent of total body mass in the average female and around 12-25 per cent in the average male. Conversely, men have more lean body mass (mainly muscle) and

more body water than women.

These proportions change with age, degree of muscularity, fitness levels and a range of other factors, but they generally represent the difference between the sexes. Each gram of fat is the equivalent of around 9 kcal of energy. Hence, a person with 15kg of body fat will have the equivalent of 135 000 kcal of potential energy. To put this in perspective, for a person who weighs 70kg, it requires about 100 kcal of energy to walk 1km. It also requires around 2000 kcal of energy to keep the average sized person alive for a day. This is called the resting metabolic rate (RMR). So theoretically, fat stores provide enough energy to keep the average person alive for around 60-70 days. For the average person to perform normal daily activity and to meet energy requirements of metabolism, a daily consumption of around 2000-2500 kcal of food is required.

Glucose (a basic form of carbohydrate) is the other main source of energy used by the muscles. This is a more immediate source of fuel than fat in that it can be used in the absence of oxygen. An analogy would be the difference between using paper (glucose) and wood (fat) as fuels in a fire. Paper is quick to start, but also burns out quickly. Wood is slower to get going, but is a much more effective form of fuel over a longer period.

Glucose is present in the body in three main locations: liver, muscle and blood. In the blood it circulates as glucose and in the liver and muscle it is stored as glycogen, which is a series of glucose molecules joined together. Total storage volumes in each of these locations is shown in

In contrast to fat, glycogen has to be stored with water. Each gram of glycogen is stored with about 3 grams of water, making it very bulky as an energy store.

Although this storage capacity can be increased, and perhaps even doubled through various nutritional techniques such as carbohydrate loading, glucose stores can’t be relied upon for long term survival in times of food shortages. Indeed, if we were to store as much glycogen as provides the equivalent energy of fat stores in the body, humans would need to be the size of elephants to accommodate the volume of glucose and water! Fat, therefore, is a very efficient and valuable store of energy.

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