About Stored Energy in the Body

We obtain our energy from the food we eat. The energy contained within the food is measured in calories. This energy, according to the 2nd Law of Thermodynamics, will be converted to another form once we consume it. A typical daily diet consists of food from the three major classes of foods, namely carbohydrates, proteins, and fats and oils. Once in the body, these foods are used to build the body, metabolized to provide energy or stored to produce energy for future use. Some of the metabolic pathways are energy efficient, others are not. The human body follows the thermodynamic principles relying on the most efficient pathways to store and produce energy.

Potential

Each food consumed has a potential amount of energy it can provide: Carbohydrates provide about 4 calories per gram; fats, 9 calories per gram; proteins, 4 calories per gram. The energy contained within the food we consume is mostly chemical energy and potential energy. An average diet should consists of 2000 calories a day, however a typical American consumes closer to 3000 calories a day, which is a lot of potential energy.

Effects

The body stores its energy in the simplest molecules derived from the consumed food. Carbohydrates are broken down into their simple form, glucose. The glucose is released into the blood stream where it can be converted immediately to energy in the cells where it is needed through a multi-step process known as glycolysis. Extra glucose that isn't needed is converted to glycogen and stored in the liver and muscle tissues. When blood glucose levels drop below the optimal level (as glucose is used up), the liver converts the glycogen back to glucose and releases it into the blood stream.

Considerations

In a fasting situation when all the available glucose has been used up, the body seeks out alternative sources of energy such as protein and fats and oils. Protein once ingested is broken down to its simplest component: amino acids. The amino acids are primarily used in the building of muscles, but during an energy crisis, the amino acids undergo gluconeogenesis converting the carbohydrate skeleton of the amino acid to a substrate that can be used in glycolysis. Fats undergo a similar type reaction; they are converted to triglycerides which undergo lipolysis to form glycerol which can be further converted for use in glycolysis.

Significance

The most efficient energy producing pathway is glycolysis. Glycolysis is important because it results in the formation of adenosine triphosphate (ATP). ATP is important as it is commonly referred to as the energy currency of the human body. It contains the energy rich phosphate bond that when broken releases energy for what ever purposes the body requires. Once the ATP looses the phosphate, it is now called adenosine diphosphate (ADP), the ADP enters back into the glycolysis pathway where it receives another energy rich phosphate bond converting it back to ATP. Active cells, such as muscle cells normally contain high levels of ATP.

Warning

Several diseases have been linked to excessive glycogen stored in the cells. This condition is normally caused by a genetic defect. The diseases are characterized by the lack of important enzymes required in the conversion of glycogen to glucose. The common symptom for these disorders is low blood sugar. When excessive glycogen is present in the muscle cells, the patient experiences muscle weakness and an inability to perform exercises.