Fuels for Energy Generation

Frank Plasschaert

Energy sources and transfer

The human body utilises energy to support metabolic processes such as muscle contraction and relaxation and for cellular reactions such as active transport systems, hormone receptor signalling and protein synthesis. The fuel for the body’s energy requirements comes from ingested nutrients and stored glycogen and fat. The energy needed is released by oxidising our food, burning it with oxygen that we extract from the air. The amount of energy liberated by this catabolic process appears as external work, energy storage and heat.

The human body mainly uses carbohydrates (sugars) and fats as the primary energy source. (Protein is not used as a primary source of fuel for energy except under extreme conditions such as starvation.) The chemical energy of carbohydrates and fats is not used directly for metabolic work. There is a transfer first to the high energy bond of adenosine triphosphate (ATP). Muscle utilises phosphate bond energy to slide the actin and myosin protein filaments relative to each other, leading to shortening and increased tension of the muscle. Under normal conditions, ATP is produced by glycolytic and oxidative reactions in the cytosol and mitochondria of the cell. Oxidative (aerobic) ATP production provides most of the body’s energy needs at rest and at submaximal workloads. Energy for muscular contraction (ATP) predominantly comes from aerobic metabolism that involves the continuous oxidation of metabolites in the mitochondria. Only a small amount of the total energy is provided by the glycolytic (anaerobic) metabolism. In the absence of oxygen, ATP production occurs exclusively in the cytosol of the cell by glycolysis. These cytosolic sources of energy last on average for 30 seconds and are only available for short duration, high intensity work. At submaximal exercise the ATP produced by anaerobic glycolysis only represents a very small amount of the total amount ATP produced. At higher work rates, both aerobic and anaerobic ATP generation share in energy generation. Anaerobic metabolism provides an increasing proportion of the energy requirements at higher work rates. At high work rates lactic acid is produced at a higher rate that it can be eliminated, and lactate accumulation causes intracellular and extra-cellular acid-base imbalance with unfavourable consequences such as muscle fatigue and hyperventilation. Anaerobic oxidation is limited by the individual’s tolerance to acidosis resulting from the accumulation of lactate.

The amount of oxygen consumed and the amount of carbon dioxide produced in producing ATP depends on the type of food being catalysed. For carbohydrates and fats the following chemical equations are obeyed:

Sugars : C6H12O6 + 6O2 = 6CO2 + 6H2O

Fat : 2C51H98O6 + 145O2 = 102 CO2 + 90H2O

The above described equations generate an exogenic thermal reaction (generation of heat). The amount of energy in our food is measured in terms of calories. One calorie is defined as the amount of heat energy necessary tot raise one gram of water one degree of Celsius. The respiratory quotient describes the ratio of oxygen consumed to carbon dioxide produced. For carbohydrates and fats the following chemical equations are obeyed:

Sugars : RQ = 6/6 = 1.0

Fat : RQ = 102/145 = 0.703

From the respiratory quotient it is possible to estimate which substrates are being consumed at a given time. However, a more accurate measure protein consumption needs to be assessed. This requires a measure of urinary nitrogen production.

The energy production depends on the substrate being consumed and is therefore subject to minor variations. The approximate energy liberation per litre of oxygen consumed is 4.82 Kcal. One should also realise that during more demanding physical activity, the body switches to anaerobic metabolism.

During strenuous exercise the body adapts by increasing the availability of oxygen and substrate to the tissues. At rest about 250 ml of oxygen per minute enters the blood. In extreme exertion this can increase to 4 liters. To do so there is an increase in tidal volume and breathing frequency. There is no significant increase in the proportion of oxygen which can be extracted from the inspired air. At rest the normal respiratory volume is 12 breaths per minute. The tidal volume for each breath is 500 ml on average. Of this tidal volume not all of the air comes into equilibrium with blood. The last part of the inhaled air and the first part of the exhaled air occupies the dead space in the large airways and does not take part in gas exchange. This volume is bout 150 ml in adults. In order to measure gas exchange it is necessary to have a system that measures gas concentrations in the alveolae of the lungs after equilibration. Sampling the end-tidal volume usually performs this.