There is no single strict definition for ‘life’ or ‘living’ but one of the criteria is a metabolism. In order to function from the cellular level and as a whole multicellular organism, trillions of chemical processes have to continue efficiently – in a systematic and continuous manner. In the process it needs fuel (food) and an oxidant (oxygen) in addition to all the other chemicals (nutrients) that are needed for every activity in the human body.
What is metabolism?
Metabolism is a collective term for all the chemical processes in every cell in the body. It is essentially the sum of all these processes. The body’s processes are essential to conduct a wide variety of functions but the ultimate goal is to sustain life. The chemical processes utilizes energy and also gives off energy usually in the form of heat.
What is the metabolic rate?
The metabolic rate is the amount of heat that is generated during the various chemical processes. It is proportional to the energy utilized – some energy will be used in other forms like mechanical energy (for example when walking) while the rest will be dissipated as heat.
A person with a higher metabolic rate will utilize more energy and dissipate more heat. This, however, is carefully regulated within a narrow range (normal body temperature) by the body’s ‘cooling’ system (thermoregulation). Conversely, a person with a lower metabolic rate will utilize less energy and ultimately dissipate less heat. Thermoregulation also comes into effect here to maintain the body temperature within a narrow range.
So simply measuring the person’s body temperature will not necessarily be a true indication of the metabolic rate – other approaches need to be adopted to calculate the metabolic rate.
Adenosine Triphosphate (ATP)
Adenosine triphosphate (ATP) is the basic energy unit in the body. It is a compound formed in the body by the breakdown of foods and can be utilized by every cell to yield energy which is needed complete the various chemical processes that make up the body’s metabolism.
When ATP is utilized, energy is released. The measure of the energy is either a calorie (more accurately kilocalorie ~ kCal) or joule (kilojoule ~ kJ). The two high energy phosphate bonds in ATP can release between 7,000 and 12,000 calories per mole. Calorie is also the measure of energy content in food since the breakdown of food (metabolism) yields ATP molecules.
Food and Oxygen
Food has an an energy potential. When it is broken down and processed in the body, high-energy compounds are transported to the cells that are further processed to form ATP. Since these ATP molecules can yield energy, it is logical to standardize the units of measure by looking at food in terms of calories.
There is also another component in these exothermic reactions and that is oxygen. In the ideal state, the body will use oxygen and ATP to yield energy. While energy production can occur for a short period of time without sufficient oxygen, this is not the ideal state and if the oxygen is insufficient, the metabolism will cease. Therefore it is also possible to look at oxygen in terms of its energy potential.
Measuring the Body’s Metabolic Rate
Measuring the metabolic rate is known as calorimetry. It is essentially the practice of estimating the calories utilized by measuring other factors involved in metabolism.
Energy by Body Heat
As discussed, measuring heat dissipated from the body by using a thermomenter is not an accurate way of determining the body’s metabolism. Thermoregulation maintains the body temperature around 37.5 degrees Celsius (give or take about one degree). Direct calorimetry by putting a person in an enclosed chamber with a constant temperature and then monitoring temperature changes in the environment is difficult to do.
Energy by Oxygen Used
Monitoring oxygen utilization is another way of estimating the body’s metabolic rate. This is known as indirect calorimetry. It also depends to some degree on the type of food being metabolized. It is however a fairly accurate although not convenient means of measuring the metabolic rate.
Basal Metabolic Rate
One of the best ways of estimating the body’s metabolic rate is by considering the entire body weight. Every cell in the body, whether a muscle or fat cell, has to consume energy even at rest to stay alive. Some cells like energy cells are high energy consumers even a rest. About 60% of the energy utilization is a day is just to maintain the life processes even if a person is sedentary and sleeps all day. This minimum energy level is known as the basal metabolic rate (BMR).
Therefore the basal metabolic rate (BMR) can be defined as the minimum daily energy utilization to maintain life. Logic will dictate that in order to maintain this BMR, a person has to eat at least the same minimal amount of calories per day. Eat more than is needed and the body will store it as fat and contribute to weight gain or obesity. Eat less than what is needed and the body will start consuming its own tissues and there will be weight loss – death may be the ultimate consequence. Eat approximately the same amount that the body uses and a person, in an ideal world, will neither gain nor lose weight. These concepts are important for weight management particularly with regards to calorie counting.
Calories by Body Weight
It is difficult to base the basal metabolic rate on weight by a kilogram per kilogram basis. Muscle cells use more energy. Fat cells use less energy. The body is composed of complex tissues and organs with varying metabolic rates. Therefore the weight of the body must be considered as a whole.
On average, a man weighing 70kg will need about 1,600 calories per day to sustain life adequately and a 70kg woman will require about 1,450 calories. This equates to about 65 to 70 calories per hour in men and 60 to 65 calories per hour in women.
However, a change in activity can increase this rate above the basal metabolic rate (BMR). For example standing still will increase the metabolic rate to about 105 calories per hour. Since most human beings are not static but are also not extremely active in the modern world, it is fair to consider that the average 70kg male will need 2,500 calories per day and a 70kg woman equally as active will require about 2,000 calories per day.