In order for humans (that would be us) to survive we require energy to keep our brain, heart and other vital organs functioning properly. Humans require energy in the form of calories found in the food we eat and drinks we consume. Thermodynamics is the study of the conversion of energy from one form to another (i.e. food that we eat to energy to survive).
As it relates to body composition, the popular equation of “calories in = calories out” is a basic explanation for the first Law of Thermodynamics but it is not universal. The first Law of Thermodynamics, or energy balance, basically states that in a closed system, energy can neither be created nor destroyed, only transformed or transferred. In simpler terms, the energy taken into the body can either be stored (in the form of fat) or transformed to kinetic energy and heat.
The calories that we eat and the calories we use when we are physically active are only two components of energy balance, but there are several other variables to consider (that are not under our control). Because of these uncontrollable variables it is easy to understand why calories cannot be balanced like a checkbook, and why people never seem to gain or lose as expected when they use the “calories in = calories out” calculation.
Balance in an open system, like the human body, is when all energy going into the system equals all energy leaving the system (plus the storage of energy within the system). But energy in any thermodynamic system includes kinetic energy, potential energy, internal energy, flow energy, heat and work processes.
In other words, in real life, balancing energy includes a lot more than just the calories we eat and the calories we burn according to those exercise charts. The essential parts of the energy equation include:
- Calories consumed
- Calories converted to energy and used in involuntary movement
- Calories used for heat generation
- Calories used with immune processes
- Calories used in growth, tissue restoration and various other metabolic processes
- Calories used in voluntary movement
- Calories not absorbed in the digestive tract and matter expelled
- Calories stored as fat
- Fat converted in the liver to glucose (and there’s more)
When looking at a person’s body weight:
A person will maintain their current body weight if the “total number of calories consumed per day is EQUAL TO the total number of calories expended per day”.
A person will gain weight if the “total number of calories consumed per day is MORE THAN the total number of calories expended per day”.
A person will lose weight if the “total number of calories consumed per day is LESS THAN the total number of calories expended per day”.
Estimated Calorie Needs Per Day
To get started, determine your estimated calorie needs per day using the chart below. The values in this chart are based on age, gender and physical activity levels (rounded to the nearest 200 calories) to maintain calorie balance and your current weight.