Metabolic energy, more simply referred to as energy is a common topic amongst many when discussing physical and mental conditions. Some days you don’t feel like yourself, and routine tasks become challenging. Other times, the opposite happens, as you get an “extra” bounce in your step and feel everything is within reach. These feelings are often referred to as personal energy levels, translating into a certain level of performance. But, what is metabolic energy? Where does it come from? How do we use it? And how can we get more of it?
Metabolic energy is power, derived from chemical resources used towards generating work or activity. Human body uses energy for all daily tasks including physical movements, breathing, blood circulation, brain activity, organ function, digestion, metabolic processes and everything in between. To carry out all these processes the body generates a chemical form of energy called, adenosine triphosphate (ATP). This molecule is found within and used by all cells in the body.
The source of raw materials for energy production come from our diet and produced through three metabolic pathways. These pathways produce ATP through their own chemical processes and time frames. The three pathways are: the phosphagen system, glycolysis and the aerobic system.
Energy 101
We will not discuss the principles of thermodynamics here, but simply attempt to apply these principles in reference to energy. In physics, energy is an entity that makes up an object. Energy can be transferred (from object to object) or converted (from one form into another). These unique characteristics of energy (E) translate into the ability of any system to perform work. The human body is a system which performs work in physical and mental capacity. Simply stated, energy is equal to work.
Since every object contains energy, state of that object also determines the type of energy it contains. For example: a stationary ball contains Potential energy, and moving ball contains a form of energy known as Kinetic energy. If that ball is picked up and squeezed, essentially storing energy inside known as Elastic energy.
Chemical compounds or molecules – like protein, carbs and fats – are combination of different atoms held together by bonds. The bonds that hold these molecules together contain Chemical energy. When compounds are broken down, chemical energy is released, like digesting food into basic amino acids, sugars and fats. Our mighty ATP is another example of stored chemical energy. When ATP is used, a phosphorus atoms are broken off and chemical energy is released creating heat, also known as Thermal energy.
Energy within a closed system cannot be created nor destroyed but rather transformed from one form to another (e.g. chemical E converted to kinetic E). The measurements for energy such as work, velocity, and force are named after physicists who researched these entities. For example, English physicist James P Joule discovered that mechanical work generates same amount of heat and published his findings in 1845, a paper titled “The Mechanical Equivalent of Heat”. Since then energy quantifying unit is named in his honour as Joule or J, and is the current international (SI) designation. Having said that, other units are used when discussing energy. For instance, food energy is often quantified in metric system unit Calorie (Cal). Our bodies burn calories by doing everyday activities such as walking, breathing, thinking, digesting.
Food and Energy
Food is a condensed from of energy containing caloric and nutritional value. Often, we see on product labels containing portion sample with allocated total energy in terms of calories, as well as carbohydrate, protein, fat and vitamin/mineral contents. Nutrient rich food not only fuels the body but provides raw materials towards numerous metabolic processes. Ingested food goes through mechanical and chemical breakdown into small molecules used by the body, with energy production being the main objective. The body uses protein, carbohydrates and fats and converts them to energy.
Metabolic Energy Storage
Through evolutionary design, mainly food scarcity in the past, our bodies are designed to store energy for dire times of need. The body stores energy in different forms and converts it when required into the final energy package ATP.
ATP
ATP has three phosphate groups attached in a row and is maintained and balanced with stabilizers (buffers) and other products such as ADP (adenosine diphosphate). Our bodies continuously utilize energy so, ATP reserves within cells don’t last long before hydrolysis. However, only small amounts of ATP is stored in cells for immediate use.
Glycogen
The second form of energy stored inside the body is sugar – by way of glycogen molecule. Glycogen consists of many glucose molecules attached to each other creating a multi-branched sugar molecule (polysaccharide). It is stored primarily in the liver and muscle, and can be converted into ATP through two separate systems – anaerobic or aerobic system (with or without presence of oxygen). Anaerobic process of ATP production is faster however, it produces less ATP than aerobic system. Glucose is an important energy storage available for fast production of ATP.
Fats
The third form of energy storage is fat. Fat is stored in form of triglycerides (three fatty acids connected to glycerol molecule). Majority of triglycerides stored are within fat cells, also known as adipose tissue, but some can also be stored in liver, muscle, heart and pancreas. For energy, triglycerides are broken down into glycerol and free fatty acids (FFAs) which are shuttled to targeted cells and converted to ATP through aerobic energy pathway. Adipose tissue is much more than energy storage facilities, as it is recognized as a major endocrine organ, producing a number of hormones and cytokies. Fat is low maintenance energy form, requiring no water, and contains twice as much energy (9 kcal) per gram compared to sugar (4 kcal). Fat oxidation is the slowest process to produce ATP but yields immense amount of ATP.
ATP Energy for Cells
Humans are able to eat and utilize variety of foods for energy, store excess energy in from of sugar and fat, transport materials into cells and then convert it to ATP for final energy usage. The question then arises, why is ATP the ultimate energy source used by cells for all actions? The precise answer is still unknown, but several logical ideas explain the reason.
Many molecules store energy however, cells have adapted to use ATP as a preferred energy choice. As a molecule ATP has stable adenosine nucleotide and less than stable tail composed of three phosphates. The bond between the last two phosphates in the tail is sensitive and oxidizes (breaks), resulting in energy release while generating a stable tail within adenosine called ADP (for diphosphate). The conversion of ATP to ADP and vice versa is continuous within a cell, which also makes this energy package particularly attractive delivery system within cell system.
Final Thoughts
Energy is all around us, and it can not be created nor destroyed but simply changes form. We utilize energy stored in food, converting it into chemical currency used in all cells – ATP. In the body, small amount of immediate, ready-to-go, energy is stored as ATP. Larger volumes of energy are stored as glycogen and triglycerides. Sugar metabolism is capable of fast production of ATP, but yields less final product. Fat lipolysis and oxidation is slower and more involved process, however generates the most amount of ATP.
We further discuss this topic, energy and training, in depth along with others in our Stronger Leaner Faster Books. In the next series of articles on our blog, we’ll look at ways the body produces energy and the systems that synthesize ATP. We will also look into how each energy system (article coming soon) contributes overall to physical demands.
