How does food become energy?

Optimizing fat burning while simultaneously building muscle should be the goal of any effective exercise program, the critical linkage between exercise and the food we eat is ATP.  This article describes the importance of understanding what ATP is, and how different foods are converted to ATP, and when different fuels (protein, carbohydrate, or fat) are used for energy.   Understanding these concepts provides the blueprint for the diet and exercise guidelines.

I frequently talk to patients about the foods they eat, and how it impacts their health.  I think its very important to understand that converting food to usable energy is far more than simply chewing big pieces of food, which are digested into small pieces, and used directly by the cells.   Your body does not directly take the food particles it consumes and transfers them to your cells for metabolism.  It converts these particles to fundamental substrates, which are then converted to ATP (adenosine triphosphate), which is the actual fuel for your cells.


(Other substrates are used for structural growth and for cofactors, as well as immune recognition purposes; but the apple you eat does not become a little apple that your cells consume, it instead converts to sugars, proteins, and fats which are metabolized to ATP, which power your cells.)

The universal currency for life is ATP, the energy-carrying molecule found in the cells of all living things.  ATP functions like a tiny battery debit card, an ATP molecule consists of adenosine and three negatively charged inorganic phosphate groups when it is fully charged and ADP (adenosine diphosphate) when it has released its energy and has only two phosphates.



Recharging ADP to ATP requires fuel substrates, which is the converted food that you eat, whether it be protein, carbohydrate, or fat.  The exact recharging is coupled to the molecular destruction of Carbon bonds, similar to gasoline combustion in your car.

The recharging of ADP to ATP is what couples the food you consume to usable energy, the discharging of ATP to ADP transfers energy to usable cellular function for either chemical or mechanical reactions.  Available ATP (and Phospho Creatine- a related Phosphate storage bank in muscle) is depleted within 15 seconds of vigorous exercise.




Substrate to ATP

There are three significant fuel sources or substrates of energy to reform ATP from ADP.



Carbohydrates are metabolized to glucose, which quickly regenerates ADP to ATP through a process called glycolysis.  Consumed carbohydrates are directly available as glucose, or excessive consumed carbohydrates are stored in muscle and liver as glycogen for later reconversion to glucose. (Excessive carbohydrates in the presence of insulin are also converted to fat in fat cells.) Use of carbohydrates for energy does not require large amounts of oxygen and can occur even during anaerobic conditioning.  Production of lactate and lactic acid occur with use of carbohydrates for fuel.  Available carbohydrates provide energy to recharge ATP for the first 45-60 seconds of high intensity therapeutic exercise.


Fat is a slow energy release form, typically stored in fat cells.  Lipolysis is the term used to describe the breakdown of fat (triglycerides) into the more basic units of glycerol and free fatty acids, which then undergo beta-oxidation. Combustion of fatty acid molecules produces significantly more ATP, compared to glucose molecules. However, because fatty acids consist of more carbon atoms than glucose, they require more oxygen for their combustion.  Fatty acids are oxidized by most of the tissues in the body; except, the brain, red blood cells, and the adrenal medulla.  It is a huge storage reservoir, but is too slowly released for immediate energy utilization.  Lipolysis provides energy after the first 60 seconds of exercise, but only effectively in the presence of large amounts of oxygen.


Protein can also be used as an energy source, but it must first be broken down to amino acids before being converted to glucose.  Protein is thought to make only a small contribution (< 5%) to energy production, but up to 18% of total energy requirements during long strenuous activity, sometimes characterized as rhabdomyolysis.