OK! Let’s start with the basic science. ( And bare with me… I took College Biology & Biochemistry almost 10 years ago now…. Eek!)
Every cell in your body (including your muscles) need some source of energy to keep them going – that energy is called Adenosine triphosphate ( i.e. ATP). This ATP is the biochemical way your body stores and uses energy. ATP is turned into energy by breaking the bonds that hold together the phosphate molecules to adenine nucleotide. When a cell needs energy, it breaks the bonds in ATP to form ADP and 1 free phosphate molecule. Subsequently, when the cell has too much energy, it stores it by forming ATP from ADP and phosphate. As the work of the muscle increases, more and more ATP gets consumed and must be replaced in order for the muscle to keep moving.
ATP: Adenosine + 3 Phosphates
Bond is Broken = ADP (Adenosine + 2 Phosphates) AND Energy
Because ATP is so important, the body has a bunch of different systems in order to synthesize it and these systems all work together in phases. AND!!!! What’s really cool is that different forms of exercise use different systems! (No? Not cool? Just me that thinks that???)
OK, so.. Like I was saying.. There are 3 different biochemical energy systems in your body:
(1) The Phosphagen system
(2) The Glycogen-lactic acid system
(3) And Aerobic respiration
The Phosphagen System (A Sprinter’s Best Friend)
Every muscle cell has some amount of ATP laying around that it can use up immediately, BUT not a whole lot (only about enough to last for about 3 seconds). To replenish the ATP levels quickly, muscle cells contain a high-energy phosphate compound called creatine phosphate. The phosphate group is removed from creatine phosphate by an enzyme called creatine kinase, and is transferred to ADP to form ATP. The cell then turns ATP into ADP, and the phosphagen rapidly turns the ADP back into ATP. (I know it’s a lot of back and forth right???) This phosphagen system can supply the energy needs of working muscle at a high rate, but only for 8-10 seconds.
The Glycogen-Lactic Acid System (A Swimmer’s Buddy)
Muscles also have big reserves of a complex carbohydrates called glycogen, which is a chain of glucose molecules. In order to use glycogen for energy, a cell splits it into glucose and then uses anaerobic metabolism to make ATP and a byproduct called lactic acid. About 12 chemical reactions take place to make ATP under this process, so it supplies ATP at a slower rate than the phosphagen system. This system can produce enough ATP to last about 90 seconds. Since this system is ANaerobic (i.e. does not require oxygen), it is helpful to the body because it takes the heart and lungs some time to catch up.
NOTE: 1 molecule glycogen = 3 ATP
Aerobic Respiration (A Marathoner’s BFF)
By the time you’ve exercised for 2 minutes the body begins to supply working muscles with oxygen (i.e. the heart and lungs have now “caught up”). When oxygen is present, glucose can be completely broken down into carbon dioxide and water in a process called aerobic respiration. The glucose needed for this can come from 3 different sources: leftover glycogen supplies in your muscles, glycogen in your liver, and glucose from food in your intestine. Aerobic respiration can also use fatty acids from fat reserves in your muscles and the body to produce ATP (but this is only after all the glycogen has been used up). Aerobic respiration takes the most chemical reactions out of ALL the energy reactions mentioned to produce ATP. Because of this, aerobic respiration produces ATP at the slowest rate of all three systems, BUT it can continue to supply ATP for several hours or longer – just as long the fuel supply lasts.
NOTE: 1 molecule glycogen = 32 ATP
So… What have we learned??? That aerobic respiration is much more efficient (32 ATP to 3 ATP!!!) at converting glycogen to energy than the previous two energy systems (Phosphagen & Glycogen-Lactic Acid) AND it can provide more sustained energy as well. Depending on the activity, it can take up to an hour to use up all the glycogen stored in your cells and liver, at which point the body has no choice but to start burning fat as fuel. (Which is the goal for most of us.) This is why you often hear how low carb diets work well for fat loss (especially when coupled with exercise). The more carbohydrates that are eaten before a workout, the more glycogen remains in your system to burn off before fat can be used as fuel.
I feel like that’s enough information for now. Catch you guys next week for the second installment:
Take it from a Chemist: Cardio Before or After a Workout? (Part 2 – The Pros and Cons!)