Energy Sources, When and Where
Let’s touch on one of the more common misconceptions of exercise metabolism; where does energy come from and what is the best intensity to burn fat?
This is the number one question I get from people early in their fitness journey. What I see is the misconception that at a given intensity you are “burning fat” and at another you are using carbs. This is an overly simplistic view of metabolism. The truth is all energy sources are being used at all times and intensities. Which begs the question, where did this misconception come from? As we can see in the graph different sources have relative contributions that vary based on intensity and duration.
What we see looking deeper, in the shortest most intense bouts the immediate fuel sources stored in our muscles are the first to be used. This is molecular Adenosine Triphosphate or ATP, the energy currency of the cell. The action of removing a phosphate from ATP to yield adenosine diphosphate enables muscular motion that we see in exercise. The baseline supply is very low as ATP is a high energy reactive molecule and consumed within the first seconds of activity. Additional phosphate to convert ADP back to ATP is stored in the form of creatine phosphate. This is the idea behind creatine supplements. Carbohydrate and creatine supplementation are among the few compounds that can be stored and provide some level of performance benefit.
Moving forward we see the peak in contribution of anaerobic glycolysis. Glycolysis or carbohydrate (glucose) metabolism is a great source of high energy very quickly. This is why we see higher peaks for output. The initial breaking of glucose does not require oxygen and is very fast, 100 times faster than aerobic metabolism. On the other hand we only gain 2 ATP molecules of the 36 total that can be gathered through aerobic metabolism. The downside of this fast energy extraction is that the remaining product, pyruvate, will pool and become lactate. This is not sustainable. Further aerobic contribution increases as we move into longer durations. These processes are slower, but contribute much more energy. This is for all energy sources primarily glucose and fat. The takeaway is that there is no point at which all energy comes from one source. All sources contribute at all intensities while their relative contribution will vary based on time and intensity as more energy dense pathways take time to provide the ATP needed for exercise.
In part 2 we will delve further into the discussion of the why and how we can monitor these phases
