Sources of energy for the body
In the last article on understanding lactate threshold, we discussed the three different energy systems of the body and how ATP, the energy currency of the body, was produced in all three systems concurrently. Each system provides different amounts of energy to the working muscles at varying rates for distinct intensity demands. To further shore up this concept, it would be helpful to be somewhat familiar with where our bodies derive the energy to power our movement and basic body functions.
Throughout the day, your body has three possible sources of energy that it stores from the food you eat: carbs, fat, and protein. To keep things simple, we won’t talk about protein, as it is used as a last resort. Just know that muscles are the only place protein is stored, and the last thing we want is for the body to break down our precious protein stores to use as energy!
Typically, the body uses a combination of carbohydrate and fat stores as fuel for energy. Think of energy substrate use (the substrate is the fuel used) within the body as a sliding scale where duration and intensity determine the ratio of fat to carbohydrates that will be used to produce the required energy for that activity. The relationship between intensity and duration is what determines the relative contributions from anaerobic and aerobic energy sources; this (plus dietary factors) will also determine substrate utilization.
When you are moving or working out, your body prefers to get its energy through the aerobic pathway, known as the oxidative system. At lower levels of intensity, your body doesn’t require energy as quickly, so this pathway easily provides the energy required. Aerobic metabolism draws on a higher percentage of fat for fuel rather than carbohydrates. It has a low throughput but a high output, meaning energy production and supply is slower, but each molecule of fat yields more than twice the units of energy as a molecule of glucose, so it is a much more efficient means of supplying energy.
However, there are cases when activity levels are more intense and the body requires energy at a much faster rate. In this scenario, the body relies more heavily on the glycolytic system, an anaerobic metabolic pathway that uses carbohydrates, in the form of glucose, as energy. This system has a high throughput but low output, meaning energy is available much more quickly, but yields less energy per molecule. During these intense work efforts, the body will use carbs exclusively as its energy source for glycolysis.
As a general rule, anaerobic energy delivery is limited to intense efforts lasting from 30 seconds to 2 minutes before it begins to downshift to an aerobic-based metabolic component. There is always a cost of doing big business, and when exercising at near-maximal efforts, that cost comes in the form of lactate production. This is where lactate threshold comes into play.