The Biochemical Dynamics of Marathon Running

Running a marathon is not just a test of physical endurance but also a fascinating exploration of the body's biochemical responses. The body undergoes a series of intricate hormonal and metabolic changes to fuel the muscles and maintain performance. This article explains the changes in plasma concentrations of key hormones and energy substrates during a marathon, including insulin, adrenaline, lactate, free fatty acids, and glucose.

Insulin and the Regulation of Blood Glucose

Insulin is a hormone made of protein that is very important for controlling the amount of sugar in the blood. It helps the body's cells to absorb sugar, which reduces the sugar level in the blood. But when someone is running a marathon, the amount of insulin starts to go down from the beginning of the run.

This reduction in insulin levels happens because the muscles are using up more sugar as they work harder. As the muscles use this sugar, the amount of sugar in the blood goes down too. If insulin kept reducing the sugar levels, it could lead to dangerously low blood sugar, known as hypoglycemia. So, the body naturally lowers the production of insulin to stop the blood sugar level from dropping too much.

Adrenaline: The Body's Response to Stress

Adrenaline, also known as the "fight or flight" hormone, works in the opposite way to insulin. It stops insulin from being released and raises the amount of sugar in the blood, making it easier for muscles to get the energy they need.

During a marathon, the amount of adrenaline in the body goes up. This increase is especially noticeable when runners hit the 30-kilometer mark, by which time the body's sugar stores in the liver and muscles are almost used up. Adrenaline activates certain actions like turning glycogen into glucose, making glucose from some amino acids, and breaking down stored fats. These actions are important for making up for the lack of glucose and providing the muscles with the energy they need.

Lactate Accumulation and Muscle Fatigue

When muscles lack oxygen, they produce a substance called lactate, or lactic acid. If the body doesn't get rid of this substance efficiently, it can harm the cells. Although the body tries to manage lactate levels, it can't handle too much of it.

During intense activities, like running a marathon, muscles get tired mainly because of the buildup of lactate. As you exert more energy in such activities, lactate levels rise. It's important to keep these levels in check to avoid overloading the body.

The lactate threshold is the exercise intensity at which lactate begins to accumulate in the bloodstream faster than it can be removed. This threshold is often reached at around 50-80% of an individual's maximum heart rate, depending on their fitness level and training. Training just below or at the lactate threshold can help improve the body's ability to process and clear lactate, effectively increasing the threshold over time.

The Role of Free Fatty Acids and Glucose

Adrenaline helps break down fat, releasing fatty acids from our body's fat storage. Even though fat gives us less energy than sugar for the same amount of oxygen used, we have more fat stored than sugar. That's why our bodies usually use fat for energy and start using more sugar when we exercise harder.

However, turning fat into energy also needs sugar to work properly, and we need to keep some sugar available all the time. To avoid using up our sugar stores too quickly and running out of energy during a long run, it's best to keep a steady and moderate pace throughout a marathon.


Understanding the biochemical dynamics of marathon running provides valuable insights into how the body adapts to physical stress. The intricate interplay of hormones and energy substrates underscores the body's remarkable capacity to sustain prolonged physical exertion. With this knowledge, runners can better strategize their training and race plans to optimize performance and endurance.

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