Exercise Lies: Running for Hours = Fat Loss

Over the past few years, one of the most common mistakes by fitness seeking individuals is that running for hours will result in weight loss. This common mistake is often made by people wanting to get fit and achieve their goal of losing body fat percentage. They believe that running for miles upon miles for hours on a treadmill or track will help them achieve this goal. These people will then get frustrated when they lose a little weight, but their percent body fat has not changed. Why is this?

Aerobic energy systems are activated when a person exercises for low to moderate intensity (40-75% of VO₂ Max) for a duration of 60 seconds to 3 hours. During this aerobic exercise oxygen is synthesized and hormones such as cortisol and testosterone are produced the concentration of these hormones is determined by the intensity and duration of the exercise. Cortisol is a hormonal response to stress on the body, which helps to increase the concentration of glucose in our blood so it is a readily available energy system for muscles to utilize. Cortisol can be a good thing, but only when released infrequently and for short periods so that it can help with the body’s threat to internal imbalance. However, if a person is exposed to cortisol for a long period of time (30 minutes or more) under a stressful act, such as running, they may experience long-term consequences.

When over-exposure of cortisol is seen in long durations of exercise, it may overtake the levels of testosterone in the body. Cortisol is associated with breakdown of muscle tissue, while testosterone is associated with building muscle. When cortisol is produced at such a high level for such a long time, it is very hard to flush out of the body’s system. When the ratio of testosterone and cortisol is unfavorable (e.g. T < C), muscle is lost. When muscle, which helps to burn fat, is lost fat percentage is increased. It has been found that there was no increase in cortisol after 10 minutes of aerobic exercise (75% of VO₂ Max) in 17 male physical education students but increased by 54 percent compared to their pre exercise levels.1 This demonstrated cortisol was influenced by prolonged intense exercise. In another study conducted in 201, 42% of analyzed cortisol levels in 304 amateur endurance athletes had an average additional secretion above the control (as seen in the graph below).2  Which means, that of those athletes running for 30 minutes or more, cortisol amounts doubled which could lead to a very high ability for the body to store fat in the future, for more running. It is a vicious cycle. All one has to do is compare archetypes of sprinters to marathon runners (see above).

As for alternatives to the myth of running for fat loss, simply walk. Walking for 40% of VO₂ Max for 1 hour actually lowers Cortisol levels. This occurs because the intensity is lower, and the cortisol has had a chance to be removed from the body’s system faster than it can be secreted. If you still miss the heart pounding excitement that you get from running, try high intensity intervals such as 1 to 16 seconds of sprinting then equal or more time walking for removal of Cortisol. This type of training may actually stimulate muscle growth at the same time.

So if you are a person with the goal of fat loss, but just don’t have the time to commit to hours and hours of running, GOOD NEWS, you should not do it anyway. Commit to slow walking for 45 minutes to an hour, or a few intervals of sprinting, and you will be on your way to fat loss without the threat of cortisol.

1Kindermann, W., Schnabel, A., Schmitt, W., Biro, G., Cassens, J., & Weber, F. (1982).

Catecholamines, growth, hormone, cortisol, insulin, and sex hormones in anaerboic and aerobic exercise. European Journal of Applied Physiology and Occupational Physiology, 49(3), 389-399.

2Skoluda, N., Dettenborn, L., Stalder T., & Kirschbaum C. (2011). Elevated hair cortisol  

concentrations in endurance athletes. International Journal of Psychoneuroendocrinology, 37(5), 611-7.