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Does overtraining reduce mitochondrial function and glucose tolerance?

A recent paper (1) studied how a progressively increasing training program affected various measures of health and performance. It suggested that too much training could be bad for overall health.

Overtraining decreases performance and mitochondrial function

What exactly did they do?

The authors recruited 11 participants (including both males and females) who were active and performed endurance and strength training regularly, though were not high-level athletes. They completed a 4-week training program involving doing high intensity interval training (HIIT) on a cycle ergometer. Each week, the number of sessions increased, as did the length of some of the sessions, thereby increasing the total volume of training each week. At the end of the study, the participants also did a recovery week, with lower training volumes. They took various measures before and after the program, as well as at the end of each week (such as blood samples and muscle biopsies).

What did the study find?

As would be expected, the whole training program increased performance, resulting in higher power output. However, although performance improved week-to-week, in the week of the highest training volume, there was no improvement in power output. Similar results have been seen before; that excessive training (sometimes called overreaching) stagnates training improvements. However, after the recovery period, performance peaked, in a process called ‘super-compensation’.

The study was particularly interested in looking at changes to the mitochondria in the muscles. The mitochondria (often described as the ‘powerhouse of the cell’) are responsible for producing the majority of the energy (as ATP) required for aerobic or oxidative exercise. Most exercise training (including endurance training and HIIT) increase the number of mitochondria found in the muscle, as well as many of the proteins and enzymes that are responsible for energy production. This is one of the main adaptations to training that results in improved performance, as more energy can be produced by the muscle to power exercise.

Mitochondrial function and overreaching

From week-to-week, many of these mitochondrial markers increased, but stopped increasing, or even fell after the week of excessive training. Many of these mitochondrial markers are linked to good health and are dysregulated in conditions such as type-2 diabetes. Intrinsic mitochondrial respiration (IMR) is one such marker and is the capacity of mitochondria to function and produce energy. This marker was reduced by 40% in the week with the highest training! IMR is much lower in type-2 diabetics, though it is not clear whether diabetes causes poorer mitochondrial function, or vice versa.

Glucose tolerance and overreaching

To investigate whether the changes in IMR might affect insulin sensitivity, participants also did an oral glucose tolerance test (OGTT). The OGTT involves drinking a high concentration sugar (glucose) drink and analysing the blood for blood glucose levels. During the training program, the blood glucose response to the OGTT did not change from week to week, but after the week of excessive training, the levels rose. Higher blood glucose levels suggests that the body was less effective at removing glucose from the blood stream, meaning less insulin sensitivity. This is also what underpins type-2 diabetes.

Mitochondrial function and glucose tolerance, assessed via intrinsic mitochondrial respiration and an oral glucose tolerance test, respectively, were reduced during the highest intensity training week.

This aspect of the study showed that a week of excessive training, or overreaching, not only caused mitochondrial dysfunction at a molecular level, but also reduced glucose tolerance, which is an important marker for metabolic health. This was alongside a blunted improvement in exercise performance.

Follow up: blood glucose in elite athletes

As part of the same study, the authors did a second, smaller investigation. Instead of looking at the impact of just a few exercise sessions, they recruited national level, elite athletes to see if the changes seen with excessive training result in a real-world problem. Their blood glucose levels were measured for 24 hours a day, for 2 weeks using a continuous glucose monitor (a small device on the body that continuously measures blood glucose) and compared to a group of healthy non-elite volunteers.

Interestingly, although the average blood glucose concentrations across the whole day of the elite athletes and healthy volunteers were almost identical, there were some other major differences. The elite athletes spent much longer than the control group in hyperglycaemic (high blood glucose) and hypoglycaemic (low blood glucose) conditions.

Having long periods of time with high blood glucose levels is a sign of type-2 diabetes, and it is strange that these elite athletes were worse off on this measure than the control group, despite larger amounts of training and exercise which leads to better insulin sensitivity. However, elite athletes often spend large amounts of time overreaching in their training, which is now understood to cause mitochondrial dysfunction, and perhaps related to changes in blood glucose control.

Continuous glucose monitoring data revealed the elite athletes spent a greater period of time in hyperglycaemic and hypoglycaemic conditions compared to the control group.

Could I be training excessively?

It is unlikely that many people who are not high-level athletes would do similar amounts of training to the participants who undertook the excessive training week. It involved almost daily sessions of all out-intensity exercise, and a training volume of over 150 min/week of this style of exercise. Even in this study, just a week of low volume training allowed most markers of health and mitochondrial function to recover. For elite athletes who might perform regular overreaching to maximise performance, intensive training is unlikely to be a major concern for long-term health, as studies have shown that former endurance athletes have lower mortality rates and longer life expectancies than the general population.

Following an acute period of overreaching, a week of low volume training allowed most markers of health and mitochondrial function to recover.

A critical review of this paper was published a few weeks after publication. Read more here.


  1. Flockhart M, Nilsson LC, Tais S, Ekblom B, Apro W, Larsen FJ. Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers. Cell Metab. 2021.

  2. Ruiz JR, Fiuza-Luces C, Garatachea N, Lucia A. Reduced mortality in former elite endurance athletes. Int J Sports Physiol Perform. 2014;9:1046-9.


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