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