It was once believed that any kind of exercise was beneficial to health. Apparently these beliefs were naive. As scientists focus on the molecular and cellular changes that occur in response to different approaches to exercise, it became clear that one type of exercise, high-intensity interval training, also known as HIIT, is potentially harmful.
A recently published study looked at people who followed a typical HIIT exercise routine. Scientists are studying the relationship between mitochondrial dysfunction, metabolic health, and glucose intolerance in healthy subjects as they gradually increase the exercise load.
Female and male subjects took part in a four-week HIIT training course. Muscle biopsies and oral glucose tolerance tests were performed throughout the exercise. The HIIT protocol used in their study has been shown to be effective in improving aerobic fitness. Previous studies have speculated that the short-term benefits of HIIT could be due to an increase in the number of mitochondria in muscle cells. The assumption is that the extra mitochondria allow the muscle to generate energy from body fat during intense workouts. However, studies of subjects who did HIIT routines three times a week for six weeks did not improve their blood pressure or reduce their total body fat as much as those who exercised far more moderately five times a week. What explains the lack of benefit from HIIT?
The HIIT subjects developed sudden and severe declines in the function of their mitochondria, as well as emerging signs of blood sugar dysfunction. Fortunately, their metabolic problems began to reverse when they switched back to moderate levels of exercise. However, the metabolic problems did not go away. These results suggest that the benefits of extremely energetic exercise may depend on how much is done.
The typical cellular response of a given unit of muscle tissue after a moderate level of exercise is to increase the amount and breathing capacity of its mitochondria. This has the effect of increasing the breathing capacity of the muscle. In contrast, more sophisticated studies on human subjects show that mitochondrial breathing is actually reduced after excessive exercise, even if the density of the mitochondria increases.
Given this finding, it is not surprising that elite endurance athletes have impaired glucose control with an increased risk of developing type 2 diabetes. There are numerous ways to explain this phenomenon. Elite athletes consume too many calories, and too many of those calories come from carbohydrates. Elite athletes spend more time with significantly higher blood sugar levels than controls. A typical control object spent 22 minutes in the hyperglycemic zone, while elite athletes spent almost twice as long in the hyperglycemic zone. In top athletes, hyperglycaemia tended to appear in the early afternoon; In contrast, they were hypoglycemic while sleeping between 3 a.m. and 7 a.m.
This recent study joins a long list of other research showing that there is an upper limit to the amount of vigorous exercise that can be done without disrupting metabolic homeostasis. Beyond this limit, negative effects on metabolic health and the adaptation of physical performance, which are apparently caused by a partial mitochondrial shutdown of normal breathing. Exercise moderation is still the smartest choice.
Wenk GL (2021) Your brain while exercising. (Oxford University Press).
Flockhart M et al. (2021) Excessive exercise leads to an impairment of mitochondrial function and decreases glucose tolerance in healthy subjects. Cell Metabolism, Vol. 33, pp. 957-970.e6.