Can people get too much exercise? Mitochondria hint that the answer is yes

Elite athletes help to reveal the costs of intense physical activity.
Regular exercise is a boon to health, but fanatical workouts might have a metabolic downside. A new analysis suggests that highly intensive exercise blunts the body’s ability to regulate blood sugar because it impairs mitochondria, the powerhouses of the cell.
Mikael Flockhart and Filip Larsen at the Swedish School of Sport and Health Sciences in Stockholm and their colleagues had 11 volunteers do 3 weeks of increasingly strenuous exercise. The participants’ athletic performance improved after the transition from light to moderate training, but stagnated with the most intense regimen.
Weekly muscle biopsies revealed that the energy-producing capacity of mitochondria initially increased with exercise intensity, but dropped drastically with vigorous training. This reduction in mitochondrial activity coincided with an impaired ability to metabolize the glucose in a glass of sugar water that participants gulped.
The researchers also studied a separate group of 15 professional endurance athletes and 12 non-athletes, and found that the average blood-sugar levels for each group over 24 hours were nearly identical. But the professionals experienced longer periods of high and low blood sugar than the non-athletes.