Study: High-Intensity Interval Training May Be Hazardous to Your Health

Once upon a time, it was believed that any type of exercise was beneficial to health. Apparently, those beliefs were naïve. As scientists focus on the molecular and cellular changes that occur in response to different exercise approaches, it has become clear that one particular type of exercise, high-intensity interval training, also known as HIIT, has the potential to be hazardous to your health.

One recently published study investigated people who followed a typical HIIT exercise routine. The scientists examine the relationship between mitochondrial dysfunction, metabolic health, and glucose intolerance in the healthy subjects as they progressively increased training loads.

Female and male subjects took part in a four-week HIIT training course. Muscle biopsies and oral glucose tolerance tests were performed throughout training. The HIIT protocol used in their study has been proven effective at enhancing aerobic fitness. Previous studies have speculated that the short-term benefits of HIIT may be due to increases in the number of mitochondria in muscle cells. The assumption is that the additional mitochondria allow the muscle to generate energy from body fat during the intensive training sessions. However, studies of subjects who completed HIIT routines three times a week for six weeks did not improve their blood pressure or reduce their total body fat as much as people 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, along with emerging signs of blood sugar dysfunction. Fortunately, their metabolic issues started to reverse when they dialed back on their workouts to a moderate level. However, the metabolic issues did not disappear. These results suggest that the benefits of extremely energetic exercise may depend on just how much is performed.

The typical cellular response of a given unit of muscle tissue following a moderate level of exercise is to increase the amount and respiratory capacity of its mitochondria; this has the effect of increasing the respiratory capacity of the muscle. In contrast, more sophisticated examinations of human subjects demonstrate that mitochondrial respiration is actually reduced after excessive training even in the presence of an increase in the density of mitochondria.

Given this finding, it is not surprising that elite endurance athletes have a disturbed glucose control with an elevated 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 glucose levels than do control subjects. A typical control subject spent 22 minutes in the hyperglycemic range, while elite athletes spent almost twice as long in the hyperglycemic range. In the elite athletes, the hyperglycemia tended to occur 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 investigations that show that there is an upper limit of the amount of intensive exercise that can be performed without disrupting metabolic homeostasis. Beyond this limit, negative effects on metabolic health and adaptation of physical performance, seemingly caused by a mitochondrial partial shutdown of normal respiration. Moderation in exercise is still the wisest choice.