It is a well-known and well-established fact that regular exercise is good for your health. In fact, there is a great deal of scientific evidence demonstrating its benefits: from improved cardio-vascular function to positive effects on mood, to the possible prevention of neurodegeneration and consequently different forms of dementia. All forms of physical activity, however, are not the same. Short, very intense workouts are becoming increasingly fashionable, for example, but are they really useful? Or is there a limit beyond which the intensity of physical activity performed may even become counterproductive? According to a study published in Cell Metabolism, there is a limit. Here’s why.

What is High Intensity Interval Training?

The research authors asked a group of 11 healthy volunteers, consisting of six women and five men, to undergo High Intensity Interval Training (HIIT) sessions of gradually increasing duration and intensity for a total of four weeks.

HIIT training consists of short sessions of high intensity anaerobic activity interspersed with short breaks or short sessions of lower intensity aerobic training. It can be any type of exercise, from running to exercise bikes, what matters are precisely the intervals and peaks of very intense activity.

For this study, the authors designed the following type of training: in the first week the participants performed a total offirst week the participants performed a total of 36 minutes of activity (not including minutes devoted to warm-up and defatigue) spread over two training sessions, each consisting of 4-8 minute intervals of very intense activity during which participants had to try to push their performance to the maximum; in the second week the total minutes of activity increased from 36 to 90 (spread over three training sessions), and then reached the 152 minutes in the third week (divided into five training sessions); then

, activity was reduced to 53 minutes total in the fourth and final week.

What are the consequences of high intensity training?

During this period, participants underwent various types of tests, from the ability to “process” glucose following the intake of highly sugary drinks, to muscle biopsies to measure the activity of the mitochondria, the energy powerhouses of the cell.

It was found that starting from the third, most intense week of training, participants developed a form of insulinHormone produced by Langerhans island cells of the pancreas. It stimulates the entry of glucose into the cells, subtracting it from the blood and thus lowering blood sugar. It induces the use of glucose both to produce energy and for the synthesis of glycogen and fats. resistance similar to that seen in people about to develop diabetes, as well as a 40 percent reduction in mitochondria function. These values improved, although they did not regress completely, during the fourth week at reduced intensity.

However, it must be taken into account that the study involved a limited number of participants and that the relatively short duration did not allow for an assessment of any long-term consequences. Nonetheless, these are useful and thought-provoking observations: as always, moderation seems to remain a wise companion.

What are the effects of very intense exercise on the immune system?

Incidentally, a recent study published in Military Medical Research has reignited the debate that intense physical exertion may have a negative impact on immune defenses and may predispose to respiratory tract infections. This is a controversial issue, for which the scientific literature and expert opinion are basically split down the middle.

The novelty of the study is to show the biochemical changes the body would seem to undergo after a rather heavy performance. This research team analyzed the levels of 4,700 molecules (proteins, lipids and other metabolites) in the saliva, urine and blood of 11 firefighters before and after they went through 45 minutes of very intense exercise.

Among their various findings, the researchers specifically observed changes in the composition of the oral microbiota-that is, the populations of bacteria that are regularly present in the mouth and that, among other things, help fight infection. In addition, levels of three pro-inflammatory molecules in saliva were found to be reduced after performance.

Inflammation is detrimental to health when it is chronic, but it is also a physiological mechanism for fighting infection. The latter result could therefore indicate a lower readiness of the body to deal effectively with external “incursions.” However, the real meaning of these observations will need to be further clarified. In addition, again this is a study that involved a limited number of participants, so the results will need to be confirmed on a larger and more heterogeneous sample.