Overdoing HIIT Sessions Can Affect Your Health

Picture by Ving Henson from The Pit

Many of my patients tell me they are doing HIIT classes now. If you interested about your fitness and have been exercising regularly, you will definitely know what HIIT stands for. High intensity, interval training.

When I was a teenage runner, those intervals we did certainly did not have such a fancy name like HIIT. You run 15 intervals or repetitions of 400m with a one minute rest in between or the coach will say run 6 x 1km going every 5 minutes (meaning if you run your kilometer in 3:50 min, you get 1:10 min rest before starting again).

I've written how HIIT can be beneficial and how it can even reduce growth in cancer cells. Since HIIT is a super time efficient method for improving aerobic fitness and other health outcomes and metabolic conditions such as diabetes, high cholesterol or high blood pressure. 

One such outcome is increasing our cell mitochondria (power generators in our cells) in our muscles. With more mitochondria in our cells, we become stronger. 

Before you think more HIIT is better, a recent small study showed that people who tried to do 5 HIIT sessions in a week ended up with sudden and severe declines in their mitochondria function, along with blood sugar dysfunction. 

When the subjects decreased their HIIT sessions, their metabolic condition started to improve but did not disappear, hinting that the benefits of extremely strenuous exercise may still depend on how much we do and how we allow ourselves to rest and recover.

The researchers first tested the subjects in their lab for the current fitness levels and metabolic health, which included blood sugar levels throughout the day. In the first week, 2 sessions of HIIT were performed, 5 reps of 4 minute long bike intervals on a stationary bike with 3 mins rest in between. The subjects pedaled as hard as they could while their power output was tracked. Their leg muscles were biopsied while a 24 hour blood sugar control was investigated.

In week 2, they did 3HIIT sessions while increasing some of their intervals to 8 minutes. During week 3, they did 5 HIIT sessions with a mix of 4 and 8 minute intervals of all out cycling. Week 4 allowed for some recovery and they subjects and the amount and intensity of exercises were halved. All the tests were repeated weekly.

The findings showed the subjects getting stronger by week 2. They were pedaling harder and getting fitter, better daily blood sugar control and more mitochondria in the cells. These mitochondria were also more efficient, producing greater energy compared to baseline.

However, by week 3, the subjects were not able to generate more power, their muscle biopsies showed that their mitochondria were only producing 60 percent of the energy of week 2. Blood sugar control levels were spiking and dipping throughout the day.

After allowing for recovery in week 4, the following week showed the mitochondria producing more energy, but still 25 percent less than week 2. Blood sugar levels stabilized too, but again not to the same extent as week 2. The subjects could however ride with the same or even greater vigor as with week 2.

The researchers did not find out what exactly caused the decline in results (glucose intolerance and insulin secretion)  that occurred after week 3. It may be the combination of total biochemical changes that weakened the mitochondria which in turn disrupted the blood sugar control levels. 

This study was done on normal healthy subjects and not athletes nor people with metabolic conditions such as diabetes, high cholesterol or high blood pressure. The latter group attempting such training must get the go ahead from your medical professional. For those of us who just want to be healthier, don't do HIIT sessions excessively since previous studies and formal recommendations suggest 3 intense sessions in a week as maximum. Otherwise, injuries may come knocking on your doors too. 

The researchers managed to assess blood glucose profiles in world class endurance athletes and found that they, too had impaired glucose control when compared with a matched control group. So for athletes who want to be stronger, faster quicker, do allow ample time for recovery between training sessions.

Reference

Flockhart M, Nilsson LC, Tais S et al (2021). Excessive Exercise Training Causes Mitochondrial Functional Impairment And Decreases Glucose Tolerance In Healthy Volunteers. Cell Metab. DOI: 10.1016/jcmet.2021.02.017