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Low volume of HIIT and its cardiometabolic impact

Regular physical activity has been shown to bring many benefits for the prevention and treatment of many chronic diseases. Unfortunately, among the modifiable risk factors for life expectancy, the prevalence of physical inactivity is the highest. The "lack of time" is one of the main reasons cited by respondents to explain their sedentary lifestyle. Current physical activity guidelines (WHO) recommend at least 150 minutes at moderate intensity (3-6 METs) or 75 minutes at high intensity (> 6 METs) per week, in increments of 10 minutes minimum, in addition to two resistance training sessions.

As discussed in a previous article, the HIIT has become an interesting alternative because thanks to its intermittent nature, it allows higher intensities and a training time shorter than the classic cardiovascular endurance (ie, running for 30 minutes or more at constant and moderate speed). And despite the different training volume, it allows benefits similar to those seen with the classic "cardio". However, with the warm-up, some HIIT protocols still require a half-hour session. And some scientists have wondered if HIIT is really so time efficient. But what if the HIIT session was even shorter? Is the physiological impact diminished? And compared to classic "cardio"?

The Study

To answer these questions, Professor Gibala's team at McMaster University, Canada compared two cardiovascular training protocols with different volumes and intensities and their impact on several metabolic parameters. For this, they recruited 25 sedentary men (<600 MET-minutes per week) and divided them into 3 groups : HIIT (n = 9), MICT (n = 10) and Control (n = 6).

For 12 weeks, both training groups participated in 3 sessions per week (except for the 1st and 2nd week where only 1 and 2 sessions were performed, respectively). During the 7th week, two sessions were replaced by laboratory evaluations. The Control group only came to the laboratory for the evaluations. The training program was conducted on a cyclo ergometer and was as follows :

• HIIT : 3 x 20s "all-out" (resistance : 0.05 kg / kg body mass) separated by 2 minutes of low-intensity cycling (50W)
• MICT : 45 min at 70% FC_MAX

For each session, a warm-up of 2 min and a cool-down of 3 min at 50 Watts were included. The total duration of each session was 10 minutes for HIIT and 50 minutes for MICT.

Before and after the 12 weeks of the protocol, the researchers assessed fat mass via plethysmography, cardiorespiratory fitness via VO_2PEAK, glucose tolerance with a fasting intravenous test over 50 minutes (IVGTT) to determine the insulin sensitivity index (CS_I), the area under the glucose curve during the 50 minutes and the insulin resistance (HOMA-IR). They also performed a muscle biopsy to measure the enzymatic activity of citrate synthase (CS) (enzyme involved in the Krebs cycle), and 3-Hydroxyacyl-CoA dehydrogenase (β-HAD) (an enzyme involved in β-oxidation) and to measure the content of various proteins including GLUT4, a glucose transporter protein.

Results & Analyzes

The main results of this study show that in sedentary men, even in small doses, HIIT allows cardiometabolic improvements similar to those observed with MICT. Both groups significantly improved their body fat percentage, VO_2PEAK (+ 19%) (Fig. 1), insulin sensitivity, enzyme content, and GLUT4 content. No improvement was observed for the control group.

In comparison with the "snack training" which was discussed in a previous article, the improvement of the VO_2PEAK corresponds here to 6 ml/kg/min, ie ~1.7 METs. This improvement is clinically significant since, as some studies show, it is comparable to a decrease of 7 cm in waist circumference, a decrease in systolic blood pressure of 5 mmHg or a reduction of 1 mmol/L in fasting blood glucose, in terms of relative risk reduction in all-cause and cardiovascular disease mortality.

While the volume of the two experimental protocols was totally different, 1 minute of maximal effort over 10 minutes for the group HIIT vs. 50 continuous minutes at moderate intensity (~60 vs. ~310 kJ per session), this study demonstrates the effectiveness of brief and intense efforts to improve cardiometabolic parameters. In addition, the maximal activity of citrate synthase is a marker strongly associated with mitochondrial content in skeletal muscle, and the results of this study suggest that mitochondrial improvements are strongly related to exercise intensity rather than volume.

Practical Applications

3 x 10 minutes per week is all that is needed to significantly improve sedentary markers of cardiorespiratory and metabolic health, at least as much as practicing 150 minutes (the equivalent of official recommendations). Nevertheless, the 3 minutes of effort distributed in these 30 minutes must be spent at maximum intensity. This does not necessarily suit everyone from the point of view of personal preferences. Some people will not like to make a long effort, and others will not like to practice at maximum intensity.

The time for physical activity, which may seem to some to be THE main barrier to access, is finally minimal and it only needs good willpower to find the time to work it out.

References

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