Why HIIT is extremely effective in overweight and obese young people

Physical activity has multiple health benefits and is an important component in the management of pediatric obesity. WHO guidelines recommend that children and young people (aged 5 to 17) should have an average of 60 minutes or more of daily physical activity, mainly aerobic, at moderate to vigorous intensity. However, one of the main obstacles cited by overweight young people for not meeting these recommendations is a lack of time.

Moderate aerobic exercise generates improvements in some of the cardiometabolic risk factors, including fasting insulin, blood glucose content, systolic and diastolic blood pressure and lipid profile, as well as increasing aerobic capacity (VO2max).

Recent studies have addressed the problem of lack of time by proposing high-intensity workouts, which reduce the time needed for exercise compared to moderate intensity programs. Although it is known that moderate intensity aerobic exercise generates improvements in some of the cardiometabolic risk factors, including fasting insulin, blood glucose content, systolic and diastolic blood pressure and lipid profile, as well as increasing aerobic capacity (VO2max), the practice of high intensity interval training (HIIT) has increased significantly in recent years.

The greater interest in this type of training in recent years is particularly relevant for the younger groups, especially considering the high percentage of overweight or obese children in the world and the scarcity of time dedicated to their physical activity.

High-Intensity Interval Training, or HIIT, consists of high-intensity efforts alternated with rest periods.

In planning high-intensity interval sessions, according to a 2013 study by Buchheit and Laursen, some variables need to be considered and "manipulated", including:

  • Intensity and duration of the work interval
  • Intensity and duration of the recovery interval
  • Exercise modality
  • Number of repetitions (determining overall set duration)
  • Number of sets
  • Inter-set recovery

The increased interest in this type of exercise protocol in recent years is particularly relevant for younger groups, especially considering the high percentage of obese or obese children in the world and the scarcity of time dedicated to their physical activity.

HIIT for young people

A recent review of the literature reported that HIIT offers similar or greater benefits for the cardiometabolic profile than other forms of exercise. Although the scientific research dealing with HIIT in obese populations presents inconsistent results, there is an evidence-base to support the greater benefits of HIIT than continuous aerobic exercise.
A meta-analysis conducted by Garcia-Hermoso and collaborators in 2016 studied the effectiveness of HIIT interventions compared to other forms of exercise on improving cardiometabolic risk and aerobic capacity in obese and obese young people. In this study, the authors showed that high-intensity training programs induce greater reductions in systolic blood pressure than continuous and interval training at moderate intensity.
Although the mechanisms underlying these improvements are not yet completely clear, high-intensity interval training may generate a greater reduction in sympathetic tone than moderate continuous activity. In addition, following HIIT programs, there are greater increases in VO2max compared to those obtained with other training techniques.

VO2max is an important marker for cardiometabolic health and is associated with decreases in the risk of morbidity and mortality in the general population, as well as preventing the risk of cardiovascular disease in old age.

The importance of HIIT program duration

The duration of HIIT programs also seems to be an important variable; in fact, it has been shown that long-term, high-intensity training regimens over a period of more than 12 weeks produce greater benefits than shorter program lengths. The effect of HIIT programs on body composition, however, has led to contradictory outcomes.
For example, Batacan and colleagues demonstrated in 2017 that HIIT, in the long term, increases VO2max  and reduces resting heart rate in obese/obese populations, but did not find significant differences in total cholesterol (TC), high-density lipoprotein (HDL), low-density lipoprotein (LDL) or triglycerides (TG).
Keating and colleagues (2014) also reported that 12 weeks of HIIT had no significant effect on fat mass in obese and inactive adults, Gillen, and co-workers (2013) found an improvement in body composition in response to a 6-week HIIT in a similar population group. Several authors have suggested that the reason for the increase in aerobic capacity as a result of HIIT programs is an increase in the availability of oxygen, thus greater cardiac output, total hemoglobin, and plasma volume, or a better ability to extract and use oxygen due to an increase in muscle oxidative potential.
Previous research has shown that HIIT is a very effective training method for improving aerobic capacity and cardiometabolic risk in young people and adults compared to control groups (Baquet et al., 2001; Lambrick et al., 2015).
Trapp and colleagues (2008) demonstrated that 15 weeks of 3 HIIT sessions per week favored weight loss in young women. Song and co-workers (2010) also reported a decrease in body weight and fat mass percentage after a 4-week intervention in slightly obese young women. Racial and colleagues (2013) found a significant reduction in body weight and waist circumference (-3.58%) in a group of obese adolescent females in their study as a result of following a 12-week HIIT training program.
Racial and colleagues (2013) found a significant reduction in body weight and waist circumference (-3.58%) in a group of obese adolescent females in their study as a result of following a 12-week HIIT training program.

The correlation between HIIT and physical inactivity

Other studies have not shown any effect of HIIT interventions on body composition or body weight in sedentary obese adults. This contrast in the reported results indicates that the beneficial effect observed as a result of the HIIT program may depend on the degree of initial obesity of the subjects. In fact, significantly positive correlations were found between initial body weight, body mass index, waist circumference (as an indicator for abdominal obesity) and fat mass at the end of the intervention. The authors also noted that the beneficial effects can be explained by the fact that the sample was composed of highly sedentary and inactive participants.

In fact, as recently pointed out, HIIT may have a greater effect on body composition and weight in obese sedentary individuals than inactive ones.

These beneficial results may also be explained by the recently described effects of HIIT on appetite control and eating behavior. It has recently been shown that HIIT interventions help to reduce hunger and the desire to eat in obese individuals.
In conclusion, high-intensity interval training programs can be effective in specific populations such as obese or obese. There is conflicting evidence in the scientific literature about the benefits of HIIT on aerobic capacity, cardiometabolic and cardiovascular parameters and body composition. A growing body of evidence, however, highlights the beneficial effects of HIIT on markers of health in obese and obese young people In some cases the research indicated HIIT as being more effective than traditionally proposed training methods (moderate aerobic activity of longer session duration). In addition, it is worth consideration that HIIT training could be a good solution to the problem of "lack of time" barrier identified by many young adolescents.
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