Overweight is an important risk factor for chronic non-communicable diseases. ^{1 – 3} The global prevalence of overweight in adolescents is approximately 20–25%, and is related with high consumption of ultra-processed food (UPF). ^{4 – 8} Generally, UPF are industrially manufactured foods that contain large amounts of calories, trans fats, sugars, sodium, and chemical additives. ^{9 – 11} Overweight is also associated with lower intake of vitamins, minerals, fiber and protein. ^{7 , 8} In this context, reducing the consumption of UPF, as well as improving the amount of physical activity and exercise practice are essential to decreasing the prevalence of overweight in children and adolescents.

Exercise is associated with food consumption and energy intake. ^{12 , 13} Drenowatz et al. ¹⁴ reported in their review that longitudinal analyses indicate that habitual exercise is beneficial on dietary intake and intake of specific foods. In addition, exercise has been shown to lead to several health benefits in school-aged children and adolescents. ^{15 – 17} To reach wide health benefits, increasing moderate and vigorous activities are indicated, the highest advantage given when exercise is at vigorous intensity. ^{18 , 19} In accordance with the current recommendation for children and young people (5–17 years old), vigorous-intensity activities are critical “including those that strengthen muscle and bone, at least three times per week”. ²⁰ Cardiorespiratory fitness and muscular strength are central indicators of health in infants and youth. High cardiorespiratory fitness and muscular strength are related to healthy outcomes and a lower risk of developing diseases in the next decades. ^{19 , 21 – 23}

Currently, within vigorous intensity activities, there is a surge of interest in high-intensity interval training (HIIT), in both research and clinical perspectives views. In this regard, a recent review ²⁴ compared the effects between moderate-intensity continuous training and interval training and showed that interval training provided greater reductions in total absolute fat mass. HIIT refers to alternating short bursts of high intensity activities with recovery periods or light exercise ²⁵ and ‘near maximal’ efforts performed at ≥80% of the maximal heart rate or the equivalent as expressed in the function of the maximal oxygen consumption. ²⁴ Although HIIT has shown several benefits (e.g. increase in VO ₂ peak ²⁶ ; improving cardiorespiratory fitness and body composition ²⁷ ; beneficial effects on cognitive performance ^{28 , 29} and mental health ²⁹ ), to the best of our knowledge, no systematic review has focused on HIIT effects on UPF consumption and the relationship between the two remains inconclusive. To fill this gap in the current literature, our study objectives aimed to answer the following question: is HIIT associated with UPF consumption in adolescents? We executed a systematic review that aimed to investigate the influence of HIIT on UPF consumption in adolescents.

The eligibility criteria were specified according to the Population, Intervention, Comparison, Outcome (PICO) framework: “P”, adolescent; “I”, the practice of high-intensity training; “C”, the use of other therapies or placebo; and “O”, UPF.

The eligibility criteria were articles with the following features: (a) adolescent samples (10–19 years old, as defined by the World Health Organization ³⁰ ); (b) assessment of high-intensity interval training; (c) assessment of UPF consumption; (d) studies with observational and clinical trials designs. No language and year of publication restrictions were applied. Articles with incomplete results and systematic reviews were excluded.

We used three databases (SCOPUS, PubMed, and Embase) for our searches. The search strategy pattern of three blocks of keywords used in PubMed are provided in Box 1. Identical keyword patterns were searched in SCOPUS and Embase. We ran the search for each database in June 2020. The search was performed by two independent researchers (MN and CRM), who also evaluated all references of included articles to find additional sources. Moreover, we also searched in Google Scholar for potential additional articles Box 1.

(“adolescent”[MeSH Terms] OR “minors”[MeSH Terms] OR “adolescent”[Title/Abstract] OR “minors”[Title/Abstract] OR “Youth”[Title/Abstract] OR “juvenile”[Title/Abstract] OR “teen”[Title/Abstract] OR “teenager”[Title/Abstract] OR “Childhood”[Title/Abstract] OR “adolescence”[Title/Abstract] OR “schoolchild”[Title/Abstract] OR “Schoolchildren”[Title/Abstract] OR “young”[Title/Abstract])

AND

(Ultra-processed[Title/Abstract]) OR “ultra processed”[Title/Abstract]) OR ultraprocessed[Title/Abstract]) OR food processing[MeSH Terms]) OR ready-to-eat[Title/Abstract]) OR “ready to eat”[Title/Abstract]) OR “ready-to-consume”[Title/Abstract]) OR fast food[MeSH Terms]) OR “fast-food”[Title/Abstract]) OR “fast food”[Title/Abstract]) OR fastfood[Title/Abstract]) OR “junk food”[Title/Abstract]) OR “Ready Prepared Food”[Title/Abstract]) OR “Convenience Food”[Title/Abstract])

AND

(“High-intensity training”[Title/Abstract] OR “High-intensity interval training”[Title/Abstract] OR “high-intensity interval training”[MeSH Terms] OR “high intensity intermittent training”[Title/Abstract] OR “Repeated sprint training”[Title/Abstract] OR “interval training”[Title/Abstract] OR “intermittent training”[Title/Abstract] OR “high intensity sprint”[Title/Abstract] OR “aerobic interval training”[Title/Abstract] OR “High-intensity interval training”[Title/Abstract] OR “high-intensity interval exercise”[Title/Abstract] OR “high-intensity interval exercise”[Title/Abstract] OR “high-intensity intermittent exercise”[Title/Abstract] OR “high-intensity intermittent exercise”[Title/Abstract] OR “high intensity intermittent training”[Title/Abstract] OR “high intensity intermittent training”[Title/Abstract] OR “HIIE”[Title/Abstract] OR “HIIT”[Title/Abstract]).

After executing the search strategy, articles from all databases were organized, and duplicates eliminated with Mendeley software (Desktop version - 2021). Two reviewers (MN and CRM) separately screened the titles and abstracts of all studies identified in the literature search to see if they met the eligibility criteria. Disagreements were resolved by a third reviewer (PN). All three reviewers have backgrounds in systematic reviews and were trained on the eligibility criteria.

Eleven suitable articles were recognized (PubMed, 2 studies; SCOPUS, 4 studies; Embase, 5 studies) ( Figure 1). After removal of duplicates, five articles remained. ^{31 – 35} None of the five articles met our eligibility criteria; elimination reasons included that the population was adults and not adolescent, or UPF consumption was not included as an outcome.

In accordance with Yaffe et al., ³⁶ if no study meets the eligibility criteria, the review is referred to as an ‘empty review’; empty reviews are known to contribute to inspiring future research. ³⁶ Despite the empty review, we believe that it is pertinent to publish our results as our study identifies a gap in the literature concerning HIIT, adolescents and UPF consumption, and may motivate new investigations on this topic and contribute to progress in this research. The present lack of evidence highlights how essential additional epidemiological research will be, especially those with high quality methods and longitudinal and controlled trial designs.

In adults under a fast food diet, HIIT seems to prevent weight gain and cardiometabolic deterioration. ³² These initial findings demand more investigation, as they show promising results against the negative effects of fast foods. Most ready-to-consume foods that are present in fast foods can be considered UPF in the novel classification according to the processing level. ^{10 , 11} In adolescents with obesity, a single HIIT session is associated with reduced energy intake and subsequent food reward. ³⁷ Thus, investigating strategies to counteract the deleterious effects of UPF in health is required and HIIT may have a role to play.