Introduction

Globally, the prevalence of individuals with overweight and obesity, defined by a body mass index (BMI) of 25 or higher, is steadily increasing, presenting a significant health threat linked to both morbidity and mortality [1, 2]. The condition contributes significantly to the development of non-communicable diseases, with obesity and being overweight acting as notable risk factors. The occurrence of vascular dysfunction is significantly more common in adults who are overweight or with obesity compared to those of with normal weight. Research shows that individuals with obesity exhibit a 54–57% decrease in flow-mediated dilation (FMD), indicating vascular dysfunction [3, 4]. This condition is a major risk factor for cardiovascular diseases, including hypertension, coronary artery disease, and stroke, metabolic abnormalities, including dyslipidemia, hypertension, oxidative stress, insulin resistance, and heightened inflammation [5, 6]. These factors increase the risk of vascular dysfunction by diminishing nitric oxide bioavailability and disrupting vascular homeostasis [5].

The term “vascular function” refers to the ability of the vessel to expand in response to stimulation or shear stress, achieved through the heightened synthesis of nitric oxide. This fundamental process serves as the cornerstone for vessel relaxation and vasodilation, crucial elements in maintaining optimal blood flow and overall cardiovascular health [7, 8]. The assessment of vascular function plays a pivotal role in understanding the intricate dynamics of the circulatory system and can be assessed invasively or non-invasively. Non-invasive techniques are particularly essential, providing a comprehensive understanding of vascular health without the risks associated with invasive procedures. Approaches such as Doppler ultrasound, flow-mediated dilation (FMD), arterial stiffness measurement (e.g.: Pulse Wave Velocity (PWV), Augmentation Index (AIx), Ankle-Brachial Index (ABI)), carotid intima-media thickness (cIMT) and arterial tonometry offer valuable insights into the efficiency and responsiveness of blood vessels [9]. These methods contribute significantly to our understanding of vascular health, enabling a nuanced exploration of the factors influencing circulatory dynamics.

Exercise is one of the therapeutic interventions that has shown significant improvement in cardiovascular health, particularly in enhancing vascular health [10]. It has demonstrated the ability to increase nitric oxide availability, improve endothelial function, reduce oxidative stress, and aid in vascular remodeling [10,11,12,13,14]. Moderate Intensity Continuous Training (MICT) has been extensively studied. It involves prolonged periods of activity at 40–80% of maximal oxygen consumption and has proven effective in reducing the risk of cardiovascular disease [15]. Another form of exercise known as high-intensity interval training (HIIT) entails brief and intense intervals of activity interspersed with periods of rest or low-intensity exercise and has gained interest in recent years [16]. Remarkably, HIIT has been shown to improve fitness and health among young adults with overweight and obesity [17, 18]. However, current literature presents mixed findings regarding the comparative effectiveness of HIIT and MICT in improving vascular function. Some studies suggest that HIIT is more effective than MICT in enhancing FMD in individuals with obesity, indicating better vascular adaptations due to the high-intensity bursts of activity in HIIT [19,20,21]. This suggests that HIIT may stimulate more significant improvements in endothelial function compared to the steady, moderate exercise of MICT. Conversely, other studies report no significant difference between HIIT and MICT as FMD remained unchanged [22]. This inconsistency extends to other measures of vascular function, such as arterial diameter, blood flow, and velocity. For instance, some research indicates that both HIIT and MICT produce similar changes in arterial diameter and flow, while others find no measurable changes at all, regardless of the exercise regimen [23]. Despite these contradictory findings, studies suggest that HIIT can yield comparable, if not superior, improvements in cardiorespiratory fitness, insulin sensitivity, and endothelial function compared to standard MICT [20, 21]. This suggests that while the specific effects on vascular function may vary, HIIT generally offers substantial cardiovascular and metabolic benefits. These benefits are particularly pronounced in populations with obesity, where HIIT has shown promise in improving several key health markers more efficiently than traditional moderate-intensity exercises. This highlights the complexities and varying outcomes associated with different exercise regimens, underlying the need for further research to fully understand the distinct benefits and limitations of HIIT and MICT on vascular health.

Previous reviews have consistently affirmed that HIIT is a more effective strategy for enhancing vascular function than MICT. However, these conclusions are predominantly based on studies that have concentrated on populations such as those with coronary artery disease (CAD), hypertension, metabolic syndrome, congestive heart failure (CHF), and so on [21, 24, 25]. These studies have contributed valuable insights into the impact of HIIT and MICT programs on vascular function. Notably, a comprehensive systematic study conducted by Way et al. [26] arrived at a distinct conclusion, stating that no significant differences were observed in central arterial stiffness between HIIT and MICT. This diversity in results underlines the ongoing uncertainty regarding the most effective exercise modality for enhancing vascular function in individuals with overweight and obesity. Therefore, this systematic review aims to investigate the influence of both HIIT and MICT on vascular function among individuals with overweight or obesity.

Methods

Eligibility criteria

This systematic review has adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [27]. The inclusion criteria were developed using the Cochrane guidelines for conducting systematic reviews [28]. All the authors decided upon and approved the inclusion and exclusion criteria. Following the original selection of studies, the eligibility assessment was carried out independently, blindly, by two authors (SKB and TB) who screened the abstracts and titles. The following requirements had to be met for the manuscript to be considered for inclusion:

  1. (1)

    Population—adults (18–60 years) who are overweight or with obesity, with a BMI of 25 kg/m2 and above without any health issues such as CVD, neurological disorders, diabetes, hypertension, metabolic disorders, musculoskeletal injuries, and sleep apnea.

  2. (2)

    Intervention—high-intensity interval training, which alternates between periods of low-intensity exercise or rest and short bursts of extremely intense activity, such as running, cycling, or workouts with a VO2max or MHR more than 80% of intensity and moderate-intensity interval training involving prolonged periods of activity of walking or cycling with 40–80% of VO2max both given for a minimum period of 4 weeks.

  3. (3)

    Comparison—high-intensity interval training versus moderate-intensity interval training. Studies that involve interventions other than HIIT and MICT or those that include only one of these were excluded.

  4. (4)

    Outcome variables—assessing vascular functions such as endothelial dysfunction, arterial stiffness, flow-mediated dilation, and pulse wave velocity.

  5. (5)

    Design—randomized trials or non-randomized trials like quasi-experimental, pre-post study involving the human population and published in English before January 2023 were included. Other study designs such as cross-over trials, case reports, case-control, cross-sectional designs, animal studies, letters to the editor, conference abstracts, and literature reviews were excluded.

Literature search strategy and information sources

The computerized English-language literature search was conducted using the Manipal Academy of Higher Education electronic library, PubMed (MEDLINE), Scopus, SPORT Discus® (via EBSCOhost), CINAHL, and Web of Science electronic databases. To find pertinent data on HIIT, MICT, and vascular function in the titles, abstracts, and keywords of the indexed publications, the following search syntax was used with Boolean operators: (“high-intensity interval training”[Mesh] OR “high intensity interval training” OR HIIT) OR (“aerobic interval training” OR “continuous training” OR “moderate-intensity continuous exercise” OR MICT OR “endurance” OR “walking” OR “running” OR “cycling”) AND (“obesity” OR “overweight”) AND (“Vascular Stiffness” OR “Blood Flow Velocity” OR “endothelial function” OR “vascular function” OR “vascular resistance” OR “flow-mediated dilation” OR “peak wave velocity” OR “pulse wave velocity” OR “intima-media thickness” OR “nitric oxide” OR “Arter* complian*” OR “arter* elasticit*” OR “vascular* stiff*” OR “vascular* complian*” OR “vascular* elasticit*”) (Appendix 1). Furthermore, truncation, proximity searching, and other sophisticated search techniques were included. Covidence software was used to manually screen the articles and reference lists of all included papers for additional relevant publications (SKB & TB) as part of the secondary search. In order to investigate the possibility of leveraging authors and citations for follow-up investigations, forward reference searching was also done. To mitigate potential selection bias, TB carried out the study selection searches on their own. The flow of articles by the study selection process is shown in Fig. 1 using the PRISMA 2020 flow diagram [27].

Fig. 1: PRISMA 2020 flow diagram indicating the number of studies retained and excluded at each stage of the review process.
figure 1

PRISMA 2020 flow diagram for new systematic reviews which included searches of databases, registers, and other sources.

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Study selection

When the title and abstract of an article were not sufficient to evaluate its relevance to the review, the complete text of the article was retrieved and reviewed. This allowed the writers to determine if the work satisfied the primary inclusion conditions. If the primary goal of the publications did not entail a study looking at how HIIT and MICT affect vascular function, they were excluded from the review.

Data extraction

The two reviewers (SKB and TB) retrieved data independently, while a third (KV) checked the data with the following data extracted from the included papers: (1) the study authors and date; (2) the number of participants and participant’s characteristics (age, gender, body weight, and BMI); (3) exercise protocol (frequency, intensity, time, and type of HIIT and MICT); (4) outcome measure; (5) equipment used; (6) main findings—vascular function variables which were assessed.

Quality assessment

The quality of all randomized studies was assessed using the Risk of Bias (ROB) 2.0 tool that addresses five domains of the randomization process, deviation from intended intervention, missing outcome data, measurement of the outcome, and selection of the reported result. The quality of the non-randomized study was assessed using the Risk of Bias in Non-randomized Studies—of Interventions (ROBINS-I) tool by the recommendation of the Cochrane Scientific Committee. This tool addresses 7 domains namely: confounding, selection of participants, classification of interventions, deviations from intended interventions, missing data, measurement of outcomes, and selection of the reported result. Three categories—“high,” “some concerns,” and “low” risk of bias—are used to group these assessments that are based on the responses to the signaling questions in both tools to determine the overall bias judgment. A visualization of the outcomes was produced using “traffic light” plots of the domain-level judgments for each result, following the guidelines provided by each evaluation tool.

Results

Search results

The initial database search yielded 7705 articles, and a further 866 via citation and organization searches. The number of articles discovered in each electronic database or by using other techniques is shown in Fig. 1, along with a comprehensive flowchart of the literature search’s phases. After duplicates were eliminated, 5397 titles collected from databases were still included in the Covidence software. Following a review of all the papers’ titles, abstracts, and keywords, 94 papers were sought for retrieval and retained for full-text analysis. The systematic review included 7 articles after determining that the inclusion criteria were eligible, and 87 articles were excluded. Figure 1 shows the explanations for exclusion. The eligibility of a total of reports found through organizations and citation searching was evaluated, and of these 4 were included in the review.

Study characteristics

A detailed study characteristics of eleven studies that were included in this review are shown in Table 1, the publication year of these studies ranged from 2008 to 2023. Ten of the included studies were randomized trials and one resulted in a quasi-experimental study. The review incorporated three studies focusing on individuals with overweight [22, 23, 29], five studies on individuals with obesity [19, 30,31,32,33], and three studies involving individuals with both overweight and obesity [34,35,36]. Eight of these studies were supervised training [19, 22, 23, 31, 33,34,35,36] whereas three of them were a combination of supervised and unsupervised training [29, 30, 32].

Table 1 Summary of the articles reviewed for individuals with overweight and obesity (n = 11) with an overview of the participant characteristics, exercise protocol, exercise supervision, outcome variables, equipment used, and the main findings related to vascular functions.
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Participant characteristics

The detailed characteristics of the participants in the included studies are presented in Table 1. A total of 346 participants with an average of 31 participants per study were included. The number of participants in each study ranged from 12 to 90. The age of the participants ranged from 18 to 55 years and the BMI of the participants ranged from 25 to 36 kg/m2. Both genders were included in 6 studies, 4 included only men and 1 included female participants only. In this review, healthy individuals with overweight and obesity were included.

Intervention characteristics

The intervention characteristics of the included studies are described in Table 1. The frequency of the training ranged from 3 to 5 sessions per week with 3 sessions per week being the most common among 7 studies [19, 22, 29, 31, 32, 34, 36]. Most of the research used HIIT sessions that ranged in intensity from 85% to 95% of MHR; a small number of the studies utilized 90% Heart Rate Reserve (HRR) and active recovery periods. Two studies chose to use Sprint Interval Training (SIT), which is characterized by intense bursts of activity, in contrast to most of the included studies that used HIIT [23, 31]. MICT protocols included 60%–70% MHR and an HRR for a duration of 45 min that ranged between 30 min [19] to 60 min [31]. One study compared strength training with an intensity of 90% 1 RM along with HIIT and MICT [32]. The intervention duration ranged from 4 weeks [31] to 12 weeks, with 12 weeks being practiced in 4 studies [23, 30, 32, 36]. Most of the studies used cycling, treadmill walking, and running as the modality of intervention. Five studies included a control group among which four studies were advised to maintain their routine [22, 23, 35, 36] whereas in one study control group was educated about the dangers of obesity, its causes, and ways to improve through lectures and videos, and online communication groups [33].

Measurement of vascular function

Vascular function among individuals with overweight and obesity was assessed in 11 studies, and significant improvements were observed in at least one of the assessed parameters, including FMD, venous compliance, Pulse Wave Velocity (PWV), and Augmentation Index (AIx) in eight of the studies. Focusing on the comparative effects of HIIT and MICT on FMD in five studies, two studies revealed a substantial enhancement in FMD by 3.8% following HIIT [19, 32]. Significant improvements in post-occlusion peak diameter (5%), mean artery diameter (23%), shear rate (28%), and mean blood flow velocity (23%) were also seen with HIIT, while these remained unchanged in MICT [19, 32]. Three additional studies found no significant alterations in FMD following either exercise modality [23, 34, 35]. Additionally, among six studies examining arterial stiffness, two reported significant improvements in AIx 126.7% [30, 31], one in PWV [31], and one in Ankle-Brachial Index (ABI) 15% [36] within the HIIT group. Conversely, two studies found no changes in AIx [22, 29], and one study found PWV and carotid distensibility remained unchanged [23]. Particularly, Shi et al. [33] noted significant improvements in Wall Shear Stress (21.6%), circumferential strain (18.1%), and a decrease in arterial stiffness (18.8%), pulsatility index (7%), arterial diameter (6%), and oscillatory shear index in the HIIT group [33]. These findings underline the varied impact of HIIT and MICT on vascular parameters.

Methodological quality control and publication bias

The risk of bias (ROB) 2.0 tool was used in ten studies and the entire results are shown in Fig. 2. All ten studies in outcome measurement (Domain 4), nine studies in reported result selection (Domain 5), eight studies in missing outcome data (Domain 3), seven studies in randomization process (Domain 1), and five studies in deviation from intended intervention (Domain 2) have a low risk of bias. Whereas five studies in the domain of deviation from the intended intervention (Domain 2), three from the randomization method, two studies in missing outcome data (Domain 3), and one research in the domain of selection of reported outcomes (Domain 5) raise some concerns. The ROBINS tool was used in one non-randomized study [36] (Fig. 3). Across all domains, bias due to confounding (Domain 1) was discovered to have a moderate bias risk, whereas all other domains exhibited a low bias risk.

Fig. 2: Risk-of-bias ViSualization (robvis).
figure 2

Risk of bias of the ten included studies, according to the RoB 2.0 tool using the “traffic light” plots of the domain-level judgments for each individual result [48].

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Fig. 3: Risk-of-bias ViSualization (robvis).
figure 3

Risk of bias of one included study, according to the ROBINS-I tool using the “traffic light” plots of the domain-level judgments for each individual result [48].

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Discussion

This study aimed to systematically review and compare the effects of high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT) on various measures of vascular function in individuals with overweight and obesity. In this review, data from 11 trials were analyzed to determine the effects of HIIT and MICT interventions on vascular functioning. The following are the significant findings from this assessment: firstly, a considerable proportion of the publications (72%, N = 8) showed significant differences in at least one vascular function variable. Second, most studies found that HIIT interventions outperformed MICT in terms of improving vascular functioning.

In six studies, HIIT outperformed MICT, strength training, and control interventions in improving vascular function. A meta-analysis undertaken by Sabouri et al. [25] supports this corroborative tendency by highlighting HIIT as a particularly beneficial technique for increasing FMD in individuals dealing with overweight or obesity. This meta-analysis data suggests that HIIT increases FMD by 2.6% compared to MICT and by 1.83% compared to no exercise intervention [25]. Studies by Sawyer et al. [19] and Schjerve et al. [32] both demonstrate significant improvements in FMD—3.89% after 8 weeks and a noticeable improvement after 12 weeks of HIIT, respectively [19, 32]. Importantly, studies established that a mere 1% increase in FMD corresponded to an 8–13% decrease in the risk of cardiovascular disease [37]. Exercise-related increases in FMD may be caused by an increase in NO bioavailability brought on by increased blood flow and higher shear pressures on the endothelium [38]. Increased exercise training lowers pro-inflammatory and oxidative stress factors linked to vascular dysfunction [39]. Accordingly, HIIT’s superior ability to improve vascular function over MICT might result from its capacity to increase blood flow to working muscle, which facilitates higher NO bioavailability generated by shear stress [40, 41]. In line with this, Shi et al. [33] discovered that after 8 weeks of HIIT, there was a rise in blood flow and wall shear stress (11.5% and 21.6%) [33]. However, few studies showed that there were no differences in FMD between HIIT, MICT, and control groups [34, 35]. This could be attributed primarily to elevated baseline endothelial function, potentially impeding FMD improvement [34]. Additionally, some participants in these studies had consumed doughnuts, potentially hindering the anticipated improvements [35].

On the other hand, a study conducted by Shenouda et al. [23] demonstrated that 12 weeks of traditional MICT was more effective than HIIT in producing notable enhancements in brachial artery FMD and artery diameter, which were not observed in HIIT. The potentially pivotal factor behind this discrepancy could be the lower volume of HIIT compared to MICT, possibly contributing to the superior results observed with the latter [23].

Arterial stiffness was evaluated by five studies, Cheema et al. [30] observed a significant reduction in the augmentation index (AIx) after 12 weeks of high-intensity boxing sessions lasting 50 min each, accompanied by enhancements in endothelial function and peripheral resistance [30]. Shi et al. [33] reported a notable decrease in arterial stiffness with a higher circumferential strain and a lower oscillatory shear index following HIIT, contrasting with MICT [33]. Although not statistically significant, the improvement in arterial stiffness after HIIT may be attributed to an increase in circumferential strain, as it has been shown to negatively correlate with arterial stiffness [42]. Farahati et al. [36] discovered a significantly higher right Ankle-Brachial Index (ABI) in the HIIT group after 12 weeks of training at an intensity of 85%–95% of maximal heart rate [36]. These findings align with previous assessments demonstrating that aerobic exercise significantly reduces PWV and AIx in adults, as indicated by studies like Ashor et al. [43] and Huang et al. [43, 44]. Contrarily, Cocks et al. [31] research indicated that both HIIT and MICT significantly increased AIx and carotid-femoral Pulse Wave Velocity (cPWV) [31]. However, inconsistencies exist, as reflected in a systematic review by Way et al. [26] which found no discernible difference in changes to central arterial stiffness measured by AIx, AIx@75, and PWV between HIIT and MICT [26]. In accordance with this, Shepherd et al. [29] also reported no change in AIx after using either training mode [29].

When compared to MICT, HIIT is known to provide 40% gain in time efficiency [17, 45, 46]. The greatest hurdle to exercise is time, which prevents people from engaging in it regularly [47]. The time-effectiveness of HIIT improves the possibility of overcoming this barrier and encouraging better adherence to regular exercise. An intriguing aspect of this comparison is that both MICT and HIIT result in comparable levels of energy expenditure [32, 35]. This demonstrates the adaptability of HIIT, proving its ability to provide physiological benefits comparable to typical moderate-intensity exercise in significantly less time. The parallel energy expenditure strengthens the case for HIIT as a feasible alternative, particularly for persons with time constraints or wanting a more time-efficient workout schedule.

Strengths and limitations

This systematic review’s strength is that it distinguishes itself through a robust methodology, adhering closely to the PRISMA 2020 guidelines. The strength of the review is enhanced by the comprehensive integration of a diverse range of databases in the search strategy, coupled with the meticulous application of specific search terms. Noteworthy is the stringent adherence to inclusion criteria, ensuring the inclusion of only those studies that evaluated HIIT and MICT while also assessing vascular function. However, it is crucial to acknowledge a limitation inherent in this systematic review, arising from substantial methodological differences. A notable difference is the exclusive use of MHR to determine HIIT intensity, without considering VO2max. Future studies should consider including VO2max to provide a more comprehensive exercise intensity assessment. Additionally, the diverse approaches employed in assessing vascular function across the 11 included studies further contribute to this limitation. These variabilities rendered it impractical to conduct a meta-analysis and amalgamate the observed datasets to comprehensively evaluate evidence regarding the impact of HIIT and MICT on vascular functioning. The impediment primarily emanated from the methodological and clinical heterogeneity inherent in these 11 studies, underscoring the need for cautious interpretation due to these intrinsic differences.

Conclusion

The systematic review suggests that HIIT is more effective than MICT in improving vascular function (FMD, arterial stiffness, and wall shear stress) and can be a time-efficient exercise model. The findings align with prior studies showing a link between cardiovascular fitness and vascular function. Consistent with a recent meta-analysis, HIIT is highlighted for its superior ability to enhance cardiovascular fitness. Importantly, these benefits extend to low weekly exercise volumes, exhibit rapid manifestation, and remain consistent across diverse BMI statuses. Overall, HIIT emerges as a promising strategy for efficiently improving vascular function across various conditions.