Body Composition
Meta-Analysis
2019
Is interval training the magic bullet for fat loss? A systematic review and meta-analysis comparing moderate-intensity continuous training with high-intensity interval training (HIIT)
By Ricardo B. Viana, Joao Pedro A. Naves, Victor S. Coswig, Claudio A.B. de Lira, James Steele, James P. Fisher and Paulo Gentil
British Journal of Sports Medicine, 53(10), pp. 655-664
<h2>Abstract</h2>
<p><strong>Background:</strong> <a href="/terms/hiit/" class="term-link" data-slug="hiit" title="High-intensity interval training">High-intensity interval training</a> (HIIT) has been promoted as a time-efficient superior alternative to moderate-intensity continuous training (MICT) for fat loss. However, the comparative efficacy of these two modalities has been inconsistently characterized across individual studies.</p>
<p><strong>Objective:</strong> This <a href="/terms/systematic-review/" class="term-link" data-slug="systematic-review" title="systematic review">systematic review</a> and <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> compared the effects of HIIT versus MICT on total body fat mass and body fat percentage in adults.</p>
<p><strong>Methods:</strong> MEDLINE, EMBASE, and SPORTDiscus were searched for randomized controlled trials directly comparing HIIT with MICT in adults. HIIT was defined as repeated intervals at ≥80% maximal heart rate (HRmax) or ≥80% VO2max; MICT was defined as continuous exercise at 40–70% HRmax for ≥20 minutes. The primary outcome was absolute change in fat mass. Random-effects models were employed for pooling.</p>
<p><strong>Results:</strong> Thirty-six trials encompassing 1,012 participants were included. Both HIIT and MICT produced significant within-group reductions in fat mass. However, direct between-group comparisons revealed no statistically significant difference in total fat mass reduction (weighted mean difference: −0.08 kg, 95% CI: −0.49 to +0.33, p = 0.70). Subgroup analyses by intervention duration, sex, and BMI category did not reveal significant moderating effects.</p>
<p><strong>Conclusions:</strong> HIIT and MICT produce comparable fat loss outcomes despite HIIT requiring substantially less time per session. The choice between modalities should be guided by individual preference, injury history, recovery capacity, and adherence likelihood rather than a presumed superior efficacy of either approach.</p>
<h2>Introduction</h2>
<p>Fat loss remains one of the most common goals among individuals who exercise. For decades, moderate-intensity continuous training (MICT)—commonly known as steady-state cardio—was the dominant exercise prescription for fat reduction, endorsed by major health organizations based on its demonstrated efficacy and safety across diverse populations [1]. A typical MICT protocol involves 30–60 minutes of sustained exercise at 55–70% of maximal heart rate, producing substantial acute caloric expenditure.</p>
<p><a href="/terms/hiit/" class="term-link" data-slug="hiit" title="High-intensity interval training">High-intensity interval training</a> (HIIT) emerged as a prominent alternative in the exercise science literature during the 2000s and gained considerable popular appeal based on claims of superior fat loss efficacy despite requiring a fraction of the time commitment [2]. HIIT protocols alternate between short bouts of near-maximal effort exercise (typically 85–95% HRmax) and brief recovery periods, with total session durations often ranging from 10–25 minutes. The proposed advantages of HIIT over MICT include greater excess post-exercise oxygen consumption (EPOC), enhanced fat oxidation capacity, superior improvements in cardiorespiratory fitness, and favorable effects on appetite regulation [3].</p>
<p>However, these mechanistic advantages do not necessarily translate to superior fat loss in controlled trials where total energy expenditure may differ substantially between conditions. When HIIT sessions are shorter and involve less total work than matched MICT sessions, any per-calorie advantage may be offset by reduced total caloric expenditure. Conversely, when sessions are matched for caloric expenditure, HIIT's time-efficiency advantage is diminished.</p>
<p>This <a href="/terms/systematic-review/" class="term-link" data-slug="systematic-review" title="systematic review">systematic review</a> and <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> was designed to resolve these inconsistencies by directly comparing fat loss outcomes between HIIT and MICT across randomized controlled trials, with attention to study design characteristics that may moderate the relative efficacy of each approach.</p>
<h3>References</h3>
<p>[1] Donnelly JE, et al. ACSM Position Stand on physical activity and weight loss. <em>Med Sci Sports Exerc</em>. 2009;41:459–471.
[2] Gibala MJ, et al. Physiological adaptations to low-volume, high-intensity interval training in health. <em>J Physiol</em>. 2012;590:1077–1084.
[3] Batacan RB, et al. Effects of high-intensity interval training on metabolic syndrome. <em>Br J Sports Med</em>. 2017;51:494–503.</p>
<h2>Methods</h2>
<h3>Search Strategy and Eligibility</h3>
<p>Electronic database searches of MEDLINE, EMBASE, SPORTDiscus, and the Cochrane Central Register of Controlled Trials were performed from inception through December 2018. The search strategy combined terms for <a href="/terms/hiit/" class="term-link" data-slug="hiit" title="HIIT">HIIT</a> (e.g., "high-intensity interval training," "sprint interval training," "HIIT"), MICT (e.g., "moderate-intensity continuous training," "steady-state exercise," "aerobic exercise"), and body composition (e.g., "fat mass," "body fat," "adiposity").</p>
<p>Randomized controlled trials were eligible <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="if">if</a> they: (1) directly compared HIIT to MICT within the same study; (2) defined HIIT as exercise at ≥80% HRmax or ≥80% VO2max; (3) defined MICT as continuous exercise at 40–70% HRmax for ≥20 consecutive minutes; (4) enrolled adults ≥18 years; (5) measured fat mass or body fat percentage using objective methods (DXA, BodPod, or underwater weighing); and (6) had a minimum duration of 6 weeks.</p>
<h3>Operational Definitions</h3>
<p>HIIT protocols in included studies encompassed various formats: traditional HIIT (e.g., 4×4 minutes at 85–95% HRmax with <a href="/terms/active-recovery/" class="term-link" data-slug="active-recovery" title="active recovery">active recovery</a>), sprint interval training (SIT; e.g., Wingate-based all-out sprints), and tabata-style protocols. These were distinguished in sensitivity analyses to examine potential differential effects.</p>
<h3>Data Extraction</h3>
<p>Primary extracted variables included: sample size, participant characteristics (age, sex, BMI, fitness level), intervention duration, <a href="/terms/training-frequency/" class="term-link" data-slug="training-frequency" title="training frequency">training frequency</a>, session duration, intensity prescriptions, dietary control methods, and body composition outcomes. Two reviewers independently extracted data; discrepancies were resolved by a third reviewer.</p>
<h3>Statistical Analysis</h3>
<p>The primary outcome was absolute change in total fat mass (kg). Secondary outcomes included percentage body fat change and <a href="/terms/lean-body-mass/" class="term-link" data-slug="lean-body-mass" title="fat-free mass">fat-free mass</a> change. Weighted mean differences (WMDs) were calculated where outcomes were measured on the same scale; otherwise, standardized mean differences were used. Between-modality differences were analyzed using random-effects <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a>. A priori subgroup analyses were performed for intervention duration (12 weeks vs. ≥12 weeks), BMI (normal weight vs. overweight/obese), and sex.</p>
<h2>Results</h2>
<h3>Study Inclusion and Participant Characteristics</h3>
<p>The database search identified 2,199 unique records. After removal of duplicates and sequential screening, 36 randomized controlled trials comprising 1,012 participants were included in the quantitative synthesis. Mean participant age was 32.4 ± 8.1 years, mean BMI was 26.8 ± 4.2 kg/m², and interventions ranged from 6 to 24 weeks (median: 12 weeks). Forty-four percent of participants were female. Aerobic <a href="/terms/training-frequency/" class="term-link" data-slug="training-frequency" title="training frequency">training frequency</a> ranged from 2 to 5 sessions per week across included studies.</p>
<h3>Fat Mass Reduction: <a href="/terms/hiit/" class="term-link" data-slug="hiit" title="HIIT">HIIT</a> vs. MICT</h3>
<p>Both HIIT and MICT produced significant within-group reductions in fat mass (HIIT: −1.58 kg, 95% CI: −2.03 to −1.13; MICT: −1.34 kg, 95% CI: −1.79 to −0.89). The direct between-group comparison revealed no statistically significant difference in fat mass reduction attributable to training modality (WMD: −0.08 kg, 95% CI: −0.49 to +0.33, p = 0.70). Heterogeneity was high (I² = 71%), indicating substantial variability across studies.</p>
<h3>Body Fat Percentage</h3>
<p>Similarly, neither HIIT nor MICT produced significantly greater reductions in body fat percentage than the other (WMD: −0.15%, 95% CI: −0.65 to +0.35, p = 0.55).</p>
<h3>Subgroup Analyses</h3>
<p>No significant subgroup interactions were detected for intervention duration, BMI category, or sex. However, studies employing caloric-expenditure-matched protocols (controlling for total energy burn between conditions) showed even smaller between-group differences (WMD: −0.02 kg, p = 0.92), further supporting the equivalence of the two modalities when total energy expenditure is controlled.</p>
<h3>Time Efficiency</h3>
<p>HIIT protocols required significantly less training time per week than MICT (HIIT: mean 75.2 min/week vs. MICT: mean 152.6 min/week), representing approximately 51% less time investment per unit fat loss outcome.</p>
<h3>Safety and Adherence</h3>
<p>Dropout rates were marginally higher in HIIT groups (11.8%) compared with MICT groups (8.5%), though this difference did not reach statistical significance. Adverse events were rare in both groups.</p>
<h2>Discussion</h2>
<p>The central finding of this <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> is that <a href="/terms/hiit/" class="term-link" data-slug="hiit" title="HIIT">HIIT</a> and MICT produce statistically equivalent fat loss outcomes when implemented as structured training programs in adults. This finding holds across intervention durations, BMI categories, and sexes, suggesting that the choice between these modalities should be driven by practical and individual considerations rather than assumptions about differential efficacy.</p>
<h3>Why Fat Loss Is Equivalent Despite Different Intensities</h3>
<p>The equivalence in fat loss outcomes despite HIIT's proposed mechanistic advantages (EPOC, fat oxidation, hormonal responses) likely reflects compensatory differences in total energy expenditure. Many HIIT protocols in research settings are substantially shorter than matched MICT sessions, reducing total caloric expenditure per session. The metabolic advantages of HIIT—including elevated EPOC and improved mitochondrial biogenesis—appear to be roughly offset by the greater total energy expenditure of longer MICT sessions [1].</p>
<p>Additionally, the "afterburn" effect of HIIT (elevated post-exercise oxygen consumption) has been found to contribute only 6–15% of additional energy expenditure beyond the session itself in most protocols, which, while real, is modest in absolute terms [2].</p>
<h3>Practical Implications</h3>
<p>For time-constrained individuals, HIIT offers a clear advantage: comparable fat loss in approximately half the weekly time investment. This time efficiency may translate to superior long-term adherence for individuals with scheduling constraints, potentially offsetting any theoretical efficacy differences [3]. For individuals with joint pain, injury history, or who are new to exercise, lower-impact MICT may be preferable due to its lower injury risk per session.</p>
<h3>The Role of Adherence</h3>
<p>It is increasingly recognized that the "best" exercise modality for fat loss is the one that an individual will consistently perform over months and years. The small, non-significant difference in dropout rates between HIIT and MICT groups in this analysis suggests that neither modality inherently drives superior long-term adherence when both are structured as controlled research interventions [4].</p>
<h3>Limitations</h3>
<p>Study heterogeneity was high (I² = 71%), suggesting that substantial variability in study populations, HIIT and MICT protocol definitions, and dietary control methods influenced outcomes. The lack of standardized caloric-expenditure matching across studies remains the most significant methodological limitation.</p>
<h3>References</h3>
<p>[1] Keating SE, et al. Continuous exercise but not high intensity interval training improves fat distribution in overweight adults. <em>J Obes</em>. 2014;2014:834865.
[2] Børsheim E, Bahr R. Effect of exercise intensity, duration and mode on post-exercise oxygen consumption. <em>Sports Med</em>. 2003;33:1037–1060.
[3] Tjønna AE, et al. Aerobic interval training vs. continuous moderate exercise. <em>Circulation</em>. 2008;118:346–354.
[4] Viana RB, et al. Is interval training the magic bullet for fat loss? <em>Br J Sports Med</em>. 2019;53:655–664.</p>