Abstract

Purpose The process in which specific exercises reduce localized adipose tissue depots (targeted fat loss) and modify fat distribution is commonly termed spot reduction. According to this long-held popular belief, exercising a limb would lead to greater reduction in the adjacent adipose tissue in comparison with the contralateral limb. Aside from popular wisdom, scientific evidence from the 20th and 21st century seems to offer inconclusive results. The study aim was to summarize peer-reviewed literature assessing the effects of unilateral limb training, compared with the contralateral limb, on the localized adipose tissue depots in healthy participants, and to meta-analyse its results. Methods We followed the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses. We searched PubMed, Web of Science, and Scopus electronic databases using several relevant keyword combinations. Independent experts were contacted to help identify additional relevant articles. Following the PICOS approach, we included controlled studies that incorporated a localized exercise intervention (i.e., single-leg training) to cohorts of healthy participants (i.e., no restriction for fitness, age, or sex) compared with a control condition (i.e., contralateral limb), where the main outcome was the pre-to-post-intervention change of localized fat. The methodological quality of the studies was assessed with the Physiotherapy Evidence Database scale. Pre- and post-intervention means ± standard deviations of the fat-related outcome in the trained and control groups (limbs) were converted to Hedges’ g effect size (ES; with 95% confidence intervals [CI]) by using a random-effects model. The impact of heterogeneity was assessed with the I2 statistic. Extended Egger’s test served to explore the risk of reporting bias. The statistical significance threshold was set at p < 0.05. Results From 1833 search records initially identified, 13 were included in the meta-analysis, involving 1158 male and female participants (age, 14–71 years). The 13 studies achieved a high methodological quality, and presented results with low heterogeneity (I2 = 24.3%) and no bias (Egger’s test p = 0.133). The meta-analysis involved 37 comparisons, with 17 of these favouring (i.e., greater reduction of localized fat) the trained limb, and 20 favouring the untrained limb, but the ES ranged between –1.21 and 1.07. The effects were consistent, with a pooled ES = –0.03, 95% CI: –0.10 to 0.05, p = 0.508, meaning that spot reduction was not observed. Conclusions Localized muscle training had no effect on localized adipose tissue depots, i.e., there was no spot reduction, regardless of the characteristics of the population and of the exercise program. The popular belief concerning spot reduction is probably derived from wishful thinking and convenient marketing strategies, such as influencers seeking increased popularity and procedure sellers interested in increasing advertising.