Search results and overview of included studies
Our search led to 462 relevant studies. After duplicates removing, a wide range of screening of the titles and abstracts, the careful assessment was performed on 454 related articles. Of these, 23 articles remained after considering the inclusion and exclusion criteria for the eligibility. Eventually, 18 articles, including, 14 effect sizes for adiponectin and 14 effect sizes for leptin, which studied a total of 1042 and 1073 participants respectively and aged 28.9 – 69 years, were identified in the current systematic review and meta-analysis. One study34 just reported baseline data in the article paper and we did not receive raw data for after trial data from the authors. This listed number is inclusive of people who were dropouts in some studies. Figure 1 represents the selection process and reasons for excluding the studies. The data inTable 1 illustrates the main characteristics of the articles in our systematic review and meta-analysis.
In brief, the studies were published between 2009 and 2019. The duration of these RCTs ranged between 4 weeks to 24 months. The total number of participants who completed the studies in inclusion criteria was 511 participants in the intervention and 531 in the placebo groups for adiponectin and 544 participants in the intervention and 529 in the placebo groups for leptin. Soy protein have been used in a variety of forms in these studies like soy-based meal replacement18-20,25,26,35,36, soy powder or formula15-17,21,22,24,27,37and capsule supplement19,23,38. However most of studies have been used soy powder or formula. The dose of soy protein supplementation was 6 to 50 g/day among these studies.
Three studies used a randomized crossover design18,24,27, two of them was also single blind18,27and the other study had the design of double-blind24. Among 18 studies 9 studies were conducted in n the United States15,18-21,24,25,27,36. The effect of soy protein on both adiponectin and leptin concentration was examined in 10 studies; and 4 studies only reported adiponectin17,18,25,38and 4 studies only reported leptin35-37,39. Overall, 18 effect sizes were extracted from these studies for adiponectin and leptin concentration.