Background: Apple cider vinegar (ACV) is a naturally fermented beverage with potential metabolic health benefits; however, its effects on weight loss remain controversial. This systematic review and meta-analysis of randomized controlled trials (RCTs) was conducted to assess the effect of ACV on anthropometric measurements in adults. Methods: We performed a systematic search of PubMed, Web of Science, Scopus, and CENTRAL up to March 2025 for randomized controlled trials (RCTs) in adults (≥18 years) evaluating the effects of ACV for ≥4 weeks on body composition parameters. Primary outcomes included changes in body weight, BMI, waist circumference, and other anthropometric measures. Risk of bias was assessed using the Revised Cochrane Risk-of-bias tool. Results: Out of 2961 reports screened, 10 RCTs comprising a total of 789 participants were eligible for inclusion in this meta-analysis. The pooled results using a random-effects model showed that daily ACV intake significantly reduced body weight [SMD: −0.39; 95% CI: −0.63, −0.15; p = 0.001; I² = 62%], BMI [SMD: −0.65; 95% CI: −1.05, −0.26; p = 0.001; I² = 83%], and WC [SMD: −0.34; 95% CI: −0.67, −0.02; p = 0.04; I² = 61%]. However, no significant effects of ACV were observed on the other body composition parameters analyzed. Sensitivity analyses excluding high-risk-of-bias studies confirmed the robustness of ACV’s beneficial effects on body weight and BMI. Subgroup analyses suggested that ACV consumption significantly improved anthropometric parameters when administered for up to 12 weeks, at a dose of 30 mL/day, and in adults who were overweight, obese, or had type 2 diabetes. Conclusions: Overall, this meta-analysis suggests that ACV supplementation may be a promising and accessible adjunctive strategy for short-term weight management in adults with excess body weight or metabolic complications.

Accurate dietary assessment is essential in nutritional research and epidemiology. Food Frequency Questionnaires (FFQs) are widely used to estimate habitual intake; however, their performance may vary according to questionnaire design, administration procedures, and the dietary components being assessed. This systematic review and meta‐analysis evaluated the relative validity of FFQs designed to assess overall dietary intake in adults and explored methodological characteristics associated with their performance. A meta‐analysis was conducted on studies validating FFQs against reference methods, including 24‐h recalls and food records. A comprehensive search of the PubMed, Web of Science, and Scopus databases was conducted for publications from January 2000 to February 1, 2026. Pooled estimates were calculated for correlation coefficients and mean differences for energy and nutrients between FFQs and reference methods. The literature search identified 43 articles that included 8467 participants. Subgroup analyses according to the sample size, administration methods, FFQ items, quality of the studies, and regions were also performed. Overall, FFQs showed moderate‐to‐good relative validity for most nutrients. Performance was relatively consistent for energy and macronutrients, particularly carbohydrates, whereas greater variability was observed for micronutrients, although some, including magnesium and riboflavin, showed high correlations. Systematic overestimation of certain nutrient intakes was observed, particularly when FFQs were compared with 24HRs. Interviewer‐administered FFQs and questionnaires with a greater number of items tended to yield slightly higher correlations, while studies of higher methodological quality also tended to report stronger correlations. These findings support the use of FFQs primarily for ranking individuals according to habitual dietary intake and suggest that administration mode, questionnaire comprehensiveness, and methodological quality should be carefully considered in future FFQ development and validation. Greater caution may be warranted when overall‐diet FFQs are used to estimate specific micronutrient intakes.