Data synthesis and statistical analysis
The comparison tests were categorized into two groups: 1) Heated (vegetable oil) and 2) control group. Hence, there are six separate comparison tests for each respective oil to determine its summary effect estimate. Review Manager 5.4.1 was used to perform the statistical analyses with the extracted mean ± standard deviation and sample size as data inputs. A random-effect model was used as the studies were heterogeneous from each other in the scope of intervention, population and outcome, hence making it impossible to impose a fixed-effect model that pools all of the study findings to arrive at a summary estimate. The selected studies were very different by the results of adverse health effects caused by consuming thermally oxidized vegetable oil but other areas such as the type of oil used in interventions are sufficiently similar nonetheless. Weighted mean differences, the statistical significance of each included paper, and heterogeneity between studies were highlighted. Chi-square test was used to determine significant heterogeneity (P < 0.1). In addition to that, I2 statistic was used to quantify heterogeneity through the percentage of total variation across studies attributable to heterogeneity rather than chance. Cut off points of percentage are such: >75% (high heterogeneity), 50 – 75% (moderate heterogeneity), and <50% (low heterogeneity).