2.3.2 Meta-analysis to explore group differences Although the purpose of the analysis was to assess whether there is a threshold that defines a sodium intake below which there is no effect on blood pressure, a meta-analysis was undertaken to confirm whether the inclusion and exclusion criteria used in our analysis would yield a dataset that reflected previous findings. Sub-group meta-analysis was done for factors which might be important in affecting the change in BP and which would therefore need to be taken into account before attributing effects to sodium intake. These factors were: hypertensive status of the subjects, posture of resting BP measurement (sitting versus supine because both postures were commonly classed as resting), parallel versus cross-over design and use of dietary advice to achieve the difference in intake versus placing all subjects on a low sodium diet and randomising to sodium or placebo pills (a surrogate for blinding). A random effects meta-analysis was done. The results were graphed using a forest plot and a funnel plot was done to examine whether publication bias might be evident. 95% CI were calculated der Simonian-Laird method [].The heterogeneity among the studies was described using I2. describes the of total variation across studies that is due to “I2 “percentage heterogeneity rather than chance” and 0%, 25%, 50% and 75% could be interpreted as indicating no, low, medium and high heterogeneity respectively [] . In the case that there is a dose-response relationship, variation in aspects such as dose or study duration among studies may cause heterogeneity. The inverse variance weights were saved and used in subsequent analyses. The inverse variance weights from the meta-analysis were used to weight the averages for SBP per 500mg decrease in Na and 24 hr Na excretion in the various analyses using categories of Na excretion to explore confounding by differences in the range of Na doses tested among the studies. Random-effects meta-analyses were also conducted for total cholesterol, HDL cholesterol and LDL cholesterol data using the previously described methods. Further statistical analysis of data on diastolic blood pressure was not done due to time constraints. The EWG prioritised the analysis if systolic blood pressure data as this has been demonstrated to be of greater clinical importance [].
2.3.2 Meta-analysis to explore group differences Although the purpose of the analysis was to assess whether there is a threshold that defines a sodium intake below which there is no effect on blood pressure, a meta-analysis was undertaken to confirm whether the inclusion and exclusion criteria used in our analysis would yield a dataset that reflected previous findings. Sub-group meta-analysis was done for factors which might be important in affecting the change in BP and which would therefore need to be taken into account before attributing effects to sodium intake. These factors were: hypertensive status of the subjects, posture of resting BP measurement (sitting versus supine because both postures were commonly classed as resting), parallel versus cross-over design and use of dietary advice to achieve the difference in intake versus placing all subjects on a low sodium diet and randomising to sodium or placebo pills (a surrogate for blinding). A random effects meta-analysis was done. The results were graphed using a forest plot and a funnel plot was done to examine whether publication bias might be evident. 95% CI were calculated der Simonian-Laird method [].The heterogeneity among the studies was described using I2. “I2 describes the “percentage of total variation across studies that is due to heterogeneity rather than chance” and 0%, 25%, 50% and 75% could be interpreted as indicating no, low, medium and high heterogeneity respectively [] . In the case that there is a dose-response relationship, variation in aspects such as dose or study duration among studies may cause heterogeneity. The inverse variance weights were saved and used in subsequent analyses. The inverse variance weights from the meta-analysis were used to weight the averages for SBP per 500mg decrease in Na and 24 hr Na excretion in the various analyses using categories of Na excretion to explore confounding by differences in the range of Na doses tested among the studies. Random-effects meta-analyses were also conducted for total cholesterol, HDL cholesterol and LDL cholesterol data using the previously described methods. Further statistical analysis of data on diastolic blood pressure was not done due to time constraints. The EWG prioritised the analysis if systolic blood pressure data as this has been demonstrated to be of greater clinical importance [].
Sodium%20Supporting%20Document%202%20%20Statistical%20Analyses.docx