Evidence-based nutrition -- using a meta-analysis to review the literature.

Abstract

One of the purposes of this series on evidence-based nutrition1,2 is to elucidate how students and researchers in nutrition, practitioners, health care providers and policy makers can apply basic principles and standardised methods to synthesise and make sense of large, often unmanageable amounts of information. This is necessary in order to draw conclusions from empirical studies and make decisions which will be evidence-based and not unduly influenced by bias and chance effects. The purpose of this paper in the series is to show how a meta-analysis can be used to do this by statistically combining results from independent but related studies into a composite measure of effect. A meta-analysis is a statistical technique used to combine the results of studies addressing the same question into a one number summary.3, 4 The meta-analysis method can be applied in the social-behavioural and biomedical sciences5 and is therefore particularly suitable for nutrition data. The term meta-analysis should not be confused with a systematic review. Egger and Smith4 suggest that the term meta-analysis should be used ‘to describe the statistical integration of separate studies whereas systematic review is most appropriate for denoting any review of a body of data that uses clearly defined methods and criteria’. According to this definition a meta-analysis can, if appropriate, be part of a systematic review. It is always appropriate and desirable to systematically review a body of data, but it may at times be inappropriate to pool results from separate studies.4 Because a systematic review is a structured, systematic qualitative and quantitative integration of comparable results of several independent studies, it can provide a firm basis for planning and policy recommendations.6 A meta-analysis has been described as ‘a quantitative approach to research reviews’,7 ‘aggregating data’,8 ‘the epidemiology of results’5 and ‘the application of statistical procedures to collections of empirical findings’.3 A meta-analysis uses specific statistical methods to combine, summarise and integrate comparable results from different studies. The unit of observation is therefore the study. A major purpose of pooling results in a meta-analysis is to increase statistical power and precision of estimates in smaller studies.3-9 There are excellent sources available3-10 with detailed, step-by-step guidelines, recommendations and numerous examples of how to do a meta-analysis. In this paper the advantages and limitations of a meta-analysis and the process of conducting one will be outlined briefly to motivate nutritionists when and how to use it in their interpretation of the literature, in drawing conclusions from studies with conflicting results, and in policy formulation.

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