According to David Sackett, one of the originators of evidence-based medicine terminology, evidence-based practice is the conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients ... [it] means integrating individual clinical expertise with the best available external evidence from systematic research.3
When we are discussing the effectiveness of an intervention in healthcare (be it the effectiveness of a drug, physical therapy, dietary change, or type of dietary advice), the best available evidence comes from the top of the levels of evidence hierarchy. A good levels of evidence model for effectiveness of an intervention was produced by the Oxford Centre for Evidence-Based Medicine.4 It is summarized in Table 1.1.
We should thus base our dietary advice on the best evidence — where possible, from systematic reviews (meta-analyses) of randomised controlled trials or individ-
Levels of Evidence for a Question about Efficacy of a Dietary Intervention
Level 1a. A systemic review (meta-analysis) of randomised controlled trails (without heterogenity between trials) Level 1b. A single randomised controlled trial with a narrow confidence interval Level 2a. A systematic review (meta-analysis) of cohort studies (without heterogeneity between study results) Level 2b. An individual cohort study, or a low-quality randomised controlled trial Level 2c. Ecological studies Level 3. Case control studies Level 4. Case series
Level 5. Expert opinion without explicit critical appraisal, or based on physiology, bench research, or "first principles"
Adapted from Centre for Evidence-Based Medicine.4
Worst evidence ual high-quality randomised controlled trials. A systematic review is an impartial and unbiased review of all relevant research evidence. It is a review that has been prepared using a systematic approach to minimizing biases and random errors which is documented in a materials and methods section. A systematic review may or may not include a meta-analysis — a statistical analysis of the results from independent studies that generally aims to produce a single estimate of a treatment effect.5
Studies of antioxidant vitamins highlight the need to base our dietary interventions on randomised controlled trials or systematic reviews of randomised controlled trials rather than on cohort data. Good evidence from individual cohort studies (and meta-analyses of cohort studies) indicates that people taking more beta-carotene or vitamin E are protected from cardiovascular disease, even after controlling for other lifestyle factors. However, no evidence of a protective effect of either beta-carotene or vitamin E was seen in large long-term randomised controlled trials6 or in metaanalyses of randomised controlled trials.78 It appears that the clustering of healthy lifestyle traits and socioeconomic advantage is so strong that it is almost impossible to control for these multiple confounding factors when assessing evidence from cohorts. While evidence from cohorts may point out interventions to try in randomised controlled trials, we need to base our understanding of the effectiveness of dietary interventions on good quality randomised studies of interventions.
In addition to choosing the type of study to provide the necessary evidence, we must consider the quality of each individual study chosen (see information about the Critical Appraisal Skills Programme (CASP) Web site in Section 1.6, Useful Contacts) and, equally important, the outcomes measured. For example, a randomised controlled trial may show that eating more of a particular fruit raises levels of specific antioxidant compounds in the blood. However, a randomised controlled trial that assesses disease outcomes would provide better evidence of that fruit's usefulness in protecting against cardiovascular disease. We must have good randomised controlled trial evidence that raising these antioxidant compounds in the blood does indeed reduce cardiovascular disease outcomes to help us interpret the findings.
Evidence-based practice is important and places emphasis on systematic reviews of randomised controlled trials or large high-quality individual randomised controlled trials. For these reasons, most of the evidence presented in this chapter is based on these types of trials.
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