Note from the National Guideline Clearinghouse (NGC): A systematic evidence review was prepared by the Research Triangle Institute-University of North Carolina Evidence-based Practice Center for the Agency for Healthcare Research and Quality (AHRQ) for use by the U.S. Preventive Services Task Force (USPSTF) (see the "Companion Documents" field).
Senior investigators reviewed titles and abstracts to identify which full manuscripts to review and made the final decisions about inclusion or exclusion. Other team members then reviewed individual articles and abstracted selected information into evidence tables. When multiple articles described the same study, the most complete article was used as the main source of data; the other articles were used for supplemental information. Team members discussed disagreements with reviewers and made final decisions by consensus.
Net change in consumption, defined as change in the intervention group from baseline to follow-up minus the change in the control group from baseline to follow-up, was used as the main outcome. Unadjusted outcomes from the article were reported when they were presented. In some cases when necessary data were not presented in the article, they were calculated from other information presented.
To facilitate comparison of effectiveness of counseling on dietary change across studies that used a variety of different outcome measures, two investigators independently classified the magnitude of dietary change in each study as "small," "medium," or "large." The study team resolved disagreements by consensus. Definitions of small, medium, and large changes based on the distribution of findings from the studies and the limited information available about the relationship between dietary change and health outcomes were developed.
For saturated fat, small was defined as an absolute net difference between intervention and control groups of 0 to 1.2 percentage points, medium as a difference of 1.3 to 3.0 percentage points, and large as a difference of greater than 3.0 percentage points. When studies reported only change in proportion of calories from total fat, large was classified as a difference of greater than 10 percentage points, medium as a difference of 5.1 to 9.9 percentage points and small as a difference of less than or equal to 5 percentage points. Effect sizes were classified based on the difference in the number of servings of fruit and vegetables per day consumed by the intervention and control groups. Small was defined as a difference of less than 0.3 servings per day, medium as a difference of 0.4 to 0.9 servings per day, and large as a difference of greater than or equal to 1.0 serving per day. For fiber, a small effect size was defined as a net difference of less than 2.0 grams (g) per day of fiber, medium as 2.0 to 4.0 g per day, and large as greater than 4.0 g per day.
If studies did not provide data on main outcomes of interest, the relative change in the outcome reported (e.g., grams of fat consumed, dietary risk scores) was used to guide the definition of magnitude of change. The relative change was defined as the net change divided by the baseline value in the control group. A relative change of 25% or greater was considered large, 10% to 24% medium, and less than 10% small.
Analysis of Factors Influencing Effect Size
The effect of different intervention characteristics was examined, including intensity, the risk status of the patient populations studied, the study setting, and the use of well-proven counseling elements, on the magnitude of change in dietary behavior achieved. Trials were considered that examined multiple nutrients as separate studies for these analyses. Because of concern about double-counting studies, the authors repeated the analyses with each study’s effect counted only once (once using the largest effect and again using the smallest effect) and found similar results. Because of heterogeneity in the outcomes, meta-analysis was not attempted.
Two senior reviewers independently rated the intensity of the dietary intervention as "low," "medium," or "high" based on the number and length of counseling contacts. Interventions with only one contact of 30 minutes or less were considered low intensity, those with six or more contacts of 30 minutes or more each were considered high intensity, and all others were considered medium intensity.
Each study's intervention "setting" was classified as (1) performed within the primary care clinic (by the usual primary care provider or referral to a dietitian or nutritionist);
(2) conducted in a special research clinic; or (3) conducted using self-help materials and/or interactive health communications (e.g., telephone messages or computer-generated mailings).
Finally, the studies were examined to determine whether they included as part of their intervention any of seven counseling elements (using a dietary assessment, enlisting family involvement, providing social support, using group counseling, emphasizing food interaction, encouraging goal setting, and using advice appropriate to the patient group being studied) that have been effective in previous research on dietary behavior change.