Evidence Report/Technology Assessment: Number 25
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Under its Evidence-based Practice Program, the Agency for Healthcare Research and Quality (AHRQ) is developing scientific information for other agencies and organizations on which to base clinical guidelines, performance measures, and other quality improvement tools. Contractor institutions review all relevant scientific literature on assigned clinical care topics and produce evidence reports and technology assessments, conduct research on methodologies and the effectiveness of their implementation, and participate in technical assistance activities.
Overview / Reporting the Evidence / Methodology / Findings / Future Research / Availability of Full Report
Increasingly, behavioral research is being conducted on the role of dietary change in cancer risk reduction. To implement the findings of such research, researchers are testing numerous dietary interventions in a variety of populations. Despite these efforts, no clear understanding has emerged regarding which interventions are more efficacious in influencing dietary change and for which groups.
To clarify what is known about the efficacy and effectiveness of behavioral interventions in promoting dietary change, a systematic review was undertaken to synthesize the findings of 92 studies that report the impact of behavioral interventions on dietary outcomes considered to be relevant to cancer risk:
- Dietary fat intake.
- Consumption of fruits and vegetables.
In addition to summarizing the existing knowledge base on behavioral interventions and dietary change, the report identifies limitations in the current literature and outlines directions for future research.
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Reporting the Evidence
This evidence report addresses three key questions regarding the efficacy and effectiveness of behavioral dietary interventions for promoting dietary change:
- Is there evidence that one type of intervention or combination of interventions is more effective than another in helping individuals or groups modify their diets to consume more fruits and vegetables and less fat?
- What is the evidence for the efficacy of dietary interventions by population subgroup, particularly groups defined by ethnicity and sex?
- What conclusions (if any) can be reached about the cost-effectiveness of these types of interventions?
In addressing these questions through a review and secondary analysis of published articles, the report considers interventions of all types (i.e., individual dietary counseling, group nutrition classes, social support groups) and in all settings (i.e., school, workplace, health care, environmental modifications, community). It includes adults, adolescents, and children who were generally healthy—as well as those who were at elevated risk of cancer, or chronic disease.
The primary goal is to explore the overall efficacy and effectiveness of behavioral dietary interventions in increasing fruit and vegetable intake and decreasing dietary fat consumption. In addition, the report considers the relative effectiveness of interventions in various population subgroups, and explores the impact of key intervention characteristics (the use of theory, intervention intensity, intervention setting, the use of social support, and the involvement of families) on behavior change outcomes.
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To identify studies regarding behavioral dietary interventions, the authors performed literature searches in six databases (MEDLINE, EMBASE, PsycINFO, CINAHL, AGELINE, and AGRICOLA). Primary Medical Subject Headings (MeSH) terms used in the searches included health behavior, attitude to health, health promotion, behavior change, food habits, fat-restricted diet, diabetic diet, fruit, vegetables, prevent, counsel, cardiovascular disease, cancer, and neoplasms.
The initial search strategy excluded studies:
- Published before 1975 or in languages other than English.
- Conducted outside of North America, Europe, or Australia.
- Conducted with infants, institutionalized populations, or populations with insulin-dependent diabetes mellitus.
- With sample sizes of fewer than 40 subjects at follow-up.
- In which dietary intake was externally controlled.
Because the primary dietary outcomes selected were intake of fruits and vegetables and dietary fat, studies were also excluded that did not report results for these outcomes. Included were both randomized, controlled trials and nonrandomized studies that had control or comparison group designs. In all, the authors reviewed 907 articles and retained 104; collectively, these reported the results of 92 studies, which were used as the unit of analysis in secondary analyses.
The authors conducted three sets of secondary analyses:
- A meta-analysis (for the outcome of total fat as a percentage of energy intake).
- A standardized, quantitative analysis of the differences in dietary changes between intervention and comparison or control groups from baseline to follow-up (which is hereafter referred to as the "differences-in-deltas" approach).
- A semiquantitative analysis summarizing the statistical significance of the intervention effects (hereafter, the "summary of significant findings" approach).
The primary goal of each of these analytic strategies was to determine the overall effectiveness of dietary interventions in changing dietary behavior. Secondary goals included a determination of the relative effectiveness of different types of interventions for changing dietary behavior in different population subgroups. Together, these analytic approaches represent a spectrum of selectivity, from use of a highly selective class of studies and statistically rigorous methods to assessment of a larger, broadly inclusive set of studies by a simpler analytic technique.
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The currently available literature provided considerable evidence to address the first key question regarding the efficacy or effectiveness of different types or components of interventions in helping individuals or groups modify their dietary intake. These findings are presented below. Very few studies were appropriately designed or reported their findings to permit interpretation of the evidence for the efficacy of interventions by subgroup, particularly low-income or ethnic subgroups. No studies that met the authors' review criteria provided data on the cost-effectiveness of dietary interventions.
Fruit and Vegetable Intake
Approximately one-third (39/92) of the studies we reviewed reported results of behavioral dietary interventions on fruit and vegetable intake. Based on the small number of studies (14) reporting daily servings of fruits and vegetables as outcomes, and the high degree of variability across these studies, the authors concluded that a formal meta-analysis was inappropriate. Therefore, the remaining two analysis strategies were used to determine the impact of interventions on fruit and vegetable intake.
The results of both the differences-in-deltas approach—based on 17 of 39 studies reporting daily fruit and vegetable intake—and the summary of significant findings approach—based on 36 of the 39 studies—indicated that:
- Dietary interventions were positively associated with changes in fruit and vegetable intake.
- When fruit and vegetable intake were measured individually, changes in fruit intake were larger.
Although the results from studies that reported significant findings for fruit and vegetable intake varied (depending on the particular outcome measured), the majority of the studies reviewed reported statistically significant increases in fruit and vegetable intake (either as separate outcomes or combined). Statistically significant increases in fruit and vegetable intake (as a combined variable) were noted in 16 of 22 studies reporting this outcome. Using the differences-in-deltas approach, the authors determined that the median difference between intervention and control groups in the change in daily servings of fruits and vegetables was an average increase in fruit and vegetable intake of 0.6 servings per day.
The authors were unable to explore the relative effectiveness of interventions on many population subgroups because of the minimum cell size requirement established for specific analyses. (They determined that a minimum of five studies per cell was necessary to conduct a particular analysis.)
Nevertheless, the analyses suggested that interventions were more successful at increasing fruit intake among children and vegetable intake among adults. In addition, interventions conducted among higher disease-risk populations were consistently more likely to report statistically significant increases in fruit and vegetable intake than were studies in general populations. For example, all six of the studies conducted in high-risk populations reported significant intervention effects for fruit intake compared to only eight of 14 studies conducted among general-risk populations. A slightly smaller difference (five of seven high-risk studies compared with five of 14 general population studies) was observed for the outcome of vegetable intake.
Among the specific intervention characteristics explored, several patterns were evident. Studies employing a theoretical basis were more likely to report statistically significant increases in fruits and vegetable intakes than studies that did not utilize theory (14 of 16 studies using theory reported a significant increase in fruit and vegetable intake, while three of six studies that did not indicate the use of theory reported a significant intervention effect for fruit and vegetable intake).
In addition, the authors observed a linear relationship between study quality and the likelihood of reporting significant findings. Also, the use of social support components was associated with more favorable increases in fruit and vegetable intake (using both analytic strategies). For example, all five studies that included a social support component reported a statistically significant increase in fruit intake, compared with nine of 17 studies not using social support that reported a significant effect for fruit; the differences in the proportion of studies reporting a statistically significant effect for vegetable intake was smaller (three of five studies compared with seven of 17 studies).
Finally, studies that employed goal setting and interactive activities involving food were more likely to report statistically significant increases in fruit and vegetable intake, although the magnitude of the increases was not notably higher than studies not employing such techniques. For example, among the three fruit and vegetable outcomes explored, the differences in the proportion of studies reporting statistically significant intervention effects between studies that incorporated a goal setting component and studies that did not ranged from 8 percent to 47 percent.
The authors did not have a large enough pool of articles to explore characteristics such as intervention intensity, setting, mode of delivery, use of individual tailoring, or culturally or ethnically specific interventions.
Nearly 80 percent (80/104) of the articles reviewed reported results for dietary fat, although these outcomes varied tremendously. In determining the impact of behavioral dietary interventions on decreases in fat intake, the authors used all three analysis strategies. Based on all three techniques, dietary interventions were positively associated with changes in fat consumption. They observed similar decreases in intake of total fat and saturated fat (the two most commonly reported fat outcomes in the studies reviewed). The median difference between intervention and control groups in the change in total fat intake (as a percentage of total energy intake) translates into a 7.3 percent reduction in the percentage of calories from fat. Among a subset of articles employing biochemical indicators (i.e., measuring changes in blood cholesterol), the decrease in total fat intake was significantly correlated with concomitant decreases in total blood cholesterol (r = 0.763, p = 0.004). The change in saturated fat was not associated with statistically significant decreases in total blood cholesterol.
The large number of studies reporting results for dietary fat enabled the authors to explore two moderating population characteristics: age and disease risk status. Across the five sets of fat outcomes that they explored using the summary of significant findings approach, studies conducted in high-risk populations were not consistently more likely to report a statistically significant decrease in fat intake.
However, the differences-in-deltas analysis indicated that the magnitude of the change in dietary fat, particularly the reduction in saturated fat, was notably higher among interventions conducted in higher disease-risk populations. For example, the median difference in outcome change between intervention and control groups for saturated fat intake among studies conducted with high-risk populations was -29.3 percentage points; among studies conducted with general-risk populations, the median difference-in-deltas was only -14.5 percentage points.
Similarly, 16 of the 17 studies analyzed by this method that focused on high-risk populations reported a significant intervention effect for saturated fat intake, compared with only seven of 10 studies conducted in general-risk populations. The pattern of larger effect sizes being observed among the studies focusing on high disease-risk populations was also evident in the meta-analysis. In addition, interventions conducted among children appeared to be more successful at reducing intake of total fat and less successful at reducing intake of saturated fat than interventions conducted among adults, although a very small number of studies measured fat intake among children (six for total fat intake, five for saturated fat intake).
Unlike the pattern observed for fruit and vegetable outcomes, interventions employing a theoretical framework were not consistently more likely to report significant effects (and the magnitude of the intervention effect was actually lower among studies based on theory). Nor was study quality associated with either the likelihood of reporting significant effects or the magnitude of the intervention effect.
Among the specific intervention characteristics explored, however, several consistent patterns were evident. The use of social support, small groups, and goal setting appeared particularly effective at reducing intake of dietary fat. Greater proportions of studies employing such strategies reported statistically significant findings, and the magnitude of the change in dietary fat (using the differences-in-deltas approach) appeared higher among these studies. For example, among studies so analyzed that incorporated social support components into the intervention, all seven reported a significant intervention effect for total fat intake, and the median difference-in-deltas was -26.7.
Among the studies so analyzed that did not report the use of social support, 83 percent (35/42) reported a statistically significant intervention effect for total fat intake, and the median difference in change between intervention and control groups was only -10.4 percentage points. The differences in the impact of goal setting on total fat intake were less pronounced. Among the studies that used goal setting, 18 of 19 reported a significant intervention effect (with the median difference in change being -18.9 percentage points). Among the studies that did not report the use of goal setting, 80 percent (24/30) reported a significant intervention effect (with the median difference in change being -11.0 percentage points).
Although studies that involved families in the interventions and used interactive food-related activities were more likely to report significant decreases in fat intake (for example, all 13 of the studies using a family component reported significant decreases in fat intake, and 80 percent—29 of 36—of the studies not using a family component reported a significant intervention effect for fat intake), the magnitude of the decrease was not higher than among the studies that did not incorporate these special features.
Finally, although very few studies were designed to be culturally or ethnically specific (to the study sample), the results suggest that such studies reported more favorable decreases in dietary fat (although the authors did not have a sufficient number of articles to explore the magnitude of this decrease). For example, all five of the interventions designed to be culturally or ethnically specific reported significant decreases in both total and saturated fat intake, compared to 84 percent (37/44) of the studies that were not designed to be culturally or ethnically specific.
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The evidence review lends support to the notion that a wide variety of dietary interventions delivered in many different settings to individuals of different ages, ethnicities, and genders can have a positive impact on dietary behaviors associated with cancer risk reduction. The large proportion of studies showing favorable outcomes in varied situations suggests an overall positive effect, although the potential for publication bias may have influenced the likelihood of identifying positive effects of interventions. The language limitations in the literature review present an additional source of potential bias.
The lack of similarity across studies in outcome measures, study design, analysis strategy, and intervention technique makes it difficult to draw broad conclusions about the most efficacious behavioral dietary interventions. Nevertheless, the findings offer insight into intervention components that may hold promise for future research efforts. Several dietary intervention components appear to be promising in modifying dietary change. These factors include:
- Social support.
- Goal setting.
- Small groups.
- Food-related activities.
- The incorporation of family components.
Interventions that included "interactions with food," such as cooking or taste testing, seemed particularly promising in increasing fruit and vegetable intake and reducing fat intake.
To gain the most from intervention research, future studies should assess dietary intake at the individual level and should collect detailed process and psychosocial data to help identify determinants of dietary change. With the emergence of new technologies to enhance health communications, research is urgently needed to evaluate the efficacy of these interventions, either relative to or in combination with more traditional approaches to dietary change. Comparing the cost-effectiveness of these different intervention approaches will be critical to assessing their broader applicability.
Few of the studies followed participants for more than a year, and those that did often showed a falling off from the initial dietary behavior change. More research is needed to determine the longer-term effectiveness of dietary interventions and to evaluate programs specifically designed to encourage the maintenance of change and prevent relapse over time.
Priority funding by federal agencies for dietary intervention research among underserved and minority populations has increased the amount of activity in these areas recently. However, the deficit in good quality, published research designed to determine the relative efficacy of different intervention approaches in these high-risk, hard-to-reach populations remains serious.
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Availability of Full Report
The full evidence report from which this summary was derived was prepared at the request of the National Cancer Institute for the Agency for Healthcare Research and Quality by the Research Triangle Institute—University of North Carolina at Chapel Hill Evidence-based Practice Center under contract No. 290-97-0011. Print copies of this report are available free of charge from the AHRQ Publications Clearinghouse by calling 800-358-9295. Requestors should ask for Evidence Report/Technology Assessment No. 25, Efficacy of Interventions To Modify Dietary Behavior Related to Cancer Risk (AHRQ Publication No. 01-E029).
The Evidence Report is also online on the National Library of Medicine Bookshelf.
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AHRQ Publication No. 01-E028
Current as of November 2000