In individuals with pre- or stage 1 hypertension, two multi-component behavioral interventions, EST+DASH and EST, significantly reduced the estimated 10-year CHD risk by 12% and 14%, respectively, compared to “advice only.” Results were similar across subgroups defined by baseline variables, and improvements in 10-year CHD risk were maintained at 18 months.
The two behavioral interventions (EST and EST+DASH) had similar effects on CHD risk. One possibility is that participants received an inadequate dose of the DASH diet, as evidenced by lack of full adherence to DASH recommendations for fruit and vegetable intake.17
An alternative explanation is subadditivity;25
specifically, the combined effects of two interventions when implemented together are less than the sum of the two when implemented separately. Subadditivity can occur when there is reduced adherence in the combined intervention or when the interventions work through similar mechanisms to achieve improvements in CHD risk factors.
Despite the intuitive appeal and public health relevance of estimated CHD risk, few studies of lifestyle interventions have used change in CHD risk as an outcome variable. In a recent randomized trial of 315 participants in Canada with 10-year CHD risk of ≥ 10%, a lifestyle intervention (health report card and telephone counseling on smoking, exercise, nutrition and stress) reduced 10-year CHD risk at 1 year of follow-up by approximately 2% compared to a usual care group.26
A smaller randomized trial (N=75) reported a similar but non-significant decrease in 10-year CHD risk at 16 weeks for a nutrition program combined with exercise relative to the nutrition program alone, but loss to follow-up was high (36%).27
On pre-post analysis, one observational study with a median follow-up time of 8 months found a non-significant increase in estimated CHD risk with dietary advice given to patients without CHD in an urban clinic in the United Kingdom.28
Another uncontrolled, longitudinal study of multiple lifestyle changes (diet, stress management, and aerobic exercise) in participants with CHD risk factors from the Windber Coronary Artery Disease Reversal (CADRe) program reported a non-significant 6.8% decrease in estimated CHD risk over 1 year for participants at-risk for cardiovascular disease.29
A major strength of our study is its large and diverse study population. While the study was not powered to examine the subgroups, results were consistent across subgroups suggesting broad applicability of trial interventions. Second, the trial has high internal validity as evidenced by high rates of data collection during follow-up. In our study, only 13% of participants had missing data at 6 months, and missing data were imputed using multiple imputation. Third, data collectors were trained, and BP and serum cholesterol were measured directly in a standardized fashion.19
Lastly, the Framingham risk functions have been validated in whites and blacks in the United States.30
Our study also has limitations. Our results may underestimate the magnitude of the effect of the EST and EST+DASH interventions on CHD risk because there was incomplete adherence17
and because the “advice only” group made lifestyle changes perhaps because of high motivation17, 31
or a Hawthorne effect related to data collection visits.32
Second, smoking status was not available at 6 months, but the use of baseline smoking status carried forward is reasonable because the interventions did not include advice on smoking cessation. Third, the current Framingham risk equations14
do not include diabetes, but we obtained similar results when using the older equations which include diabetes status.18
Finally, the Framingham risk equations may overestimate absolute risk in some populations;33
for this reason, we emphasize relative risk reductions. Of note, the baseline CHD risk was highest in the “EST+DASH” group, and the 6-month CHD risk was highest in the EST+DASH group with the “advice only” and EST groups having similar CHD risks at 6 months. It is possible that the smaller decrease in 10-year CHD risk in the “advice only” group relative to the EST and EST+DASH groups may have occurred because of a “floor” below which the risk could not decrease. However, there are populations in which actual CHD risk is extremely low.34, 35
Also, 10-year CHD risk estimated by the Framingham equations can be <1% using biologically-plausible values of the variables included in the equations.
Future research should focus on understanding the individual components of the behavioral interventions that are most effective in decreasing CHD risk. For example, an analysis of the effect of individual PREMIER lifestyle changes on BP at 6 months showed that decreased urinary sodium, improved fitness, and low total fat intake were associated with a decrease in systolic BP before controlling for weight loss; in that study, it was concluded that error in measurement of dietary and urinary sodium might account for the loss of their statistical significance after the inclusion of weight in regression models.36
Also, a comparison of CHD incidence with the change in 10-year CHD risk would be useful to validate the change in 10-year CHD risk as a surrogate outcome.
In summary, in the PREMIER trial, two multi-component behavioral interventions incorporating diet and physical activity recommendations significantly lowered estimated 10-year CHD risk by 12-14% relative to a control condition. These estimated reductions in CHD risk are substantial and support research and translational efforts to implement counseling on lifestyle change as part of routine medical care. Given that heart disease remains the leading cause of death in the United States,37
translation of these findings into clinical practice should have a substantial public health impact.