Methods

Results

Mean age remained relatively steady during the period covered by the surveys (P for linear trend=0.502; Table 1). However, the percentage of men (P for linear trend=0.003) and high school graduates increased (P for linear trend <0.001), and the percentage of participants who were white decreased (P for linear trend <0.001).

The age-adjusted prevalence of low risk factor burden increased from 4.4% during NHANES I (1971 to 1975) to 10.5% in NHANES III (1988 to 1994) before decreasing to 7.5% in NHANES 1999 to 2004 (P for nonlinear trend <0.001) (Figure 1). The patterns were similar for men and women, although the prevalence among women exceeded that among men in each survey (P<0.001 for each survey). The prevalence of low risk factor burden was much higher among participants 25 to 44 years of age than among those 45 to 64 or 65 to 74 years of age (P<0.001 for each survey). In addition, whites had a significantly higher prevalence of low risk factor burden than blacks during each survey except during NHANES II (NHANES I, III, and 1999 to 2004: P<0.001; NHANES II: P=0.154). Furthermore, a larger percentage of whites had a low risk factor burden than Mexican Americans in NHANES III (P<0.001) and NHANES 1999 to 2004 (P=0.001). In a separate analysis, we defined favorable total cholesterol and blood pressure status using only the measured values of these variables. Thus, treatment with medications to lower total cholesterol or blood pressure was not used in determining risk status. The resulting estimates of the age-adjusted prevalence of low risk factor burden for the 4 periods were 4.4%, 6.2%, 10.7%, and 7.9%.

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Figure 1. Trends in the age-adjusted prevalence (95% confidence interval) of low risk factor burden for cardiovascular disease among US adults 25 to 74 years of age.

The nonlinear trend of the index reflecting low risk factor burden comprised 5 factors that did not move uniformly in time (Table 2⇓). Thus, the trend of participants who reported not having been diagnosed with diabetes decreased linearly (P<0.001), whereas the temporal changes in the other low risk factors exhibited a nonlinear component (Table 2⇓). The directions of the trends for the individual risk factors were generally similar for the demographic groups. We also calculated the prevalence of favorable blood pressure status by assigning participants to low risk status using only the measurements of systolic and diastolic blood pressures (treatment with antihypertensive medications was not used in determining risk status). The age-adjusted estimates for the 4 time periods were 21.6%, 28.8%, 48.9%, and 43.7%, respectively. In addition, we calculated the prevalence of favorable total cholesterol status by assigning participants to low risk status using only the measurements of total cholesterol (treatment with cholesterol-lowering medications was not used in determining risk status). The age-adjusted estimates for the 4 time periods were 34.5%, 37.9%, 45%, and 48%, respectively. Additional details for trends in categories of blood pressure, total cholesterol, and body mass index are shown in Figure 2.

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Figure 2. Trends in the age-adjusted prevalence of categories of total cholesterol (mmol/L) among adults not using cholesterol-lowering medications (A), blood pressure among adults not using antihypertensive medication (B), and body mass index (kg/m²) among all adults (C).

In a study conducted among young women,⁸ participants who did not meet the criteria for low risk and did not have a single elevated risk factor (systolic blood pressure ≥140 mm Hg or diastolic blood pressure ≥90 mm Hg or current use of antihypertensive medications, total cholesterol ≥6.21 mmol/L [240 mg/dL] or current use of cholesterol-lowering medications, body mass index ≥30 kg/m², diabetes, and currently smoking) had a risk for all-cause mortality similar to that for women in the low risk group. The prevalence for this group was 18.9% in NHANES I, 19.3% in NHANES II, 23.3% in NHANES III, and 23.8% in NHANES 1999 to 2004 (P for linear trend=0.003).

Discussion

Our results showed that only ≈8% or 1 in 12 US adults had a low risk profile for cardiovascular disease during 1999 to 2004. This emphasizes the huge potential for preventing cardiovascular disease that remains to be realized. Favorable trends were observed for not currently smoking and low concentrations of total cholesterol, 2 areas that have benefitted from decades-long public health initiatives.^29–31 Blood pressure, which was also the beneficiary of a major public health initiative,³² moved in an encouraging direction during the 1970s and 1980s but subsequently changed course, partly as a result of the epidemic of obesity. The prevalence of low risk factor burden for the period 1999 to 2004 was higher than it was for the period 1971 to 1975. However, it had decreased since 1988 to 1994, which is worrisome. Adverse trends in 3 factors contributed to this unfortunate turn of events: not having diagnosed diabetes mellitus, body mass index <25 kg/m², and favorable blood pressure.

In keeping with a published definition of low risk factor burden,⁸ we classified participants who used medications to lower blood pressure and total cholesterol as not being at low risk for these 2 risk factors. However, medications can lower values of blood pressure and total cholesterol into the low risk range. Consequently, how medication use is handled in assigning risk status can potentially affect the results. In contrast to our main analysis in Table 2⇑ showing a small nonsignificant decrease in favorable total cholesterol status from NHANES III to NHANES 1999 to 2004, the secondary analysis based strictly on concentrations of total cholesterol found a significantly increasing percentage of adults having a favorable total cholesterol status.

Primordial prevention holds enormous promise in decreasing the burden of cardiovascular disease. A series of studies have demonstrated large risk reductions in cardiovascular disease among participants who had a low risk factor burden.^{6–8,11–16} At least 2 general approaches to defining low risk were taken in these studies. One combined lifestyle behaviors with physiological and anthropometric factors. The other focused on lifestyle behaviors and included an anthropometric variable.

Previous studies have yielded estimates of the prevalence of low risk factor burden (based on lifestyle behaviors and physiological and anthropometric measures) ranging from ≈4% to ≈20%.^6,8,17 Studies that defined low risk factor burden on the basis of lifestyle factors and an anthropometric measure have produced lower estimates ranging from ≈3% to ≈8.5%.^{7,11,12,14,16,33,34} In comparison, our estimate of the prevalence of low risk factor burden for the period of 1999 to 2004 was 7.5%. Our finding showing that a larger percentage of women have a favorable risk factor profile than men is consistent with most other studies.^14,33,34 However, a gender difference was not present in the data from the Chicago Heart Association Detection Project.¹⁷ Furthermore, our finding that whites have a more favorable risk factor profile than blacks or Hispanics is also consistent with previous research.^14,33,34

Our results suggest that the distribution of total cholesterol progressively improved over time albeit at a less vigorous pace during the most recent period, that the distribution of blood pressure after improving during the first 3 time periods shifted in an unfavorable direction during the most recent period, and that the distribution of body mass index progressively deteriorated over time. These trends argue for vigorous population-based approaches to reverse the unhealthy shift in the distributions of blood pressure and body mass index and to sustain or accelerate the improvement in the distribution of total cholesterol.

Because a high percentage of US adults are in contact with a healthcare provider each year, the healthcare provider is uniquely situated to evaluate the risk factor profile for cardiovascular disease of each patient and, when necessary, to manage their patients through counseling, pharmacological management, and referral. Our results clearly demonstrate a great need for prevention; thus, healthcare providers should have adequate resources, time, and reimbursement to engage in the prevention of cardiovascular disease in individuals. Such efforts by clinicians need to be complemented by efforts by state and national agencies that have the responsibility to develop effective public health interventions. Potential targets for such interventions include work sites and schools where large numbers of people can be targeted and where evidence-based interventions can be implemented.

The impact of the adverse trend of low risk factor burden on the future incidence of and mortality from cardiovascular disease in the United States is difficult to predict because the impact depends on a delicate balance of the direction of the trends of the 5 components, the magnitude of the changes, and the degree that they increase risk. Furthermore, the trends in other risk factors such as physical activity that were not included in this study are also critical to predicting future trends in the incidence of and mortality from cardiovascular disease. In addition, the percentage of the other group of adults who were at low risk for cardiovascular disease mortality (those who did not meet the criteria for low risk and did not have a single elevated risk factor) increased by an absolute 4.6% during the study period, although the increase between NHANES III and NHANES 1999 to 2004 was only 0.6%. The growth of this group potentially offset some of the increase in risk imparted by the shrinking percentage of adults with low risk factor burden.

Our results should be interpreted in light of several limitations. NHANES only surveyed noninstitutionalized adults, so the true levels may be even worse. Furthermore, several definitions of low risk have included physical activity. Because physical activity was assessed completely differently in the various NHANES, we were unable to incorporate it into our analyses. Because dietary intake was assessed with a single 24-hour dietary recall in some of the surveys, deriving a suitable dietary index was also not feasible. Furthermore, the wording of questions for use of current antihypertensive medication and for physician-diagnosed diabetes changed over time. It is unclear whether these changes in wording affected our estimates. In addition, the protocol for measuring blood pressure changed from NHANES I to NHANES III.²⁶ Like other authors, we weighted the factors of our low risk factor burden index equally. Given that our index and those of others were not designed for risk estimation but were constructed to reflect the status of multiple prevention goals that are all supported by government and professional organizations, equal weighting of the individual factors seemed reasonable. If nonresponders to the survey were more likely to be in poor health, it is possible that our estimates of low risk were overestimated.

Healthy People 2010 objectives for adults call for the following targets: smoking prevalence of 12%, hypertension prevalence of 16%, mean concentration of total cholesterol of 199 mg/dL, rate of clinically diagnosed diabetes mellitus of 2.5%, proportion of adults who are at a healthy weight of 60%, and an obesity prevalence of 15%.³⁵ Our analysis suggests that achieving low risk status for most US adults remains a distant and challenging goal. Unfortunately, the limited strides that were made toward this goal during the 1970s and 1980s were eroded by the increases in excess weight, diabetes mellitus, and hypertension during more recent decades. Because prevention offers potentially great opportunities for lessening the burden of cardiovascular disease, it is critical that the percentages of US adults who have optimal blood pressure, who have never developed diabetes mellitus, and who have optimal body weight be improved.