Deteriorating Dietary Habits Among Adults With Hypertension: DASH Dietary Accordance, NHANES 1988-1994 and 1999-2004

Background  Although the DASH (Dietary Approaches to Stop Hypertension trial) diet is among the therapeutic lifestyle changes recommended for individuals with hypertension (HTN), accordance with the DASH diet is not known.

Methods  Using data from the National Health and Nutrition Examination Survey (NHANES) from the 1988-1994 and 1999-2004 periods, DASH accordance among individuals with self-reported HTN was estimated based on 9 nutrient targets (fat, saturated fat, protein, cholesterol, fiber, magnesium, calcium, sodium, and potassium) (score range, 0-9). Using data from 1999-2004, we compared the DASH score among demographic groups in age- and energy-adjusted models and modeled the odds of a DASH-accordant dietary pattern (≥4.5) using multivariable logistic regression. The DASH score, DASH accordance, and percentage of participants achieving individual targets were compared with estimates from NHANES 1988-1994 data.

Results  Based on 4386 participants with known HTN in the recent survey period (1999-2004), the mean (SE) DASH score, after adjustment for age and energy intake, was 2.92 (0.05), with 19.4% (1.2%) classified as DASH accordant. In multivariable logistic regression models, DASH accordance was associated with older age, nonblack ethnicity, higher education, and known diabetes mellitus. Accordance with DASH was 7.3% lower in the recent survey period compared with NHANES 1988-1994 (26.7% [1.1%]) (P < .001), reflecting fewer patients with HTN meeting nutrient targets for total fat, fiber, and magnesium.

Conclusion  The dietary profile of adults with HTN in the United States has a low accordance with the DASH dietary pattern, and the dietary quality of adults with HTN has deteriorated since the introduction of the DASH diet, suggesting that secular trends have minimized the impact of the DASH message.

Hypertension (HTN) is a major risk factor for cardiovascular disease, with elevated blood pressure representing the number 1 attributable risk for death worldwide.¹ The prevalence of HTN is increasing in developing countries,² a finding that has been attributed, in part, to the adoption of Western patterns of diet and physical activity.³ Though observational studies indicate that a vegetarian dietary pattern protects against high blood pressure,⁴ interventional studies suggest that individual nutrients cannot fully account for this effect.^5,6

This apparent discrepancy between observational studies and clinical trial findings of individual nutrients provided the rationale for the Dietary Approaches to Stop Hypertension (DASH) trial.⁷ The DASH investigators hypothesized that the nutrients in a dietary pattern that was rich in fruits, vegetables, and low-fat dairy products would act synergistically to lower blood pressure, and that this dietary pattern might also be rich in other important unmeasured nutrients. The DASH intervention significantly lowered blood pressure, including an 11–mm Hg fall in systolic blood pressure compared with the control diet among participants with HTN.⁸ Subsequently, the DASH dietary pattern has been shown to complement other lifestyle modifications to lower blood pressure such as sodium restriction⁹ and weight loss.¹⁰ Furthermore, the DASH dietary approach has demonstrated beneficial effects on several other parameters associated with cardiovascular risk, including low-density lipoprotein cholesterol levels¹¹ and components of the metabolic syndrome.¹²

While the design was published in 1995,⁷ the main results of the original DASH trial were published in 1997.⁸ The DASH dietary approach has been incorporated into the therapeutic lifestyle changes recommended for all patients with HTN by the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC).^13,14 However, little is known about the influence of the DASH findings and subsequent recommendations on the dietary habits of patients with HTN in the United States. We sought to (1) evaluate accordance with the DASH diet among individuals with diagnosed HTN in a national survey and (2) assess whether the diet of Americans with HTN has improved since the publication and dissemination of the DASH results.

This study used survey data from the National Health and Nutrition Examination Survey (NHANES) from 1988-1994 and 1999-2004.¹⁵ The NHANES is a series of cross-sectional surveys conducted by the Centers for Disease Control and Prevention designed to provide health information representative of the civilian, noninstitutionalized, US population. The survey uses a multistage, stratified sampling design to ensure adequate representation of ethnic minorities. Since completion of the third survey (NHANES III, 1988-1994), the NHANES began to use contiguous 2-year cycles, and the current analyses compiled data from 3 of these cycles (1999-2000, 2001-2002, and 2003-2004). The present study includes data on male and nonpregnant female adult participants (age, ≥20 years) who provided an in-home medical history and underwent physical examination and completed a dietary questionnaire at a mobile examination center. Additional details about the surveys may be found elsewhere.¹⁵

Demographic information obtained in the interview included self-identified race and/or ethnic group (non-Hispanic white, black, Mexican American, other Hispanic, or other), education level (< high school, high school, or > high school), and health insurance (private, Medicare, Medicaid, other governmental insurance, or none). Financial status was categorized using the poverty income ratio (PIR), where family income is divided by the federally defined poverty threshold for that family (PIR < 1 indicates impoverished).¹⁶ Additional medical history was obtained on cardiovascular risk comorbidities, including self-reported history of dyslipidemia, diabetes, coronary heart disease, stroke, and congestive heart failure.

Blood pressure was measured using a mercury sphygmomanometer, with systolic blood pressure (SBP) and diastolic blood pressure (DBP) determined based on up to 4 measures; if more than 1 measure was obtained, the first measure was not considered, and the remaining measures were averaged. Otherwise, the first measure was used. Participants who did not report a history of HTN were classified according to JNC-7¹⁴ criteria as follows: undiagnosed HTN (SBP ≥ 140 mm Hg or DBP ≥ 90 mm Hg); pre-HTN (SBP ≥ 120 mm Hg or DBP ≥ 80 mm Hg without HTN); or normal blood pressure (SBP < 120 mm Hg and DBP < 80 mm Hg). Because the purpose of the current study was to evaluate the diet of individuals with known HTN, individuals were considered to have self-reported HTN if they answered “yes” to the following question: “Have you ever been told by a doctor or other health professional that you had hypertension, also called high blood pressure?”

Diet history was obtained using a 24-hour recall administered by a trained interviewer in the mobile examination center. In NHANES III,¹⁵ interviewers used an automated, interactive format with built-in probes to optimize recall quality. For the 1999-2001 survey period, dietary intake data were collected using a computer-assisted dietary interview, a multiple-pass recall method that provided instructions to interviewers for recording information about foods. Beginning in 2002, the NHANES dietary recall was merged with the US Department of Agriculture (USDA) Continuing Survey of Food Intakes by Individuals,¹⁷ and the survey began using an automated multiple pass method, a fully computerized recall method that includes an extensive compilation of standardized food-specific questions and possible response options. To reflect changes in the nutrient profile of the dietary supply over time, nutrient intake estimates were based on USDA nutrient databases that most closely corresponded to the survey period: NHANES III–USDA Survey Nutrient Database (1993-1995)¹⁸; NHANES 1999-2000–USDA 1994-1998 Survey Nutrient Database¹⁷; NHANES 2001-2002–USDA Food and Nutrient Database for Dietary Studies (FNDDS) 1.0¹⁹; and NHANES 2003-2004–FNDDS 2.0.²⁰

To assess accordance with the DASH dietary pattern, we generated a DASH score based on prior work by Gao.²¹ Briefly, we identified DASH goals for 8 target nutrients (total fat, saturated fat, protein, fiber, cholesterol, calcium, magnesium, and potassium).⁷ Although dietary sodium was held constant in the original DASH study, sodium was included among the target nutrients based on the results of the DASH-Sodium trial⁹ and JNC-VI¹³ recommendations. Nutrient goals were then indexed to total energy intake (with the exception of macronutrients), and the DASH score was generated by the sum of all nutrient targets met (maximum score, 9) (Table 1). Individuals with intake meeting a goal intermediate between the DASH goal and the nutrient content of the DASH control diet were given a score of 0.5 for that nutrient. Furthermore, a categorical outcome was evaluated to estimate the number of individuals achieving modest accordance with the DASH dietary approach. Individuals meeting approximately half of the DASH targets (DASH score ≥4.5) were considered DASH accordant.

Data were analyzed using the SURVEY procedures in SAS version 9.1 (Cary, North Carolina) to account for the complex sampling design of the surveys. All comparisons were age adjusted, with weights assigned based on census data for 2000.²² Comparisons of the DASH score among demographic subgroups of individuals with known HTN were evaluated using 2 multivariable regression models: (1) Age and caloric intake (energy) adjusted, and (2) multivariable adjusted (covariates including age, energy intake, sex, ethnicity, education, PIR, and diabetes status). Similarly, accordance with the DASH diet was evaluated with logistic regression using the multivariable model.

To evaluate the diet of individuals with HTN before and after the incorporation of DASH principles in dietary guidelines (1997), we compared the DASH score and the percentage of patients with HTN meeting individual nutrient targets in NHANES III (1988-1994) and NHANES 1999-2004 using t tests, with the pooled standard error of the difference calculated by taking the square root of the sum of the squared standard errors. All statistical tests were 2 sided with an α of .05.

During the 1999-2004 period, the age-adjusted prevalence of self-reported HTN was approximately 28% (Table 2). In age- and energy-adjusted models, individuals with known HTN had a lower mean (SE) DASH score (2.92 [0.05]) than normotensive (3.12 [0.06]) (P < .001) or prehypertensive (3.03 [0.06]) (P = .003) individuals. The mean DASH scores of individuals with known HTN and those with undiagnosed HTN (2.92 [0.07]) were equivalent (P = .94). After adjustment for multiple covariates, the findings were similar, but the difference in DASH score between pre-HTN patients and those with known HTN was no longer significant (P = .07). Among those with known HTN, there were significant differences in DASH score by age category in an energy intake–adjusted model, and differences by ethnicity, PIR, education level, and body mass index (BMI) (calculated as weight in kilograms divided by height in meters squared) after adjustment for age and caloric intake (Table 3). These differences persisted in multivariable-adjusted models, with the exception of PIR. Most notably, DASH scores were lowest among young people, African Americans, and individuals with a BMI of 30 or higher. Approximately 19% of individuals with known HTN had DASH-accordant diets (defined by a DASH score ≥4.5), after adjusting for age and caloric intake. In multivariable logistic regression modeling, DASH accordance differed significantly by age category, ethnicity, education, and diabetes status (Figure 1).

There was considerable variation in the percentage of individuals with known HTN meeting individual nutrient targets, ranging from 6.3% for magnesium to 38.7% for percentage of energy from protein (Table 4). When comparing nutrient targets in NHANES 1988-1994 and NHANES 1999-2004, we found a significant decrease in the percentage of individuals meeting targets for energy from fat (absolute percentage change, −8.1%; P = .001), fiber (−5.7%; P < .001), and magnesium (−7.3%; P < .001).

Compared with the earlier survey period, the distribution of DASH scores shifted downward since the incorporation of the DASH diet into dietary recommendations (Figure 2). This was reflected in a decline in mean (SE) age- and energy-adjusted DASH score from 3.22 (0.07) to 2.92 (0.05) (P = .002). Overall, there was a significant decrease of 7.3% (P < .001) in individuals with a DASH-accordant diet, and findings were consistent after multivariable adjustment (Table 4).

Since the publication of the DASH study results, a DASH dietary pattern has been incorporated in the therapeutic lifestyle changes recommended by the National High Blood Pressure Education Panel for all adults with or at risk for HTN.^13,14 However, we found that few Americans with HTN had diets even modestly accordant with the DASH diet. This suggests that the diet of Americans with HTN has not been greatly influenced by the results and recommendations emerging from the DASH trial and instead reflects secular trends in the dietary patterns of the overall population.²³ Moreover, the DASH score was lower in subgroups likely to receive the greatest benefit from the DASH diet—African Americans²⁴ and obese individuals (BMI ≥30).²⁵ As data continue to accrue on the optimal diet to reduce risk in patients with HTN,²⁶ it is imperative that national guideline recommendations be complimented by research and policy initiatives that seek to effectively implement this knowledge.

As outlined by Rose,²⁷ the approach to disease prevention can be broadly characterized by 2 strategies: a high-risk strategy that targets those at greatest risk in the population, and a population strategy that seeks to shift the distribution of the population to a lower risk profile. Given the burden of HTN and the contribution of lifestyle factors to cardiovascular risk, the effective implementation of nonpharmacologic interventions to reduce blood pressure and cardiovascular risk will require a combination of these approaches. Thus far, efforts to influence blood pressure nonpharmacologically have focused on a high-risk strategy, relying on clinicians to identify appropriate individuals and counsel accordingly. However, only one-third of office visits for patients with HTN include dietary counseling,²⁸ which may reflect a host of deterrents to the implementation of lifestyle interventions by clinicians, including lack of time and reimbursement, lack of education in counseling techniques, lack of support staff, or a disbelief in the efficacy of nonpharmacologic therapy.²⁹ Yet there is evidence that even modest physician nutrition counseling can influence dietary habits and reduce cardiovascular risk parameters,³⁰ with more comprehensive clinic-based or case-management models offering greater potential benefits.³¹ As individuals with HTN frequently carry other cardiovascular risk comorbidities, these clinical interventions may be an effective way to lower global risk in high-risk patients.

Lifestyle patterns are shaped by environmental factors as well, and approaches to improve the diet of the population could seek to ensure that there is widespread knowledge of and access to healthy dietary choices. Unfortunately, national promotions such as the 5-a-Day program (eat 5 fruits and vegetables a day) do not appear to have effected a populationwide shift in fruit and vegetable intake,²³ owing in part to the influence of a food industry for whom energy-dense, nutrient-poor products are more profitable.³² Furthermore, demographic disparities in dietary quality are driven, in part, by inadequate access to healthy foods (eg, fresh produce) in high-risk, urban communities.³³ Community- and work site–based interventions show promise in providing feasible and effective interventions for dietary improvement³⁴ because the use of existing infrastructure and resources is an important part of improving environmental determinants of diet and physical activity.³⁵ A complementary approach is the collaboration between nonprofit nutrition organizations and industry partners to help generate consumer knowledge of and demand for healthy alternatives, as evidenced by the Whole Grains Council.³⁶ These efforts demonstrate the potential to create a healthier environment and decrease the burden of cardiovascular disease by lowering the risk burden in the population.

There are several potential limitations to the present observations. First, our data are drawn from single 24-hour dietary recalls, which are subject to misclassification. However, this survey technique is a valid approach to assess dietary intake of large groups (eg, adults with HTN).³⁷ While minor methodologic differences in the dietary recalls were present across surveys, the methods were generally consistent, and these data have been used by the Centers for Disease Control and Prevention to track dietary trends over time.³⁸ Indeed, these surveys are the best opportunity to evaluate this question in a nationally representative sample.

Another consideration is the macronutrient profile of the DASH diet. The DASH diet was designed to be rich in fiber and minerals but low in total and saturated fat. However, accumulating data suggest that an approach that targets total fat may not be the optimal dietary pattern for the reduction of cardiovascular risk.³⁹ In the OMNI-Heart study,²⁶ the substitution of protein or unsaturated fats for carbohydrates in a DASH-style diet resulted in greater blood pressure reductions. In additional analyses excluding the total fat component of the DASH score, our conclusions were unchanged (data not shown).

The DASH score was meant to reflect the comprehensive DASH dietary pattern based on estimated nutrient intake independent of total energy intake. Other approaches to establish DASH dietary goals have used food groups⁴⁰ or linear index models.⁴¹ Furthermore, the DASH score implicitly gives equal weighting to each nutrient, although the strength of the relationship between each DASH score component and blood pressure is likely to differ. However, as noted by the DASH investigators, the DASH study aimed to evaluate the nutrient synergy of a dietary pattern rich in fruits, vegetables, low-fat dairy products, and lean protein. The DASH score provides a summary metric reflecting the degree of accordance with the diet provided in the DASH intervention based on the nutrient parameters targeted in the original study diet.

In summary, the dietary profile of adults with HTN in the United States has a low accordance with the DASH dietary pattern advocated in national guidelines. Moreover, the dietary quality of adults with HTN, as measured by DASH accordance, has deteriorated since the introduction of the DASH diet, suggesting that secular trends have minimized the impact of the DASH message. These findings highlight the need for additional public health and clinical science initiatives to translate an efficacious intervention into an effective tool to lower blood pressure and cardiovascular risk.

Correspondence: Philip B. Mellen, MD, MS, Hypertension Center of the Hattiesburg Clinic, 5909 US Hwy 49, Ste 30, Hattiesburg, MS 39402 (philip.mellen@hattiesburgclinic.com)

Accepted for Publication: September 17, 2007.

Author Contributions:Study concept and design: Mellen, Gao, and Goff. Analysis and interpretation of data: Mellen, Gao, Vitolins, and Goff. Drafting of the manuscript: Mellen. Critical revision of the manuscript for important intellectual content: Gao, Vitolins, and Goff. Statistical analysis: Mellen and Gao.

Financial Disclosure: None reported.

Previous Presentation: This material was presented in abstract form at the American Society of Hypertension Scientific Sessions; May 20, 2007; Chicago, Illinois.