Dietary Sodium to Potassium Ratio and Risk of Stroke in a Multiethnic Urban Population
The Northern Manhattan Study
Background and Purpose—There is growing evidence that increased dietary sodium (Na) intake increases the risk of vascular diseases, including stroke, at least in part via an increase in blood pressure. Higher dietary potassium (K), seen with increased intake of fruits and vegetables, is associated with lower blood pressure. The goal of this study was to determine the association of a dietary Na:K with risk of stroke in a multiethnic urban population.
Methods—Stroke-free participants from the Northern Manhattan Study, a population-based cohort study of stroke incidence, were followed-up for incident stroke. Baseline food frequency questionnaires were analyzed for Na and K intake. We estimated the hazard ratios and 95% confidence intervals for the association of Na:K with incident total stroke using multivariable Cox proportional hazards models.
Results—Among 2570 participants with dietary data (mean age, 69±10 years; 64% women; 21% white; 55% Hispanic; 24% black), the mean Na:K ratio was 1.22±0.43. Over a mean follow-up of 12 years, there were 274 strokes. In adjusted models, a higher Na:K ratio was associated with increased risk for stroke (hazard ratio, 1.6; 95% confidence interval, 1.2–2.1) and specifically ischemic stroke (hazard ratio, 1.6; 95% confidence interval, 1.2–2.1).
Conclusions—Na:K intake is an independent predictor of stroke risk. Further studies are required to understand the joint effect of Na and K intake on risk of cardiovascular disease.
Dietary sodium (Na) and potassium (K) are increasingly recognized as important contributors to cardiovascular disease (CVD). There is substantial and growing evidence that an increase in dietary Na+ increases morbidity and mortality from CVD, at least in part via an increase in blood pressure (BP).1 Much of the increased Na intake stems from processed and restaurant foods.2 Dietary K, derived primarily from fruit and vegetable intake, has an opposite effect, with higher intake having a protective effect against elevated BP.3 The 2010 US Dietary Guidelines, which few are able to meet, emphasized that dietary Na should be limited to 2300 mg/d, with a lower limit of 1500 mg/d for adults aged >50 years, non-Hispanic blacks, and those with diabetes mellitus, hypertension, or chronic kidney disease; the K intake goals were ≥4700 mg/d.4 Guidelines on NA and K intake are based on clinical trials showing that reducing Na intake from moderate to low levels results in modest reductions in BP.5,6 Although the physiological mechanisms are not fully understood, a diet high in K+ may decrease the adverse effect of dietary Na+, resulting in lower BP and decreased risk of stroke.7 There are fewer studies, however, examining the role of the Na:K ratio on risk of CVD and stroke.8 Prior observational studies on the role of the Na:K ratio on the risk of CVD have been performed in European and Asian populations, which have different dietary patterns than populations living in urban centers in the United States.9,10 Furthermore, few of these studies have examined the role of the Na:K ratio in populations with large proportions of Caribbean Hispanics, who have an understudied dietary pattern. We have reported previously, for example, that few residents in Northern Manhattan meet dietary guidelines for Na intake or adhere to the Mediterranean diet.11,12 In a previous publication, we showed that NOMAS (Northern Manhattan Study) participants who consumed ≥4000 mg/d Na had an increased risk of stroke in comparison to those who consumed ≤1500 mg/d and there was a 17% increased risk of stroke for each 500-mg/d increase, independent of vascular risk factors.11 The purpose of the current study therefore was to examine whether dietary K and the Na:K ratio are associated with the risk of stroke in NOMAS. We hypothesized that low K intake and high Na:K ratio would be associated with a higher risk of stroke.
NOMAS is a population-based cohort study designed to determine risk factors for stroke and CVD in a multiethnic urban population. Eligible participants were (1) stroke free; (2) resident of at least 3-month duration of Northern Manhattan as defined by zip-codes 10031, 10032, 10033, 10034, and 10040; (3) randomly derived from a household with a telephone; and (4) age ≥40 years (changed to age ≥55years in 1998) at the time of first in-person assessment. Participants were identified by random digit dialing (91% telephone response rate) and recruited to have an in-person baseline interview and assessment between 1993 and 2001. The enrollment response rate was 75%, and 3,298 participants were enrolled. The study was approved by the Institutional Review Boards at Columbia University Medical Center and the University of Miami. All participants gave informed consent to participate in the study.
Baseline Risk Factors
Data on baseline status and risk factors were collected through interviews of participants. Race ethnicity was determined by self-identification based on a questionnaire adapted from the 2000 US census. Standardized questions were asked on hypertension, diabetes mellitus, cigarette smoking, alcohol intake, and cardiac comorbidities. Smoking was categorized as current (within the past year), former, or never smoker of cigarettes, cigars, or pipes. Moderate alcohol use was defined as current drinking of >1 drink/mo and ≤2 drinks/d. Moderate to heavy physical activity level was defined as engaging in 1 or more of selected rigorous physical activities in a typical 14-day period, as described previously.13 BP was measured twice, before and after each examination, and averaged. Hypertension was defined as BP ≥140/90 mm Hg, the patient’s self-report of hypertension, or use of antihypertensive medications. Diabetes mellitus was defined by the patient’s self-report of a history of diabetes mellitus, use of insulin or oral antidiabetic medication, or fasting glucose of ≥126 mg/dL. Hypercholesterolemia was defined as having a total cholesterol level of >200 mg/dL, use of cholesterol-lowering medications, or self-reported history of hypercholesterolemia. Calculation of the estimated glomerular filtration rate was performed using the Chronic Kidney Disease–Epidemiology Collaboration formula.
At baseline, participants were administered a modified Block National Cancer Institute food frequency questionnaire by trained research assistants, in English or Spanish. This questionnaire assesses dietary patterns over the previous year and was modified to include specific dietary items commonly consumed among Hispanics. Dietary Na and K intake were calculated based on self-reported food consumption using DIETSYS software (Block Dietary Data System: Dietsys+ analysis software, version 59, 1999) and reported as mean milligrams per day. K consumption was examined continuously per 100 mg and in quartiles. The ratio of Na:K was examined continuously. For descriptive analyses, we identified the median values for Na+ (2803 mg) and K+ (2446 mg), and we divided the cohort into 4 categories: low Na (<median) and high K (>median) as the reference, low Na (<median) and low K (≤median), high Na (≥median) and high K (>median), and high Na (≥median) and low K (≤median). Details on the NOMAS dietary assessments have been published previously.11 A score to represent adherence to a Mediterranean-style diet has been described previously for this cohort.
Cardiovascular Disease Outcomes and Mortality
The primary outcomes of interest were all stroke and ischemic stroke. Participants were followed-up annually via phone screening to detect any new neurological symptoms, hospitalizations, or death. Potential strokes were adjudicated by 2 neurologists independently after review of all data. Cause of death was ascertained through phone discussion with the participant’s family, review of medical records, and when available, a copy of the death certificate. Complete loss to follow-up occurred in <1%.14
For our analysis, we excluded participants without a completed diet questionnaire (n=132), with improbable total daily kilocalories or Na+ consumption based on food frequency responses (<500 or >4000 kcal/d or >10 000 mg/d Na, n=272) and those with an myocardial infarction before baseline (n=237) because of concerns that they may have altered their data after the initial event and because their baseline higher risk of recurrent events may have made this proportion of our sample biased. We examined the unadjusted associations of categories of NA and K consumption with sociodemographics and vascular risk factors using ANOVA and χ2 tests. The association of K consumption and Na:K with incident stroke was examined using Cox proportional hazards models in a series of models, unadjusted and adjusted for potential confounders, after confirming the appropriateness of the proportional hazards assumption. Follow-up accrued from baseline to the date of stroke or death, loss to follow-up, or until March 2017, whichever came first, and this time from baseline to event or end of follow-up was used as the time scale in the Cox models. Model 1 for the analysis of K consumption included the following covariates: age, sex, high-school completion, race ethnicity, total calories, Mediterranean diet score, moderate alcohol use, moderate to heavy physical activity, smoking, and Na consumption. Model 2 additionally adjusted for estimated glomerular filtration rate, body mass index, hypertension, hypercholesterolemia, and diabetes mellitus. The analysis of Na:K ratio only included Na+ consumption as a covariate in a sensitivity analysis in which it was added to model 2. We hypothesized a potential interaction between Na and K in relation to stroke, so we first looked at interactions between K+ (assessed continuously) and Na (<2300 mg/d versus ≥2300 mg/d).
There were a total of 2496 participants in NOMAS without a history of myocardial infarction at baseline who had dietary data from the food frequency questionnaire available. The mean age of the cohort was 69±10 years, with ≈64% women and 55% Hispanic. Baseline demographics of the cohort are outlined in Table 1. The mean dietary Na intake in the cohort was high (3057±1510 mg/d; median, 2803), with only a small proportion of the population being below dietary Na guidelines (<2300 mg/d, 36%; <1500 mg/d, 12%). The dietary K intake was low (mean, 2591±1099 mg/d; median, 2446; interquartile range, 1805–3211), with only a small proportion of the population meeting K intake guidelines (>4700 mg/d, 5%). The mean Na:K ratio was 1.22±0.43 with 38% of the sample having both high Na and K intake (above the median for both).
Participants were followed-up for a mean of 12±5 years and there were 268 strokes (227 ischemic).
Risk of Stroke With Potassium Intake
Tests of interaction suggested effect modification by Na (<2300 mg versus ≥2300 mg) for the relationship between K with stroke (P<0.10), so K was examined within strata of Na+ consumption, as shown in Table 2. Dietary K intake was evaluated as a continuous variable and by quartiles (with reference to the highest quartile). Among participants with Na intake of <2300 mg/d, there was a trend toward a positive association between K intake and total stroke risk after adjusting for confounders. K was associated with ischemic stroke risk. Among participants with higher Na intake (≥2300 mg/d), there was a marginally significant (P<0.10) inverse association between K intake and all stroke that was stronger in relation to ischemic stroke only.
Risk of Stroke With the Na:K Ratio
Table 3 outlines the association of the Na:K ratio with the risk of stroke. We found that an increased Na:K ratio was associated with a greater risk of ischemic and all strokes independently of other confounders. When we further adjusted for Na in sensitivity analysis model 3, the effect estimates were attenuated for all stroke. There were 346 study participants who reported taking a diuretic, which could modify Na and K levels. When we exclude these participants in sensitivity analyses, the conclusions remained the same. For example, the hazard ratio (95% confidence interval) for Na:K ratio in relation to all stroke in model 2 was 1.50 (1.11–2.03).
The increased risk of stroke among participants with both high Na consumption (>median) and low K (<median) versus all others was of borderline significance in model 2 (hazard ratio, 1.38; 95% confidence interval, 0.98–1.93; P=0.06).
Effect modification by race/ethnicity was not observed for either K or Na:K in relation to the outcomes.
In our elderly multiethnic urban-dwelling population with an overall high dietary intake of Na and low intake of K, we found that an increase in the ratio of Na:K was associated with an increased risk of stroke. The relationship between K+ intake and stroke appeared to be dependent on Na+ intake, such that an unexpected positive association was observed for K intake among those with <2300 mg Na/d and an expected inverse association was observed for K intake among those with ≥2300 mg Na/d. Our findings support the notion that Na and K dietary intake’s influence on risk of stroke may be best understood in the context of the amount of both in combination. This approach is similar to other research on dietary intake with risk of CVD, where patterns of diet such as the Mediterranean diet provide more meaningful information on risk in comparison to analyses on single micronutrients. Our results also suggest that among those with high Na consumption, who are therefore at an increased risk for hypertension and stroke, increased consumption of K-rich foods may help lower stroke risk, but further research to confirm this hypothesis is needed.
There is a substantial body of research on the effects of Na and K intake when considered separately,15 but more limited research on both moieties in combination, especially in populations with high Na intake such as blacks and Hispanics in urban centers.16 The data on dietary Na are well known, and public health campaigns to reduce dietary Na intake in processed foods as well as in restaurants have made significant strides in reducing the burden of disease associated with hypertension and CVD.17 Several researchers have identified an association of higher dietary K, seen with increased intake of fruits and vegetables, with lower BP.3 High K intake reduces BP in adults with hypertension and has no adverse effect on blood lipids and catecholamine concentration or renal function.18 The effect of increased K+ intake seems to act primarily through a reduction in BP, making it not surprising that there is a particularly strong effect on the risk of stroke as we, and others, have noted.2 For K, the data suggest that supplementation is best achieved through alterations in the diet, rather than by supplements, because interactions with other nutrients are associated with its health benefits.8 The relative levels of both Na and K intake may provide additional information on risk of CVD such that increasing K intake in the face of high Na intake (thereby reducing the ratio) may be effective at reducing the risk of stroke. The findings of a higher K intake being associated with an increased risk of stroke was, however, an unexpected finding although we note the overall high Na intake in our population, this is unlikely to be the only explanation. One possible explanation is that Na and K intake can be associated with difficult to control hypertension or congestive heart failure, such that those participants considered at highest risk were counseled by a physician to decrease Na intake and increase K intake. In other words, participants with low Na intake may have adopted this dietary pattern after having been counseled by a physician to lower Na intake because of hypertension or other comorbidities. In NOMAS, we did not collect measures of severity of hypertension or congestive heart failure and may therefore have residual confounding.
The strengths of our study include examining the role of Na and K intake on the risk of stroke in a multiethnic population with a very high Na intake, noting that increasing K intake could offset the risk associated with a high Na intake. Our study however has several limitations. Our Na and K intake was based on answers to a food frequency questionnaire rather than objective measures such as 24-hour urinary excretion, which may be more informative.19 Food frequency questionnaires are particularly deficient in accurately capturing Na consumption from salt added at the table and are limited because the Na content in prepared foods varies widely. However, we have shown previously Na consumption from this questionnaire to be associated with stroke risk in this study population. We only collected 1 measure of Na and K intake and do not have information on dietary patterns in midlife or after enrollment, which may provide additional information on the true effect on CVD. We were also unable to capture dietary changes that may result from preclinical diagnoses before baseline that may impact the risk of the outcomes of interest. Finally, our small sample size may limit power to detect more subtle effects on stroke subtypes in categorical analyses, resulting in wide confidence bounds.
In summary, we found that Na:K intake is an independent predictor of stroke risk in a multiethnic population with a low proportion meeting dietary guidelines. These findings emphasize the importance of considering dietary data as a whole versus examining micronutrients individually. Further studies are required to understand the joint effect of Na and K intake on risk of stroke.
Sources of Funding
This study was supported by National Institute of Neurological Disorders and Stroke (R37 NS 29993).
Guest Editor for this article was Eric Smith, MD, MPH.
- Received May 5, 2017.
- Revision received August 11, 2017.
- Accepted August 22, 2017.
- © 2017 American Heart Association, Inc.
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