Association of the combined estimated 24-h urinary Na/K ratio and body mass index with blood pressure: Cross-sectional data from the KOBE study

Abstract

Objective Several studies have suggested that high dietary Na/K ratio and body mass index (BMI) increase the prevalence of hypertension. However, there have been a few reports on the combination of these two factors and their relationship with hypertension. This study aimed to examine the association of the combined estimated 24-h urinary Na/K ratio (24h-u-Na/K) (high or low) and BMI (high or low) with the risk of high blood pressure.Method We performed a cross-sectional study involving 1,112 participants (340 men and 772 women) of the Kobe Orthopedic and Biomedical Epidemiological study (KOBE study) who had no cardiovascular diseases or current anti-hypertensive medications. Sex-specific analyses were performed. The 24h-u-Na/K ratio was calculated from an estimation formula using collected spot urine. Participants were divided into four groups based on their 24h-u-Na/K ratio (low or high) and BMI (low or high), with the cutoff points being the median and 25 kg/m2, respectively. Participants with systolic blood pressure (SBP) ≥130 mmHg or diastolic blood pressure (DBP) ≥80 mmHg were diagnosed with high blood pressure. Odds ratios (ORs) for high blood pressure according to the combined risks of high 24h-u-Na/K and BMI were examined with a logistic regression analysis.Results The average SBP/DBP for men and women was 122.7/77.9 and 113.3/69.1 mmHg, respectively, and prevalence of high blood pressure among men and women was 47.4% and 21.3%, respectively. The mean BMI was 22.8 kg/m2 for men and 20.9 kg/m2 for women. The median 24h-u-Na/K was 3.2 for men and 3.1 for women. The prevalence of high blood pressure (men, women) was the highest in the group in which both 24h-u-Na/K and BMI were high (60.0%, 62.9%; men: P=0.273; women: P<0.001). In the same group, the multivariate-adjusted ORs for high blood pressure were significantly higher for both men (2.59; 95% confidence interval [CI]: 1.15-5.86) and women (10.78; 95% CI: 4.87-23.88) compared to the reference group with both factors classified as low. Women with low BMI but high 24h-u-Na/K also demonstrated a higher risk for high blood pressure (OR: 1.62; 95% CI: 1.10-2.40).Conclusion The risk of high blood pressure was the highest when both BMI and 24h-u-Na/K were high. The current specific healthcare guidance in Japan is focused on obese individuals. However, in order to prevent hypertension more effectively, additional focus should be placed on the Na/K diet. Increased intake of vegetables and fruits and reduced intake of salt should be strongly recommended.