By Andrew Mente
Other authors: Prof. Martin J O’Donnell, Canada; Ms. Sumathy Rangarajan, Canada; Prof. Matt McQueen, Canada; Prof. Annamarie Kruger, South Africa; Prof. Jephat Chifamba, Zimbabwe; Prof. Li Wei, China; Prof. Liu Lisheng, China; Dr. Xingyu Wang, China; Prof. Khalid Yusoff, Malaysia; Prof. Prem Mony, India; Prof. Omar Rahman, Bangladesh; Dr. Romaina Iqbal, Pakistan; Prof. Witold Zatonski, Poland; Dr. Aytekin Oguz, Turkey; Prof. Annika Rosengren, Sweden; Prof. Roya Kelishadi, Iran; Prof. Afzal Yusufali, United Arab Emirates; Dr. Rafael Diaz, Argentina; Dr. Alvaro Avezum, Brazil; Prof. Patricio Lopez-Jaramillo, Colombia; Dr. Fernando Lanas, Chile; Prof. Gilles Dagenais, Canada; Prof. Koon Teo, Canada; Prof. Salim Yusuf, Canada, on behalf of The PURE Study Investigators.
Current guidelines on sodium (Na) intake assume that entire populations will have lower blood pressure (BP) similarly from reduced Na intake. In the Prospective Urban Rural Epidemiological (PURE) sodium study, we describe the association of Na and potassium (K) intake with BP overall and in key subpopulations.
Urine was collected in 97,000 individuals and BP was measured using an automated sphygmomanometer. Data on BP and on urinary Na and K are currently available on 51,290 individuals aged 35–70 years. We estimated Na and K excretion using the Kawasaki formula, validated against 24-hour urine in 649 individuals (r=0.61 For Na and 0.51 for K).
Mean (±SD) Na excretion was 4.42 ± 1.41g and K 2.20 ± 0.60 g. Na excretion ranged from 3.8g/day in Malaysia to 6.1g/day in China. Country GDP was inversely associated with Na, and positively associated with K excretion (P<0.001). Multivariable regression analyses of BP change per g increase in Na and K are in Table, which indicates that there is considerable heterogeneity of the effects of both Na and K on BP based on level of Na and K intakes, hypertensive status and age.
Dietary Na reduction and K increase likely reduce BP to a greater extent in those with high Na consumption, hypertension and elderly and have substantially lesser effects in those with moderate or low levels of Na consumption, younger individuals and non hypertensive individuals. Our findings have implications for international guidelines and public health strategies.
This large scale study provides a series of results that have multifold implications both for guidelines recommendations and for the practice of medicine at the individual patient level. An important finding is that the positive relationship between sodium (measured by sodium excretion) and systolic/diastolic blood pressure (BP) is confirmed and indeed strengthened by the huge amount of data obtained in the study.
Even more importantly, however, the data show that the slope of the relationship exhibits large between -subgroup differences . Namely, the relationship is steep when BP is elevated and sodium intake is high, but strikingly less so when BP is normal and sodium intake is lower (slope reduction:70% and 82%,respectively) the same being true for older vs younger subjects (>55 vs <45 years of age: slope reduction 65%). The clearcut clinical implication appears to be that as far as BP control is concerned there is not much to be gained by pursuing a low sodium diet strategy in the general population, while there can be important advantages in focusing this intervention on subgroups with specific demographic , dietary and clinical characteristics. The advantages can perhaps be increased if a reduction of sodium intake is associated with an increase in potassium intake, which the study shows to be negatively associated with BP values. While the interest of these results is unquestionable, a caveat needs to be added.
That is, that given its epidemiological nature, the study did not measure the effect of changing sodium and potassium intake within individuals. This is inferred by the BP differences between individuals.