Cardiovascular Journal of Africa: Vol 29 No 3 (May/Junel 2018)

CARDIOVASCULAR JOURNAL OF AFRICA • Volume 29, No 3, May/June 2018

172

AFRICA

Obesity masks the relationship between dietary salt

intake and blood pressure in people of African ancestry:

the impact of obesity on the relationship between

sodium and blood pressure

Muzi Maseko, Mercy Mashao, Abdulraheem Bawa-Allah, Edgar Phukubje, Bongubuhle Mlambo,

Thamsanqa Nyundu

Abstract

Previous studies conducted to investigate the relationship

between sodium intake and blood pressure in our African

population have yielded contradictory results. With the high

prevalence of obesity in this population, it is possible that

these contradictory findings are due to the masking effects of

obesity on this relationship. We measured 24-hour ambula-

tory blood pressure and 24-hour urine excretion on 547 South

Africans of African ancestry. A multivariate regression analy-

sis revealed no independent relationship between 24-hour

sodium excretion and blood pressure in the total population

sample, but when participants were stratified according to

body mass index (BMI) status, there was a significant associa-

tion between 24-hour sodium excretion and blood pressure in

the normal-BMI participants but not in the overweight/obese

participants. We concluded that dietary salt intake, indexed

by 24-hour urinary sodium excretion, was associated with

increased ambulatory blood pressure but this relationship was

masked because of a high proportion of overweight/obese

individuals in this population.

Keywords:

dietary salt intake, obesity, hypertension, salt reten-

tion, body mass index

Submitted 24/3/17, accepted 29/1/18

Published online 12/2/18

Cardiovasc J Afr

2018;

: 172–176

www.cvja.co.za

DOI: 10.5830/CVJA-2018-011

Cardiovascular diseases (CVD) are presently a leading cause

of death in South Africa and sub-Saharan Africa (SSA).

1,2

Hypertension remains the commonest risk factor for CVD.

3,4

number of studies have associated an increased dietary sodium

(Na

) intake and obesity with hypertension and target-organ

damage.

In an effort to lessen the worldwide prevalence of hypertension,

global strategies and population-based intervention studies,

including Dietary Approaches to Stop Hypertension (DASH),

have focused particularly on reduction of Na

intake as a means

of lowering blood pressure (BP) in a population.

6-8

This could

be of benefit to people of African descent because a number

of studies have shown that the incidence of hypertension is

increasing in black communities.

9-11

However there is a gap in our

knowledge on the role of dietary sodium on blood pressure in

this community.

Even though previous studies conducted in this population

have revealed a high prevalence of hypertension,

12,13

the

relationship between dietary salt intake and BP is still not

well understood because studies have revealed contradictory

findings on this relationship.

14,15

One study showed a modest

association,

while others showed no association.

17,8

In one

study, the investigators showed an association between BP and

the sodium-to-potassium ratio, which is also an index of dietary

sodium intake, but they could not show any direct relationship

between BP and dietary sodium.

The contradictory findings of these studies are indicative of

the complex relationship between BP and dietary sodium. In

this population, the complexity of this relationship could be

compounded by the high prevalence of obesity,

as body mass

index (BMI) has been shown to have a direct association with

BP.

20-23

A possibility exists that obesity masks the relationship

between BP and dietary sodium intake in this population.

The biggest contributor to the masking effect could be the

high proportion of overweight or obese individuals, especially

women. Therefore, in this study, in order to investigate whether

the relationship between dietary salt intake (indexed by 24-hour

urinary sodium excretion) and blood pressure is masked by

obesity, we stratified participants according to BMI status.

Methods

Informed consent was obtained from the participants, and the

principles of the Declaration of Helsinki were adhered to. The

study was approved by the University of the Witwatersrand

Committee for Research in Human Subjects (approval number:

M15-06-44) and forms part of the South African Hypertension

and Diet Study (SAHDS), which is part of the ongoing African

Project on Genes in Hypertension. The study design has been

briefly described in other publications.

24,25

We randomly recruited 1 219 South Africans of African

ancestry from a metropolitan area of Johannesburg (Soweto). Of

these participants, 547 (346 women and 201 men) were selected

School of Physiology, Faculty of Health Sciences, University

of the Witwatersrand, Johannesburg, South Africa

Muzi Maseko, PhD,

muzi.maseko@wits.ac.za

Mercy Mashao, MSc

Abdulraheem Bawa-Allah, MSc

Edgar Phukubje, BSc

Bongubuhle Mlambo, BSc

Thamsanqa Nyundu, MSc