Diet and blood pressure in South Africa: Intake of foods containing sodium, potassium, calcium, and magnesium in three ethnic groups

Abstract

Objectives

We investigated whether habitual intakes of sodium (Na), potassium, magnesium, and calcium differ across South African ethnic groups, assessed the proportion of Na intake, which is discretionary, and identified which food sources were the major contributors to Na intake.

Methods

This was a cross-sectional study of 325 black, white, and mixed ancestry hypertensive and normotensive subjects. Three repeated 24-h urine samples were collected for assessment of urinary Na, and three corresponding 24-h dietary recalls were administered by trained fieldworkers. Blood pressure and weight were measured at each visit. Secondary analyses were performed on existing dietary databases obtained from four regional surveys undertaken in South African adults.

Results

Mean urinary Na excretion values equated to daily salt (NaCl) intakes of 7.8, 8.5, and 9.5 g in black, mixed ancestry, and white subjects, respectively (P < 0.05). Between 33% and 46% of total Na intake was discretionary, and, of the non-discretionary sources, bread was the single greatest contributor to Na intake in all groups. Ethnic differences in calcium intake were evident, with black subjects having particularly low intakes. Urban versus rural differences existed with respect to sources of dietary Na, with greater than 70% of total non-discretionary Na being provided by bread and cereals in rural black South Africans compared with 49% to 54% in urban dwellers.

Conclusion

White South Africans have higher habitual intakes of Na, but also higher calcium intakes, than their black and mixed ancestry counterparts. All ethnic groups had Na intakes in excess of 6 g/d of salt, whereas potassium intakes in all groups were below the recommended level of 90 mM/d. Dietary differences may contribute to ethnically related differences in blood pressure.

Introduction

The Dietary Approaches to Stop Hypertension (DASH) randomized, controlled trial, provided unequivocal evidence that non-pharmacologic methods can decrease blood pressure as much as some antihypertensive drugs [1]. Subjects fed a diet rich in fruit, vegetables, and low-fat dairy products and decreased intakes of saturated and total fats for 8 wk had significant decreases in systolic and diastolic blood pressures (5.5 and 3.0 mmHg, respectively) compared with subjects on a typical American control diet. It was estimated that a population-wide decrease in systolic or diastolic blood pressure of the magnitude observed with the DASH diet would decrease the incidences of coronary heart disease by approximately 15% and stroke by about 27%. It is noteworthy that the effects of the 8-wk DASH diet were greatest in the hypertensive black subgroup, in which a blood pressure decrease of 13.2/6.1 mmHg was demonstrated [2]. Increased efficacy of the DASH diet among black subjects supports other data suggesting ethnic differences in blood pressure in response to diet, which may be related to differences in habitual dietary patterns [3].

The follow-up DASH Sodium study investigated the additional benefits of salt restriction over and above the merits of the DASH diet [4]. Decreasing sodium (Na) intake from a high level (150 mM/d) to an intermediate level (100 mM/d) or a low level (65 mM/d) resulted in a stepwise decrease in blood pressure, which was approximately twice as great in subjects on the control as in those on the DASH diet. Sodium restriction from high to low intake in those following the DASH diet resulted in a relatively small additional decrease in blood pressure. It may be concluded that the greatest benefits in Na restriction are seen in those who have a poor diet (i.e., typical “American” high-fat, low nutrient-dense diet), whereas subjects who consume large amounts of fruit and vegetables and low-fat diary products may be able to tolerate larger amounts of salt.

In South Africa, it is estimated that the age-adjusted prevalence of hypertension in the adult population is 25.2%, which amounts to 6 million South Africans [5]. The diagnosis, management, and control of hypertension is poor, particularly in the black population [5], [6], [7], [8], [9]. The situation is unlikely to improve because the public health sector, where most treated hypertensives receive care, is already overburdened with budgetary constraints, particularly in light of the epidemic of the human immunodeficiency virus and the acquired immunodeficiency syndrome. It is evident that non-pharmacologic approaches to decrease blood pressure at a population level are required to curb the predicted escalating increase in the rate of hypertension [5].

In this regard, reliable information on the dietary intake of Na and other nutrients that are important in the regulation of blood pressure, namely potassium (K), magnesium (Mg), and calcium (Ca), in various ethnic groups is required. In addition, information is needed to identify which food items are the major contributors to total Na intake. The present study was undertaken to 1) determine whether there are differences in habitual intakes of Na, K, Mg, and Ca across South African ethnic groups, 2) assess the proportion of salt intake that is discretionary, and 3) identify which food sources are the major contributors to Na intake in this and other studies of South African groups.