Racial differences in potassium homeostasis in response to differences in dietary sodium in girls

Abstract

Background: Racial differences in the renal disposition of potassium may be related to mechanisms for the greater susceptibility to hypertension in blacks than in whites.

Objective: Our objective was to study the racial differences in the renin-angiotensin-aldosterone system and in potassium balance in black and white girls consuming a controlled diet that was low in potassium with 2 amounts of sodium intake (low compared with high).

Design: The studies reported here were performed in 40 black and 28 white girls, aged 11-15 y, under highly controlled metabolic conditions. The studies comprised 2 sessions of 20-d metabolic balance sessions, at 2 amounts of dietary sodium intake (58 and 170 mmol . L(-1) . d(-1)), in a crossover design and with a constant dietary potassium intake of 50 mmol . L(-1) . d(-1). Repeated-measures analysis of variance was used to test for racial differences in potassium output and retention by sodium intakes.

Results: Thirty black and 20 white girls completed the study. Urinary potassium excretion was lower in blacks than in whites, regardless of sodium intake (P < 0.05), with no differences in fecal or sweat potassium excretion. Cumulative potassium retention was significantly higher in blacks while consuming the low sodium diet. Plasma aldosterone concentrations after upright posture were significantly lower in blacks than in whites but were similar when supine, as were urinary aldosterone excretion rates. On week 3, blood pressure, body weight, urinary volume, creatinine, and serum sodium and potassium were similar.

Conclusion: The well-known racial difference in urinary potassium excretion appears to be at least in part due to greater renal retention of potassium in black girls.

Figures

FIGURE 1

Mean (±SD) daily 24-h potassium (K+) intake and urinary and fecal excretion in blacks and whites. Repeated-measures ANOVA showed that blacks excreted significantly less urinary potassium from day 2 to the end of the study compared with whites while consuming either the low- or high-sodium (Na+) diet (P < 0.05). No significant differences were observed with fecal output. □, urinary output in whites; ▪, urinary output in blacks; Δ, fecal output in whites; ▴, fecal output in blacks; •, diet.

FIGURE 2

Mean (±SD) cumulative potassium (K+) retention during the study in black and white girls on low- and high-sodium (Na+) diets. Repeated-measures ANOVA showed that blacks had greater cumulative potassium retention while consuming the low-sodium diet than did whites (P < 0.05). No racial differences were observed for either group while consuming the high-sodium diet.

FIGURE 3

Mean (±SD) plasma aldosterone concentrations in black and white girls at the end of the 3-wk study period on low- and high-sodium (Na+) diets. Blood samples were drawn at 0700 after subjects had been recumbent overnight and at 0900 after they had been upright for 2 h. Data points with different letters are significantly different (P < 0.05, ANOVA).

FIGURE 4

Mean (±SD) plasma renin activity (PRA) in black and white girls at the end of the 3-wk study period on low- and high-sodium (Na+) diets. Blood samples were drawn at 0700 after subjects had been recumbent overnight and at 0900 after they had been upright for 2 h. Data points with different letters are significantly different (P < 0.05, ANOVA). Angio, angiotensin.