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The Journal of Clinical Endocrinology & Metabolism Vol. 89, No. 4 1858-1863
Copyright © 2004 by The Endocrine Society

Sodium Retention in Black and White Female Adolescents in Response to Salt Intake

Cristina Palacios, Karin Wigertz, Berdine R. Martin, Lisa Jackman, J. Howard Pratt, Munro Peacock, George McCabe and Connie M. Weaver

Departments of Foods and Nutrition and Department of Statistics (C.P., K.W., B.R.M., L.J., G.M., C.M.W.), Purdue University, West Lafayette, Indiana 47907; and Indiana University School of Medicine (J.H.P., M.P.), Indianapolis, Indiana 46223

Address all correspondence and requests for reprints to: Connie M. Weaver, Ph.D., Department of Foods and Nutrition, Purdue University, 1264 Stone Hall, 700 West State Street, West Lafayette, Indiana 47907-2059. E-mail: weavercm{at}cfs.purdue.edu.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Increased sodium (Na+) retention in blacks could be related to the high prevalence of hypertension in adult blacks. Na+ retention in response to controlled dietary Na+ has not been rigorously compared in the different race groups. The present study assessed Na+ retention in 22 black and 14 white girls, 11–15 yr old, during 3 wk on a low (1.3 g, 57 mmol)- and during 3 wk on a high (4 g, 172 mmol)-Na+ diet in a randomized order, crossover design. Subjects were matched by postmenarcheal age and weight. After a 1-wk equilibration period, the mean daily Na+ retention was 357 ± 69 mg (15.5 ± 3.0 mmol) in blacks and 239 ± 37 mg (10.4 ± 1.6 mmol) in whites on the low-Na+ diet and 991 ± 138 mg (43.1 ± 6.0 mmol) in blacks vs. 334 ± 90 mg (14.5 ± 3.9 mmol) in whites (P < 0.001) on the high-Na+ diet. The greater Na+ retention in blacks was not accompanied by an increase in fecal or sweat Na+ excretion. Blood pressure and weight did not increase despite the Na+ retention, and thus, the retained Na+ appeared to reside in a nonextracellular compartment that we speculate to be bone. In summary, black girls showed greater Na+ retention compared with white girls. The difference in Na+ handling may contribute to underlying racial differences in susceptibility to hypertension.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
HYPERTENSION IS MORE common, with an earlier onset of age, in blacks compared with whites (1, 2). There appears to be a greater dependence on sodium (Na+) retention in blacks, in that the hypertension in blacks typically shows enhanced sensitivity to variations in Na+ intake (3). Indeed, the Dietary Approaches to Stop Hypertension (DASH) study showed that blacks lowered their blood pressure more than whites in response to dietary modifications (4), especially with respect to the intake of Na+ (5).

Blood pressures have in some studies been found to be higher in blacks, even at a young age, before the onset of hypertension (6), suggesting that factors involved in the development of hypertension in blacks are functioning early in life (7, 8, 9). An excess of Na+ retention may be present as early as childhood in blacks, as suggested by lower levels of plasma renin activity (PRA) and plasma aldosterone (6, 10). In the present study, we sought more rigorous evidence for racial differences in Na+ retention. We performed a study in adolescent black and white females where Na+ intake and excretion were carefully monitored on both low and high-Na+ intakes.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

Na+ retention was measured in 22 black and 14 white adolescent females, 11–15 yr old. The subjects were resident in a Purdue fraternity house, which was transformed during the summer into a metabolic unit. Subjects were supervised at all times by trained staff. The balance study was divided into two sessions of 3 wk each during the summer of 1999, with two levels of dietary Na+. The Na+ intake periods were separated by a 2-wk period, in which subjects were free to consume self-selected diets. Subjects completed six, 24-h dietary recalls before the study began, which were analyzed using Nutritionist IV Diet Analysis (First Databank Division, San Bruno, CA 1995). Applicants were excluded from the study if they were less than 11 or more than 15 yr old, had a body mass index of less than 15th or more than 85th percentile for age, or had a history of amenorrhea, pregnancy or abortion, eating disorders, oral contraceptive use, or tobacco use. Race was determined with a screening questionnaire asking the race of parents and grandparents. Both parents and grandparents had to be white or black to be eligible in the study. Black and white subjects were matched for weight and postmenarcheal age. All subjects were studied under protocols approved by the Purdue University Use of Human Subjects Research Committee, and subjects and their guardians gave informed consent before the study began.

Diet and dietary composite processing

The first week served as an equilibration period. Subjects were studied twice in a randomized order, crossover design to diets containing either low- (1 g/d, 43 mmol/d) or high- (4 g/d, 174 mmol/d) Na+, with fixed amounts of dietary potassium (2186 mg/d, 56 mmol/d), calcium (816 mg/d, 20 mmol/d), magnesium (229 mg/d, 9.4 mmol/d), phosphorus (1100 mg/d, 36 mmol/d), protein (70 g/d), fat (73.6 g/d), and fiber (10 g/d), which represent the usual intake of these nutrients in this population. A 4-d cycle menu, with three meals and two snacks, was designed for the study. The basal diet contained 1 g/d (43 mmol/d) Na+. For the high-Na+ level, salt was added to low-Na+ soups, providing 2 g/d (87 mmol/d) Na+ and to low-Na+ Gatorade (The Quaker Oats Company, Barrington, IL), providing 0.86 g/d (37.4 mmol/d) Na+. A difference of 2.86 g/d (124.4 mmol/d) Na+ was achieved between the low- and high-Na+ diets. Foods during each study period were purchased in bulk to decrease variation due to processing batches.

Food portions were maintained constant daily and were equal for all the subjects. Subjects were strictly supervised at all times (one counselor per six subjects) to ensure compliance and to avoid consumption of other foods. The food and beverages were prepared with deionized water and weighed to the nearest one tenth gram on digital scales. Duplicates of each of the day’s meals were homogenized and analyzed. Subjects adjusted their energy intake with Na+-free items, which contained mainly sugar. Consumption of these items was recorded daily.

Data collection and analysis

Body weight was recorded daily using a Health O Meter electronic scale (Bridgeview, IL). Blood pressure was measured every other day in recumbent position using a sphygmomanometer (Hawksley and Sons, Lancing-West Sussex, UK). Korotokoff sound 1 was used for systolic and sound 5 for diastolic. Blood pressure was recorded for each subject by the same observer throughout the study, at 0700 h, while in supine position. Pubertal development was evaluated by self-assessment of breast and pubic hair stage according to Tanner (12). Blood was collected at the end of wk 3 of each diet period for analysis of PRA, aldosterone, and serum Na+. Blood samples were drawn at 0700 h after subjects had been recumbent overnight and at 0900 h after they sat for 1 h and stood for 1 h. PRA was measured using a Clinical Assay GammaCoat RIA kit (Baxter Healthcare, Cambridge, MA). Plasma aldosterone concentration was measured by RIA using kits from Diagnostic Products Corporation (Los Angeles, CA). Serum Na+ was measured by Cobas Mira Plus (Roche Diagnostic Systems, Branchburg, NJ).

Subjects were supervised by trained staff to ensure complete fecal and urine collections. Feces and urine were collected in acid-washed containers for 20 d on a daily basis on each study period. The first 24-h urine collection was used to estimate usual Na+ intake. Daily urinary creatinine was measured using an automated colorimetric method (Cobas Mira Systems). Urine was pooled as 24-h samples, which were analyzed daily for creatinine to eliminate days of noncompliance and to normalize to 24-h pools by the following equation

(1)

Fecal samples were also pooled for each 24-h period, and completeness of collections was determined using polyethylene glycol (PEG) by a turbidimetric assay (13). Correction of daily fecal Na+ was accomplished by measuring the amount of PEG that appeared in each fecal sample by the following equation

(2)

Balance was determined daily by the following equation

(3)

Whole-body sweat was collected after 2 wk of acclimation and adaptation to the diet for 24-h by a whole-body scrub-down procedure during each session as previously described (14). Sweat was included in the balance calculation from the mean retention of the last 2 wk as: average daily Na+ balance – 24 h whole-body sweat Na+.

Dietary, urine, fecal, and sweat Na+ were measured by atomic absorption spectrophotometry (5100 PC; Perkin-Elmer, Shelton, CT).

Statistical analysis

Student t test was used to assess baseline differences between the white and black girls. A mixed-model ANOVA was used to assess the effects of treatment and race using race, session, Na+ treatment, subject, order, and response in the model. Paired t tests were used to assess whether the slope of urinary Na+ excretion over time was different from zero, which reflects adaptation to the dietary treatments. Changes in weight and blood pressure during the study were also analyzed by repeated-measures ANOVA. Pearson correlation coefficients were obtained to describe the relationships of Na+ retention and general characteristics. The data are expressed as mean ± SEM. Statistical significance was set at P < 0.05. Microsoft Excel for Windows 2000 and the Statistical Analysis System (SAS Institute, Cary, NC) program were used for all the statistical analyses.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Fifteen white girls and 25 black girls were recruited for the study. Eight whites and 15 blacks completed both sessions of the study, and an additional six whites and seven blacks completed one of the two sessions. Three girls chose to withdraw during the initial study period. One girl was withdrawn from the study due to iron deficiency anemia. All girls that completed at least one of the sessions of the study were included in the analysis. Data from six black girls were not used in fecal or balance analysis due to poor PEG recovery. Black and white girls had similar baseline characteristics and were normotensive (Table 1Go).


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  		TABLE 1. Baseline subject characteristics1

 
Na+ content of the controlled diets by analysis was 1310 ± 43 mg/d (57 ± 1.9 mmol/d) for the low-Na+ diet and 3945 ± 47 mg/d (172 ± 2 mmol/d) for the high-Na+ diet. The subjects’ usual Na+ intake was estimated at baseline by two methods, 6-d dietary records and 24-h urine collections. Similar estimates were recorded from both methods and between blacks and whites by both methods (Table 1Go).

Mean urinary Na+ excretion for wk 2 and 3 is shown in Table 2Go. There was a significant race by dietary treatment interaction, in which urinary Na+ excretion was similar at low Na+ intake between blacks and whites but significantly lower in blacks compared with whites at high-Na+ intake. There were no differences in creatinine excretion or urinary volume between blacks and whites (Table 2Go). Results were similar if urinary Na+ values were corrected for creatinine and if only subjects completing both dietary treatments were included in the model.


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  		TABLE 2. Daily urinary, fecal, and sweat Na+ excretion; Na+ retention; and apparent Na+ absorption in black and white adolescents1

 
Daily urinary Na+ excretion is shown in Figs. 1Go and 2Go. By the second day on the low-Na+ test diet (Fig. 1Go), both blacks and whites had a decrease in Na+ excretion from baseline, which was significantly lower in the black compared with the white girls. Urinary Na+ excretion matched intake on d 2 in white girls, but it decreased on d 3 forward and remained lower compared with dietary intake. In blacks, urinary Na+ excretion decreased from baseline until d 4, which was lower compared with dietary intake throughout the study, and it was significantly lower than whites on several days. On the high-Na+ diet (Fig. 2Go), whites showed an increase in urinary Na+ excretion until d 4 but remained lower compared with dietary intake throughout the study. Blacks also showed an increase in urinary Na+ excretion on the high-Na+ intake until d 6, but it remained lower compared with dietary intake and compared with whites on most days.



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  		FIG. 1. Daily urinary Na+ excretion, in milligrams per day, in black and white girls from the beginning (d 1) to the end of the study (d 19) on the low-Na+ test diet. Clear bars, Whites; dark bars, blacks. Dietary Na+ intake is shown in the continuous line. To convert to millimoles of Na+, divide by 23 and multiply by 1000. *, Significantly lower in blacks compared with whites at P < 0.05. Mean ± SEM.

 


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  		FIG. 2. Daily urinary Na+ excretion (milligrams per day) in black and white girls from the beginning (d 1) to the end of the study (d 19) on the high-Na+ test diet. Clear bars, Whites; dark bars, blacks. Dietary Na+ intake is shown in the continuous line. To convert to millimoles of Na+, divide by 23 and multiply by 1000. *, Significantly lower in blacks compared with whites at P < 0.05. Mean ± SEM.

 
Fecal Na+ excretion was significantly higher in blacks compared with whites while consuming the high-Na+ diet but similar while on the low-Na+ diet (Table 2Go). However, fecal Na+ excretion represents only 10% and 6% of the low-Na+ diet and 5% and 2.4% of the high-Na+ diet in blacks and whites, respectively. Twenty-four-hour whole-body sweat Na+, measured after 2 wk of adaptation and acclimatization, was similar between the black and white girls as previously reported (14) (Table 2Go), and it was significantly higher on the high, compared with the low, Na+ diet (P < 0.05). Sweat Na+ loss accounted for only 7% and 3% of the low- and high-Na+ diets, respectively.

Daily mean Na+ retention (calculated using corrected urine and fecal Na+ values) for the last 2 wk of the study is shown in Table 2Go. There was a significant race-by-dietary-treatment interaction, in which Na+ retention was similar at low Na+ intake between blacks and whites, but significantly greater in blacks compared with whites at high-Na+ intake. Results were similar with the data uncorrected for PEG recovery and creatinine excretion and when only subjects completing both dietary treatments were included in the model. Individual data for the subjects that completed both dietary treatments are shown in Fig. 3Go. Cumulative Na+ retention for the last 2 wk of the study was 5.52 ± 0.99 g (240 ± 43 mmol) in whites and 4.76 ± 0.76 (207 ± 33 mmol) in blacks on the low-Na+ diet, and 7.36 ± 2.07 g (320 ± 90 mmol) in whites and 14.72 ± 1.91 g (640 ± 83 mmol) in blacks on the high-Na+ diet (P < 0.05). The difference between cumulative Na+ retention between the high- and the low-Na+ diet (among those subjects who completed both treatment periods) was 1.84 ± 2.19 g (80 ± 95 mmol) in whites and 9.96 ± 1.86 g (433 ± 81 mmol) in blacks (P < 0.05).



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  		FIG. 3. Mean Na+ retention for the last 2 wk of the study (d 7–19) as affected by Na+ intake. Upper, White girls; lower, black girls. Only the girls completing both dietary treatments are shown. To convert to millimoles of Na+, divide by 23 and multiply by 1000.

 
Figures 4Go and 5Go display the plasma aldosterone and PRA responses at the end of both the low-salt and high-salt protocols. As expected, the upright posture stimulated levels of plasma aldosterone and PRA, but the increase for both was less in the blacks while consuming the higher salt intake (P < 0.05).



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  		FIG. 4. Plasma aldosterone levels (nanograms per deciliter) in black and white girls at the end of the study (d 19). Blood samples were drawn at 0700 h after subjects had been recumbent overnight and at 0900 h after they had been upright for 2 h. Data cross-points with the same letter, Not significantly different at P < 0.05. Mean ± SEM.

 


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  		FIG. 5. PRA (nanograms per milliliter per hour) in black and white girls at the end of the study (d 19). Blood samples were drawn at 0700 h after subjects had been recumbent overnight and at 0900 h after they had been upright for 2 h. Data cross-points with the same letter, Not significantly different at P < 0.05. Mean ± SEM.

 
Blood pressure and body weight decreased from baseline to the end of the study (Table 3Go). To investigate the possibility of racial differences in energy intake, the extra calories consumed daily by subjects from the sweet products allowed during the study were estimated from their daily record of intakes. On average for the last 2 wk of the study, these contributed 1314 ± 163 joules (314 ± 39 kcal) and 1439 ± 234 joules (344 ± 56 kcal) in blacks consuming the low- and high-Na+ diet, respectively, and 908 ± 121 joules (217 ± 29 kcal) and 1155 ± 109 joules (276 ± 36 kcal) in whites consuming the low- and high-Na+ diet, respectively. No increase was observed in serum Na+ (data not shown).


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  		TABLE 3. Changes in weight and blood pressure during the study1

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
In a rigorously controlled metabolic study in black and white female adolescents, urinary Na+ excretion was lower in the blacks compared with the whites, especially while consuming a high-salt diet. After 1 wk of equilibration, there was on average 0.7 g/d more Na+ retained in the blacks than in the whites. Differences in fecal and sweat Na+ excretion did not account for the racial difference. The results provide direct evidence for greater retention of Na+ in black adolescent females than white adolescent females.

In previous studies of adults, Luft et al. (15, 16) showed that blacks had less urinary Na+ excretion than whites after a saline infusion. Na+ kinetic studies in men showed that the Na+ half-life was longer in blacks than whites under conditions of high salt intake (5.81 vs. 2.88 d) (17, 18). Earlier estimates of Na+ excretion in children and young adults identified no racial differences, but none controlled for intake of Na+ (6, 19).

The difference of 0.66 g/d of Na+ retention (1.65 g NaCl) between blacks and whites while consuming the high-Na+ diet would be expected to result in approximately 0.2 liters/d expansion of the extracellular volume and a gain in weight of about 0.2 kg/d (3.9 kg over 19 d) more in the blacks than in whites if Na+ was indeed accumulating in the extracellular fluid (ECF) compartment. However, body weight did not increase, nor did blood pressure. The levels of upright PRA and plasma aldosterone were lower in the blacks after consuming the higher salt intake. This was consistent with, not surprisingly, some additional ECF Na+ retention but apparently not enough to bring about a detectable change in weight or blood pressure. In adults, cardiac output and stroke volume increased with increased salt, which was accompanied by a 9% increase in plasma volume intake, and the effect was more pronounced in the seated than supine position (20). Sodium retention, in the absence of weight gain or increase in blood pressure, even on low Na+ intake, was an unanticipated result. Heer et al. (21) also found accumulation of total body Na+ without changes in the extracellular volume or in body weight when subjects gradually increased their dietary Na+ from a normal to a high-Na+ diet in white men. With respect to race, Aloia et al. (22) used delayed {gamma}-neutron activation analysis to show that black women retained significantly more Na+ than white women, which was accompanied by an increase in nonexchangeable Na+ content of bone. The decrease in excreted Na+ that is not explainable by an accumulation extracellularly (and not accounted for by greater excretion in feces or sweat) raises the question: where did the Na+ reside? We suggest that it may have become deposited in bone. Adolescence is a period of rapid bone growth with active accumulation of calcium, and we suggest that the same might occur with respect to Na+. Calcium excretion is less in blacks than whites, a difference that is not explained by the diet (23), thus further implicating bone as a potential reservoir for the retained Na+ in blacks.

Possibly the increased renal reabsorption of Na+ and calcium in blacks is linked to a common pathway. From previous kinetic studies performed in our laboratory, we found that black girls have higher bone formation and resorption rates than white girls (23). With a higher rate of bone turnover in black girls, it is conceivable that mineral ions other than calcium accumulate in bone in greater quantities in black girls. The pattern of bone accretion for Na+ likely is similar to that of calcium, which is accelerated for at least 2 yr and completed when peak bone mass is achieved. Thus, the high levels of Na+ retention observed here, even on low-salt diets without changes in ECF, may occur only during the relatively brief period of pubertal growth. Bone contains half of the Na+ in the body, entering by a hydration-shell diffusion process (25); bone water provides a sizeable pool of exchangeable Na+ (26). In animal models, bone Na+ is altered with Na+ depletion and with salt loading, especially in younger animals, in a variety of species (27, 28, 29). Bone might serve as an important reservoir for Na+ under conditions of prolonged lack of available Na+ for consumption.

Alternatively, it has been proposed that skin can serve as a reservoir for osmotically inactive Na+ as excessive dietary NaCl, resulting in increased accumulation in fertile rats but not ovariectomized rats (30). We did observe higher Na+ in sweat and exfoliated skin collected over 24 h on high-salt diets but no racial differences, in contrast to the large racial difference in Na+ retention.

In summary, we found greater Na+ retention in black girls, compared with white girls, under conditions where Na+ intake was high. The findings provide direct evidence for what appears to be more active mechanisms for Na+ retention in blacks.


   Footnotes
 
This work was supported by National Institutes of Health Grants HD 36609 (to C.M.W.), R01-HL-35795 and RO1-HL67360 (to J.H.P.), and M01-RR00750.

Abbreviations: DASH, Dietary Approaches to Stop Hypertension; ECF, extracellular fluid; PEG, polyethylene glycol; PRA, plasma renin activity.

Received August 21, 2003.

Accepted January 8, 2004.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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J. Titze, K. Bauer, M. Schafflhuber, P. Dietsch, R. Lang, K. H. Schwind, F. C. Luft, K.-U. Eckardt, and K. F. Hilgers
Internal sodium balance in DOCA-salt rats: a body composition study
Am J Physiol Renal Physiol, October 1, 2005; 289(4): F793 - F802.
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