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CARDIOVASCULAR JOURNAL OF AFRICA • Volume 29, No 3, May/June 2018
AFRICA
173
because they had complete 24-hour ambulatory BP (ABP)
and 24-hour urine samples. The minimum age for the study
participants was 18 years, and there was no upper age limit.
A standardised questionnaire was administered to each
participant to obtain demographic data, medical history such
as the presence of hypertension, and use of medication. Height
and weight measurements were recorded with the participants
wearing indoor clothes with no shoes. BMI was calculated as
weight in kilograms divided by the square of height in metres.
Twenty-four-hour ambulatory BP monitoring was
performed using oscillometric monitors (Spacelabs, model
90207). Standard cuffs with an inflatable bladder suitable for
the participant’s arm circumference were used. The monitors
were programmed to measure BP at 15-minute intervals from
06:00 to 22:00 and then at 30-minute intervals from 22:00 to
06:00. Participants kept a diary card for the duration of the
recordings to note the time of going to bed in the evening and
time of waking up in the morning. These times were used to
determine the wake and sleeping periods. Participants also
recorded the time when they took medication, smoked, drank
caffeine or alcohol, depending on which was applicable to
each participant. On completion of the recordings, data were
transferred to a computer for analysis. Ambulatory blood
pressure data were expressed as 24-hour average systolic and
diastolic BP.
Timed urine samples were collected over a 24-hour period.
Each participant was issued with a urine collection bottle and the
bottles were then collected from each participant after 24 hours.
Twenty-four-hour urine Na
+
excretion rate was calculated from
the product of urine volume and urine electrolyte concentration.
Creatinine clearance was determined from the product of urine
volume and urine creatinine concentrations divided by plasma
creatinine concentration.
The quality of urine samples was determined by constructing
regression relations between 24-hour urine creatinine and body
weight, and 24-hour urine volume and age in gender-specific
groups. Based upon the 95% confidence intervals for each group,
a 24-hour urine sample was considered acceptable if 24-hour
urine creatinine (mmol) was
>
3.5 and
<
35 for males and
>
3.5
and
<
30 for females. Samples with urine volumes
<
500 ml/day
were also assumed to be incomplete urine collections. These
are standard approaches and have been published on numerous
occasions by other groups.
Statistical analysis
For statistical analysis the SAS software, version 9.4 (SAS
Institute Inc, Cary, NC), was used. Data are shown as mean
±
SD and
p
<
0.05 was considered significant. To determine the
independent relationship between 24-hour urinary excretion
and 24-hour systolic and diastolic BP, a multivariate regression
analysis was used. Confounders such as age, gender, BMI
(as a continuous variable), alcohol consumption, smoking,
the presence of diabetes and treatment of hypertension were
included in the regression model.
Results
Table 1 gives a description of the demographic, anthropometric,
haemodynamic and general clinical characteristics of the
participants in this study. They were divided into three groups:
total sample, men and women. The characteristics include age,
BMI, 24-hour ambulatory BP, 24-hour urinary Na
+
excretion
rates, alcohol consumption, smoking, diabetes status and
hormone concentrations.
The mean age of the total population sample was 45.3
±
18.5
years. There was no age difference between males and females
(men 45.5
±
19.9 and women 45.1
±
17.7 years) with 63.2% of
participants being women. The mean BMI of the group was
29.1
±
7.8 and 71% of participants were either overweight or
obese. When BMI was calculated according to gender, more
women were in the overweight/obese category (75%) compared
to men (47%). Thirty-five per cent (35%) of the total sample
population was hypertensive, 23.0% consumed alcohol regularly,
15% smoked, and 14% had diabetes mellitus or an HbA
1c
>
6.1%.
There was no significant difference in urinary sodium
excretion in the total sample and in men; however the overweight/
obese women had significantly lower urinary sodium excretion
rates. Both insulin and leptin levels were significantly higher
in the overweight/obese individuals compared to the normal-
weight participants. Gender differences were observed in leptin
concentrations. Compared to men, leptin concentrations were
significantly higher in women. Table 2 gives the haemodynamic
characteristics of the population. Both systolic and diastolic
BP values were higher in the overweight/obese participants
compared to the normal-weight participants.
Table 3 shows the relationshipbetween 24-hour urinary sodium
excretion and 24-hour systolic and diastolic BP. There was no
Table 1. General and clinical characteristics of the study population according to gender and BMI status
Total sample
Men
Women
Parameters
All
Normal
a
Overweight
b
/
Obese
c
All
Normal
Overweight/
Obese
All
Normal
Overweight/
Obese
Age (years)
45.3
±
18.5 36.3
±
18.3 50.4
±
15.2 45.5
±
19.9 38.2
±
18.9 53.5
±
18.0 45.1
±
17.7 33.7
±
17.4 48.9
±
16.1
BMI (kg/m
2
)
29.1
±
7.8
21.6
±
2.1
34.4
±
1.4
24.9
±
5.0
21.2
±
2.0
29.1
±
4.1
31.5
±
8.1
21.9
±
2.2
34.6
±
6.7
Hypertensive (%)
35.1
17.6
43.8
24.4
19.8
30.2
39.3
14.9
47.5
Diabetic (%)
14.3
7.2
16.9
12.9
9.4
16.8
13.9
4.6
16.9
Alcohol intake (%)
23.6
33.1
18.0
41.8
46.7
36.8
12.7
17.2
11.1
Smokers (%)
15.2
26.4
9.1
33.8
43.4
23.0
4.3
5.7
3.9
Na
+
(mmol/day)
105.6
±
78.4 108.9
±
89.8 103.8
±
72.1 106.5
±
74.7 115.7
±
71.1 98.5
±
77.9 105.0
±
80.55 121.5
±
99.4
99.5
±
68.6*
HbA
1c
(%)
6.2
±
1.5
5.8
±
1.0
6.3
±
1.6
6.1
±
1.7
5.8
±
1.4
6.5
±
1.9
6.2
±
1.3
5.7
±
0.3
6.3
±
1.5
Renin (ng/dl)
36.4
±
73.8 35.9
±
75.8 34.8
±
35.7 35.2
±
52.2 33.1
±
51.6 37.5
±
53.0 37.0
±
84.1 39.5
±
98.4 31.0
±
5.0
Insulin (mmol/l)
14.4
±
16.6 10.8
±
13.8 16.8
±
18.9* 13.8
±
19.3
9.8
±
13.3 18.9
±
23.1* 14.8
±
14.8 13.1
±
14.1 15.5
±
15.0
Leptin (ng/ml)
24.2
±
25.3
9.9
±
12.4 43.0
±
29.4* 8.5
±
8.7
4.3
±
6.9
12.2
±
8.4* 35.0
±
27.2 17.6
±
13.9 40.5
±
28.1*
HbA
1c
, glycated haemoglobin; BMI, body mass index; Na
+
, 24-hour urinary sodium excretion rate.
a
Normal BMI is defined as
<
25 kg/m
2
,
b
overweight is defined as BMI ≥ 25
<
30 kg/m
2
and
c
obese is defined as BMI ≥ 30 kg/m
2
.