Characteristics of the study participants
A total of 3600 respondents were selected for a spot urine sample collection. Ultimately, 87 samples were excluded due to incomplete collections (42 no urinary samples and 45 insufficient urinary samples), with 3513 samples (1750 from men and 1763 from women) remaining for the final analysis. The characteristics of the respondents are listed in Table 1.
Table 1. Characteristics of the study participants
Variables
|
Pooled
|
Age (years)a
|
41.3±10.7
|
BMI (kg/m2)a
|
24.7±5.3
|
SIC (mg/kg)c
|
24.1(22.6, 25.4)
|
CRIS n(%)b
|
3144(89.5)
|
CRQIS n(%)b
|
3079(87.6)
|
UCC (g/L)c
|
1.1(0.6,1.6)
|
UIC (µg/L)c
|
132.0(77.8, 203.3)
|
DII (µg/d)c
|
195.7(132.6, 295.8)
|
UNaC (g/L)c
|
2.7(1.7, 3.8)
|
DNaI (g/d)c
|
4.0(2.6, 5.9)
|
DSI (g/d)c
|
10.2 (6.7, 14.9)
|
CRIS, Coverage rate of iodized salt; CRQIS, consumption rate of qualified iodized salt; DII, daily iodine intake; DNaI, daily sodium intake; DSI, daily salt intake; SIC, Salt iodine concentration; UIC, urinary iodine concentration; UCC, urinary creatinine concentration; UNaC, urinary sodium concentration; DSI, Daily sodium intake;
a Mean ± standard deviation.
b n (%).
c Median (IQR, Q1–Q3).
Distribution of DII
No significant differences were observed in the DII between the age and sex groups (P > 0.05) (Table 2). Further, the differences were not significant in women of childbearing age compared with women aged 50–59 years (P > 0.05). The highest median DII (236.0 µg/d) was found in the noncoastal rural areas compared with the other three areas. The lowest median DII (179.7 µg/d) was found in the underweight group compared with the other three BMI groups (P < 0.05). The lower median DII (166.3 µg/d) was found in the noniodized salt group compared with the adequately iodized group (P < 0.05).
Table 2. Distribution of DII based on sociodemographic and anthropometric data
Variables
|
N(%)
|
UIC
Median(IQR)
(ug/L)
|
DII
Median (IQR)
(ug/d)
|
Pooled
|
3513(100.0)
|
132.0(77.8, 203.3)
|
195.7(132.6, 295.8)
|
Gender
|
|
|
|
Male
|
1763(50.2)
|
134.8(79.0, 207.9)
|
202.1(134.5, 305.7)
|
Female
|
1750(49.8)
|
129.5(76.4, 196.9)
|
190.9(129.4, 285.0)
|
Female(18-49)
|
1325(61.4)
|
133.7(80.4, 203.6)
|
188.6(129.8, 282.0)
|
Female(50-59)
|
681(38.6 )
|
135.7(75.0, 215.9)
|
193.2(124.6, 287.0)
|
Age (years)
|
|
|
|
18-49
|
2046(58.2)
|
134.5(80.4, 201.9)
|
192.3(133.3, 288.5)
|
50-59
|
1467(41.8)
|
128.1(73.8, 204.1)
|
199.2(132.0, 304.7)
|
BMI
|
|
|
|
Underweight
|
194(5.5)
|
118.1(82.0, 212.4)
|
179.7(121.8, 285.0)a
|
Normal
|
1957(55.7)
|
130.1(74.8, 200.2)
|
197.0(133.8, 299.1)
|
Overweight
|
1078(30.7)
|
137.2(82.2, 207.9)
|
197.3(135.4, 300.9)
|
Obese
|
283(8.1)
|
134.2(74.7, 203.2)
|
184.1(114.0, 270.1)
|
Region
|
|
|
|
Coastal urban area
|
1185(33.7)
|
122.1(73.3, 187.4)
|
189.6(134.4, 273.8)
|
Non-coastal urban area
|
1002(28.5)
|
127.5(75.5, 189.5)
|
182.5(122.0, 289.4)
|
Coastal rural area
|
746(21.2)
|
128.5(72.1, 203.3)
|
199.4(127.2, 293.5)
|
Non-coastal rural area
|
580(16.5)
|
167.5(101.2,241.1)b
|
236.0(159.7, 371.2)b
|
Salt iodine content
|
|
|
|
Non-iodized
|
369(10.5)
|
104.4(63.3, 180.8)c
|
166.3(112.0, 274.8)c
|
Inadequately iodized
|
47(1.3)
|
128.6(72.2, 159.4)
|
208.1(136.8, 258.1)
|
Adequately iodized
|
3079(87.6)
|
134.6(79.7, 206.7)
|
199.0(135.1, 300.7)
|
Excessively iodized
|
18(0.5)
|
144.6(69.6, 244.8)
|
187.9(136.7, 386.0)
|
DII, Daily iodine intake; Female (18–49), women of childbearing age; IQR, interquartile range; UIC, urinary iodine concentration.
aStatistically significant difference in participants in the other three BMI groups (P < 0.01).
bStatistically significant difference in participants who lived in the other three areas (P < 0.01).
cStatistically significant difference in participants consuming noniodized salt (P < 0.01).
Distribution of daily sodium and salt intakes
No significant differences were observed in daily salt intake (DSI) among different sex, BMI, and salt iodine content groups (P > 0.05)(Table 3). The median DSI was lower (10.1 g/d) in adults aged 18–49 years compared with adults aged 49–59 years (P < 0.05). However, the differences were not significant in women of childbearing age compared with women aged 50–59 years (P > 0.05). The highest median daily salt intake (10.6 g/d) was found in the noncoastal rural areas compared with the other three areas (P < 0.05).
Table 3. Distribution of daily sodium and salt intakes based on sociodemographic and anthropometric data
Variables
|
N(%)
|
UNaC
Median (IQR)
(g/L)
|
DSI
|
Median (IQR)
(g/d)
|
n(% )<5g/d
|
Pooled
|
3513(100.0)
|
4.0(2.6, 5.9)
|
10.2(6.7, 14.9)
|
458(13.6)
|
Gender
|
|
|
|
|
Male
|
1750(47.8)
|
4.2(2.7, 6.1)
|
10.6(6.9, 15.5)
|
241(13.7)
|
Female
|
1763(50.2)
|
3.9(2.5, 5.6)
|
9.9(6.5, 14.4)
|
217(12.4)
|
Female(18-49)
|
1325(61.4)
|
3.9(2.6, 5.7)
|
9.9(6.5, 14.4)
|
180(13.6)
|
Female(50-59)
|
681(38.6 )
|
3.8(2.6, 5.6)
|
9.8(6.5, 14.2)
|
93(13.7)
|
Age (years)
|
|
|
|
|
18-49
|
2046(58.2)
|
4.0(2.6, 5.7)a
|
10.1(6.6, 14,6)a
|
276(13.5)
|
50-59
|
1467(41.8)
|
4.1(2.7, 6.0)
|
10.4(6.8, 15.2)
|
182(12.4)
|
BMI
|
|
|
|
|
Underweight
|
194(5.5)
|
3.7(2.5, 5.6)
|
9.5(6.4, 14.4)
|
32(16.5)
|
Normal
|
1957(55.7)
|
4.0(2.6, 5.8)
|
10.1(6.6, 14,8)
|
260(13.3)
|
Overweight
|
1078(30.7)
|
4.2(2.7, 6.0)
|
10.6(6.8, 15.2)
|
134(12.4)
|
Obese
|
283(8.1)
|
4.1(2.8, 5.7)
|
10.4(7.2, 14.5)
|
32(11.3)
|
Region
|
|
|
|
|
Coastal urban area
|
1185(33.7)
|
3.8(2.6, 5.6)b
|
9.8(6.6, 14.2)b
|
168(14.2)
|
Non-coastal urban area
|
1002(28.5)
|
4.1(2.6, 6.1)
|
10.4(6.6, 15.4)
|
143(14.3)
|
Coastal rural area
|
746(21.2)
|
4.1(2.6, 5.9)
|
10.4(6.6, 15.0)
|
92(12.3)
|
Non-coastal rural area
|
580(16.5)
|
4.2(2.9, 6.1)
|
10.6(7.3, 15.4)
|
55(9.5)
|
Salt iodine content
|
|
|
|
|
Non-iodized
|
369(10.5)
|
4.1(2.7, 6.1)
|
10.4(6.7, 15.4)
|
52(14.1)
|
Inadequately iodized
|
47(1.3)
|
4.2(2.6, 6.2)
|
10.6(6.5, 15.7)
|
3(6.4)
|
Adequately iodized
|
3079(87.6)
|
4.0(2.6, 5.8)
|
10.2(6.7, 14.8)
|
401(13.0)
|
Excessively iodized
|
18(0.5)
|
4.2(3.3, 5.5 )
|
10.8(8.5, 14.1)
|
2(11.1)
|
DSI, Daily salt intake; Female (18–49), women of childbearing age; IQR, interquartile range;UNaC, urinary sodium concentration.
aStatistically significant difference in participants aged 45–59 years (P < 0.01).
bStatistically significant difference in participants who lived in the other three areas (P < 0.01).
Factors associated with high salt intake
Based on the DSI, the participants were categorized into two groups: the low–salt intake group (<5 g/d) and the high–salt intake group (≥5 g/d). Multivariate logistic regression analyses were conducted using sex, age, region, and BMI as independent variables and DSI as the dependent variable (Table 4). Compared with people living in a noncoastal rural area, those living in coastal and noncoastal urban areas consumed less salt (Exp(B) = 0.639, 95% CI: 0.462–0.884, P < 0.007; Exp(B) = 0.634, 95% CI: 0.456–0.883, P < 0.007). No significant differences were observed in the other independent variables.
Table 4. Factors associated with high salt intake
Variables
|
N
|
Β
|
Exp(B)
|
95% CI
|
P
|
Gender
|
|
|
|
|
|
Female
|
1763
|
Reference
|
|
|
|
Male
|
1750
|
0.087
|
1.091
|
0.893-1.334
|
0.394
|
Region
|
|
|
|
|
|
Non-coastal rural area
|
580
|
Reference
|
|
|
|
Coastal urban area
|
1185
|
-0.448
|
0.639
|
0.462-0.884
|
0.007
|
Coastal rural area
|
1002
|
-0.455
|
0.634
|
0.456-0.883
|
0.007
|
Non-coastal urban area
|
746
|
-0.286
|
0.751
|
0.527-1.071
|
0.113
|
Age (years)
|
|
|
|
|
|
18–49
|
2046
|
Reference
|
|
|
|
50–59
|
1467
|
0.047
|
1.049
|
0.849-1.295
|
0.659
|
BMI
|
|
|
|
|
|
Underweight
|
194
|
Reference
|
|
|
|
Normal
|
1957
|
0.238
|
1.269
|
0.847-1.901
|
0.247
|
Overweight
|
1078
|
0.297
|
1.346
|
0.878-2.062
|
0.173
|
Obese
|
283
|
0.397
|
1.487
|
0.871-2.537
|
0.146
|
Correlation analysis between DII and salt intake
We also found a positive correlation between iodine intake and salt intake in pooled adults and women of childbearing age (r = 0.423, P < 0.01; r = 0.406, P < 0.01). Table 5 shows the correlation coefficients between iodine and salt intake in each area, including the coastal rural area (r = 0.369, P < 0.01; r = 0.350, P < 0.01).
Table 5. Correlation coefficients of DII and salt intake in different geographical regions
Region
|
women of childbearing age
|
Pooled
|
N
|
r
|
N
|
r
|
Coastal urban area
|
399
|
0.409##
|
1185
|
0.402##
|
Coastal rural area
|
290
|
0.350##
|
1002
|
0.369##
|
Non-coastal urban area
|
237
|
0.434##
|
746
|
0.496##
|
Non-coastal rural area
|
156
|
0.471##
|
580
|
0.464##
|
Pooled
|
1082
|
0.406##
|
3513
|
0.423##
|
r, Correlation coefficient.
##Significantly positive correlations between DII and salt intake in each area as detected by Spearman rank correlation analysis (P < 0.01).
Comparison of DII among women of childbearing age in different DSI groups
The DII was lower (<5 g/d) among women of childbearing age in the low–salt intake group compared with the high–salt intake group (≥5 g/d) (P < 0.05). The median DII among women of childbearing age in the low–salt intake group (<5 g/d) in coastal urban areas, coastal rural areas, noncoastal urban areas, noncoastal rural areas, and pooled adults was 130.3 (82.9–159.2), 124.3 (83.5–180.0), 111.7 (62.4–205.7), 154.7 (108.7–239.7), and 126.1 (83.8–183.6) µg/d, respectively. The coverage of qualified iodized salt among those in the low–salt intake group (<5 g/d) was 87.3%, 78.6%, 83.3 %, 91.3%, and 84.4% in the aforementioned areas, respectively (Fig. 1).