Table 1 presents the sociodemographic and medical weighted characteristics of the 2818 participants. Among these participants, 51.78% were male, 15.27% were Mexican American, 7.45% were Other Hispanic, 54.63% were Non-Hispanic White, and 12.72% were Non-Hispanic Black. The weighted mean age was 15.45 (2.25) years. Age, race/ethnicity, educational level, moderate activities, income to poverty ratio, total protein, total cholesterol, serum phosphorus, and total BMD were all significantly different across the different quartiles of blood mercury levels (P < 0.05).
Table 1
Weighted characteristics of population based on quartiles of blood mercury level
Blood mercury (nmol/L)
|
All
|
Q1 (≤ 0.90) (n = 146)
|
Q2 (1.00-1.85) (n = 1216)
|
Q3 (1.90–3.50) (n = 749)
|
Q4 (≥ 3.54) (n = 707)
|
P-value
|
Age, mean (SD), years
|
15.45 (2.25)
|
14.95 (1.93)
|
15.18 (2.21)
|
15.70 (2.29)
|
15.91 (2.26)
|
< 0.0001
|
Sex, No. (%)
|
|
|
|
|
|
0.7749
|
Male
|
1458 (51.78)
|
85 (52.03)
|
621 (52.07)
|
387 (50.16)
|
365 (52.95)
|
Female
|
1360 (48.22)
|
61 (47.97)
|
595 (47.93)
|
362 (49.84)
|
342 (47.05)
|
Race/ethnicity, No. (%)
|
|
|
|
|
|
< 0.0001
|
Mexican American
|
574 (15.27)
|
34 (13.97)
|
287 (16.28)
|
151 (16.10)
|
102 (12.48)
|
Other Hispanic
|
291 (7.45)
|
8 (2.57)
|
126 (7.09)
|
84 (8.89)
|
73 (8.21)
|
Non-Hispanic White
|
767 (54.63)
|
56 (69.39)
|
405 (59.04)
|
189 (50.00)
|
117 (45.25)
|
Non-Hispanic Black
|
660 (12.72)
|
31 (7.74)
|
244 (10.48)
|
227 (17.62)
|
158 (13.67)
|
Other Race/Ethnicity
|
526 (9.94)
|
17 (6.34)
|
154 (7.11)
|
98 (7.39)
|
257 (20.40)
|
Education level, No. (%)
|
|
|
|
|
|
< 0.0001
|
Less than HS
|
1213 (42.29)
|
71 (52.82)
|
592 (47.31)
|
301 (37.85)
|
249 (32.76)
|
HS
|
1411 (50.32)
|
69 (41.34)
|
562 (47.10)
|
391 (54.38)
|
389 (55.75)
|
More than HS
|
194 (7.39)
|
6 (5.84)
|
62 (5.59)
|
57 (7.78)
|
69 (11.49)
|
Body mass index, mean (SD), kg/m2
|
23.97 (5.99)
|
24.50 (6.20)
|
23.80 (5.84)
|
24.01 (6.08)
|
24.13 (6.09)
|
0.3864
|
Income to poverty ratio, mean (SD)
|
2.46 (1.60)
|
2.60 (1.45)
|
2.42 (1.55)
|
2.19 (1.60)
|
2.83 (1.71)
|
< 0.0001
|
Moderate activities, No. (%)
|
|
|
|
|
|
< 0.0001
|
Yes
|
1088 (41.19)
|
76 (57.55)
|
418 (38.55)
|
302 (41.72)
|
292 (41.02)
|
No
|
1113 (36.12)
|
67 (40.29)
|
445 (33.09)
|
304 (37.20)
|
297 (40.24)
|
Not recorded
|
617 (22.68)
|
3 (2.16)
|
353 (28.36)
|
143 (21.08)
|
118 (18.74)
|
Total protein, mean (SD), g/dL
|
7.24 (0.40)
|
7.15 (0.38)
|
7.24 (0.40)
|
7.25 (0.39)
|
7.26 (0.43)
|
0.0064
|
Total cholesterol, mean (SD), mg/dL
|
157.78 (29.52)
|
154.13 (29.72)
|
155.42 (27.78)
|
160.08 (29.17)
|
161.72 (32.84)
|
< 0.0001
|
Serum phosphorus, mean (SD), mg/dL
|
4.28 (0.66)
|
4.34 (0.70)
|
4.32 (0.66)
|
4.22 (0.65)
|
4.25 (0.67)
|
0.0048
|
Serum calcium, mean (SD), mg/dL
|
9.59 (0.30)
|
9.62 (0.27)
|
9.58 (0.29)
|
9.60 (0.31)
|
9.58 (0.30)
|
0.2528
|
Total BMD, mean (SD), g/cm2
|
1.03 (0.12)
|
1.02 (0.11)
|
1.02 (0.12)
|
1.04 (0.11)
|
1.05 (0.13)
|
< 0.0001
|
Abbreviations: HS, high school; BMD, bone mineral density. |
SI conversion factors: To convert cholesterol, phosphorus, calcium to mmol/L, multiply values by 0.0259, 0.323, 0.25, respectively; To convert protein to g/L, multiply values by 10. |
Association Of Blood Mercury With Total Bmd
Table 2 showed that blood mercury was positively associated with total BMD in model 1 [0.0017 (0.0005, 0.0028)], but negatively, albeit not significantly, in model 2 and 3 [model 2: -0.0006 (-0.0015, 0.0003); model 3: -0.0006 (-0.0016, 0.0003)]. For different quartiles of blood mercury levels, blood mercury in the Q2, Q3, and Q4 groups were all positively associated with total BMD in model 1, although the Q2 group was not significant; In the variable-adjusted models, blood mercury was inversely associated with total BMD in all other groups except in the Q3 group of model 3 [0.0008 (-0.0144, 0.0159)]. In both model 2 and 3, the trend was not significant [model 2: p for trend = 0.741; model 3: p for trend = 0.620].
Table 2
Association of blood mercury (nmol/L) with total bone mineral density (g/cm2)
|
Model 1, β (95% CI)
|
Model 2, β (95% CI)
|
Model 3, β (95% CI)
|
Blood mercury
|
0.0017 (0.0005, 0.0028)
|
-0.0006 (-0.0015, 0.0003)
|
-0.0006 (-0.0016, 0.0003)
|
Quartiles of blood mercury
|
|
|
|
Lowest quartile
|
Reference
|
Reference
|
Reference
|
Q2
|
0.0031 (-0.0147, 0.0208)
|
-0.0060 (-0.0202, 0.0082)
|
-0.0029 (-0.0171, 0.0114)
|
Q3
|
0.0267 (0.0079, 0.0454)
|
-0.0024 (-0.0175, 0.0127)
|
0.0008 (-0.0144, 0.0159)
|
Q4
|
0.0317 (0.0126, 0.0508)
|
-0.0030 (-0.0185, 0.0124)
|
-0.0002 (-0.0157, 0.0153)
|
P for trend
|
< 0.001
|
0.741
|
0.620
|
Stratified by sex
|
|
|
|
Male
|
0.0018 (0.0002, 0.0033)
|
-0.0006 (-0.0018, 0.0005)
|
-0.0009 (-0.0021, 0.0003)
|
Female
|
0.0011 (-0.0006, 0.0028)
|
-0.0008 (-0.0023, 0.0007)
|
-0.0002 (-0.0017, 0.0013)
|
Stratified by race/ethnicity
|
|
|
|
Mexican American
|
0.0029 (-0.0001, 0.0058)
|
0.0005 (-0.0019, 0.0029)
|
0.0005 (-0.0020, 0.0031)
|
Other Hispanic
|
0.0014 (-0.0017, 0.0044)
|
-0.0004 (-0.0031, 0.0022)
|
-0.0009 (-0.0035, 0.0017)
|
Non-Hispanic White
|
0.0019 (-0.0007, 0.0045)
|
-0.0009 (-0.0029, 0.0012)
|
-0.0012 (-0.0032, 0.0009)
|
Non-Hispanic Black
|
0.0026 (-0.0002, 0.0054)
|
0.0001 (-0.0022, 0.0024)
|
-0.0003 (-0.0024, 0.0019)
|
Other Race/Ethnicity
|
0.0006 (-0.0009, 0.0021)
|
-0.0009 (-0.0022, 0.0003)
|
0.0000 (-0.0013, 0.0013)
|
Model 1, no covariates were adjusted. |
Model 2, sex, age, race/ethnicity were adjusted. |
Model 3, sex, age, race/ethnicity, body mass index, income to poverty ratio, moderate activities, education level, total protein, total cholesterol, serum phosphorus, serum calcium were adjusted. |
In the subgroup analysis stratified by sex or race/ethnicity, the model is not adjusted for the stratification variable itself. |
In analyses stratified by sex, blood mercury was positively associated with total BMD in model 1 of both male and female, however the association was not significant in female [0.0011 (-0.0006, 0.0028)]. The association of blood mercury with total BMD in model 2 and model 3 of male and female adolescents was negative but not significant. In analyses stratified by race/ethnicity, blood mercury was positively associated with total BMD in three models of Mexican American adolescents. But in all three models, the association was not significant [model 1: 0.0029 (-0.0001, 0.0058); model 2: 0.0005 (-0.0019, 0.0029); model 3: 0.0005 (-0.0020, 0.0031)]. The distribution of blood mercury levels for male, female, and Mexican American stratified by sex and race/ethnicity was shown in Online Resource 6–8, respectively.
The analysis results of the complete dataset obtained by multiple imputation differed very little from the analysis results of the original dataset and did not affect the results of this study (Online Resource 5).
In addition, adjusted smooth curve fittings indicated a non-linear association of blood mercury with total BMD after stratification by sex or race/ethnicity. In Fig. 2, the smooth curve trends were different for males and females. In male, total BMD increased with blood mercury levels until an inflection point, showing an inverted U-shaped association (inflection point: blood mercury 5.44 nmol/L) (Table 3), while in female, total BMD decreased with blood mercury until the inflection point (inflection point: blood mercury 11.8 nmol/L) (Table 3). An inflection point was also observed in the smooth curve for Mexican American (inflection point: 5.49 nmol/L) (Fig. 3, Table 3). Taken together, an inverted U-shaped relationship of blood mercury with total BMD was found in male adolescents and Mexican Americans, but a U-shaped relationship in female adolescents.
Table 3
Threshold effect analysis of blood mercury level on total bone mineral density using two-piecewise linear regression model
Total bone mineral density
|
Adjusted β (95% CI)
|
Male
|
|
Blood mercury < 5.44 (nmol/L)
|
0.0038 (0.0001, 0.0075)
|
Blood mercury > 5.44 (nmol/L)
|
-0.0025 (-0.0041, -0.0008)
|
Female
|
|
Blood mercury < 11.8 (nmol/L)
|
-0.0014 (-0.0036, 0.0008)
|
Blood mercury > 11.8 (nmol/L)
|
0.0021 (-0.0013, 0.0056)
|
Mexican American
|
|
Blood mercury < 5.49 (nmol/L)
|
0.0049 (-0.0012, 0.0110)
|
Blood mercury > 5.49 (nmol/L)
|
-0.0016 (-0.0053, 0.0021)
|
Sex, age, race/ethnicity, body mass index, income to poverty ratio, moderate activities, education level, total protein, total cholesterol, serum phosphorus, serum calcium were adjusted. |
In the subgroup analysis for males, females and Mexican American, the model is not adjusted for sex or race/ethnicity, respectively. |