TABLE 3.
Plasma 25-hydroxyvitamin D concentration according to time and maternal characteristics related to endogenous synthesis of vitamin D
| Mixed model P values | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Variable | n | Trimester 1 11.5 ± 0.04 weeks GA | n | Trimester 3 33.1 ± 0.04 weeks GA | n | Delivery 39.3 ± 0.04 weeks GA | Overall mean | Variable | Interaction with time |
| Time (visit) | 1905 | 69.8 ± 0.51 | 1649 | 78.6 ± 0.71 | 1543 | 75.7 ± 0.71 | — | <0.0001 | |
| Self-reported | — | — | — | — | — | — | — | <0.0001 | 0.21 |
| White | 1647 | 71.7 ± 0.5 | 1426 | 80.4 ± 0.7 | 1350 | 77.7 ± 0.7 | 76.3 ± 0.41 | ||
| Nonwhite | 258 | 57.8 ± 1.2 | 223 | 66.9 ± 1.7 | 193 | 61.8 ± 1.8 | 61.9 ± 0.9 | ||
| Skin color2 | — | — | — | — | — | — | — | <0.0001 | 0.03 |
| Light | 1452 | 70.8 ± 0.5 | 1262 | 80.2 ± 0.7 | 1184 | 77.7 ± 0.83,4 | 76.0 ± 0.4 | ||
| Medium | 406 | 68.0 ± 1.1 | 347 | 74.7 ± 1.43 | 323 | 70.1 ± 1.45 | 70.8 ± 0.7 | ||
| Dark | 47 | 52.4 ± 3.13 | 40 | 59.0 ± 3.93 | 36 | 59.9 ± 4.8 | 56.7 ± 2.31 | ||
| Country of birth | — | — | — | — | — | — | — | <0.0001 | 0.18 |
| Canada | 1551 | 71.0 ± 0.5 | 1335 | 80.0 ± 0.7 | 1264 | 77.5 ± 0.7 | 75.9 ± 0.81 | ||
| Elsewhere | 354 | 64.4 ± 1.1 | 314 | 72.4 ± 1.6 | 279 | 67.7 ± 1.6 | 68.0 ± 0.4 | ||
| Time of year6 | — | — | — | — | — | — | — | <0.0001 | 0.13 |
| November 1 to March 31 | 867 | 67.4 ± 0.7 | 647 | 74.7 ± 1.0 | 601 | 71.9 ± 1.1 | 70.9 ± 0.51 | ||
| April 1 to October 31 | 1038 | 71.8 ± 0.6 | 1002 | 81.1 ± 0.8 | 942 | 78.1 ± 0.8 | 76.9 ± 0.4 | ||
| Latitude, n (%)7 | — | — | — | — | — | — | — | 0.41 | 0.0032 |
| <45°N | 1080 | 71.3 ± 0.6 | 946 | 79.2 ± 0.9 | 912 | 75.3 ± 0.94 | 75.1 ± 0.4 | ||
| ≥45°N | 825 | 67.8 ± 0.8 | 703 | 77.7 ± 1.0 | 631 | 76.3 ± 1.18 | 73.5 ± 0.5 | ||
Data are n, mean ± SEM (nmol/L). Each model included the respective maternal characteristic as a fixed effect and time as a fixed repeated measure; recruitment center was a random effect. Differences among groups were tested using Tukey post hoc tests, with Tukey-Kramer adjustment for multiple comparisons. Abbreviation: GA, gestational age.
Main effect P < 0.05 compared with other group(s) for each variable (time or maternal characteristic).
2Skin color was self-reported as natural color of inside of inner upper arm: light (white, fair); medium (olive, light/medium brown); dark (dark brown, black).
3Interaction P < 0.05 compared with other groups(s) within time point.
All pair-wise differences within row.
Values increase from trimester 1 to 3, delivery is not different from trimester 1.
April 1 through October 31 represents the period when solar ultraviolet beta radiation is sufficient for endogenous (dermal) synthesis of vitamin D.
Latitude of recruiting centers <45°N included Vancouver, Toronto, Hamilton, Kingston and Halifax; and ≥45°N included Edmonton, Winnipeg, Sudbury, Ottawa and Montreal.
Values increase from trimester 1 to 3, then stabilize.