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Background/Purpose: Dietary vitamin D intake has been studied as a risk factor for incidence of RA in several longitudinal cohort studies; however, only few have evaluated sunlight exposure, the primary source of vitamin D, and the risk of RA.
Methods: Using the Women's Health Initiative Observational Study (WHI-OS), we examined the association between vitamin D exposure on the prevalence and incidence of rheumatoid arthritis (RA). Prevalent RA cases were identified using the combination of self-report and medications at baseline. Incident cases were ascertained using year three data, and the comparison group consisted of women who did not report RA at either time point. Vitamin D exposure was identified in three ways: 1) vitamin D intake from food frequency questionnaire and supplementation sources, 2) previous sunlight exposure, and 3) current sunlight exposure. Previous sunlight exposure was ascertained via questions regarding time spent outdoors during summer and non-summer months as a child, in their teens, and in their thirties. Respondents answering <30 minutes were given a score of 15; respondents answering 30 minutes to 2 hours were given a score of 75, and those responding ≥2 hours were given a score of 180. The scores at each period were summed to create cumulative minutes of previous sunlight exposure. Mean solar irradiance, calculated using the clinical center location and national weather data, was used to ascertain current sunlight exposure. The three exposure variables were categorized and after controlling for several variables, the association with prevalent RA was tested using logistic regression and the association with the incidence of RA was tested using Cox-proportional hazards models.
Results: The study population included 76,743 women without RA, 727 prevalent RA cases, and 190 incident RA cases. Women with RA were significantly older, heavier, and had a larger proportion of African Americans and current smokers than the comparison group. After adjustment, no association between vitamin D exposure and the prevalence of RA was found in the total population; however different trends were observed by race (Table). Though not statistically significant in either group, compared to the highest category of summer sunlight exposure (>500 minutes), White women with the lowest exposure (<300 minutes) were 15% less likely to have prevalent RA [OR (95% CI): 0.85 (0.65, 1.10)] whereas African American women in the lowest category were 17% more likely [1.17 (0.60, 2.28)]. Similar differences in racial trends were found in the other exposure variables. No significant association between vitamin D exposure and the incidence of RA was found.
Conclusions: Self-reported estimates of lifetime and current vitamin D exposure were not associated with incidence or prevalence of RA in the WHI-OS; however, results suggest that race may modify the association between vitamin D exposure and RA.
Table. The Association between Vitamin D Intake, Sunlight Exposure and the Prevalence of Rheumatoid Arthritis by Race | |||||
| White |
| African American | ||
| N/# cases | OR (95% CI)* |
| N/# Cases | OR (95% CI)* |
Vitamin D Intake (IU/day) |
|
|
|
|
|
≥600 <600-400 <400-200 <200 | 13,674/ 114 15,683/ 143 10,527/ 76 18,440/ 125 | 1.00 1.08 (0.85, 1.39) 0.85 (0.64, 1.14) 0.75 (0.58, 0.97) |
| 358/ 4 849/ 16 589/ 8 2,084/ 35 | 1.00 1.64 (0.54, 4.96) 1.17 (0.35, 3.91) 1.32 (0.47, 3.75) |
Sunlight Exposure – summer (mins) |
|
|
|
|
|
>500 500-400 400-300 <300 | 15,028/ 126 17,574/ 143 12,100/ 91 14,124/ 101 | 1.00 0.95 (0.75, 1.21) 0.89 (0.68, 1.17) 0.85 (0.65, 1.10) |
| 1,073/ 17 1,308/ 17 750/ 14 1,030/ 18 | 1.00 0.87 (0.44, 1.71) 1.22 (0.59, 2.49) 1.17 (0.60, 2.28) |
Sunlight Exposure – other seasons (mins) |
|
|
|
|
|
>400 400-300 300-200 <200 | 15,722/ 149 8,112/ 58 20,941/ 151 13,888/ 97 | 1.00 0.78 (0.57, 1.05) 0.81 (0.64, 1.01) 0.76 (0.59, 0.99) |
| 1,471/ 23 551/ 11 1,217/ 17 884/ 14 | 1.00 1.34 (0.65, 2.78) 0.96 (0.51, 1.80) 1.01 (0.52, 1.97) |
Solar Irradiance (g-cal/cm2) |
|
|
|
|
|
500-475 430-400 380-375 350 325-300 | 11,771/ 85 9,550/ 80 6,745/ 50 12,743/ 77 18,935/ 175 | 1.00 1.18 (0.86, 1.62) 1.05 (0.72, 1.52) 0.80 (0.58, 1.10) 1.28 (0.97, 1.68) |
| 404/ 10 959/ 17 1,101/ 12 1,321/ 19 538/ 12 | 1.00 0.69 (0.31, 1.53) 0.43 (0.18, 1.00) 0.54 (0.25, 1.18) 0.88 (0.38, 2.08) |
*Adjusted for age, BMI, region at birth, education, smoking, alcohol, weight at birth wearing hat outdoors during thirties, and total recreational physical activity | |||||
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Disclosure: N. C. Wright, None; B. T. Walitt, Jazz Pharmaceuticals, 5 ; J. R. Curtis, Merck Pharmaceuticals, 5, Amgen, 5, Eli Lilly and Company, 5, Merck Pharmaceuticals, 2, Roche Pharmaceuticals, 5, Novartis Pharmaceutical Corporation, 2, Proctor and Gamble, 2, Novartis Pharmaceutical Corporation, 5, Eli Lilly and Company, 2 ; M. Pettinger, None; C. G. Parks, None; A. J. De Roos, None; J. Wactawksi-Wende, None; R. Mackey, None; R. D. Jackson, None; M. L. Melamed, None; B. V. Howard, None.