Vitamin D intake and incidence of multiple sclerosis

Background: A protective effect of vitamin D on risk of multiple sclerosis (MS) has been proposed, but no prospective studies have addressed this hypothesis. Methods: Dietary vitamin D intake was examined directly in relation to risk of MS in two large cohorts of women: the Nurses’ Health Study (NHS; 92,253 women followed from 1980 to 2000) and Nurses’ Health Study II (NHS II; 95,310 women followed from 1991 to 2001). Diet was assessed at baseline and updated every 4 years thereafter. During the follow-up, 173 cases of MS with onset of symptoms after baseline were confirmed. Results: The pooled age-adjusted relative risk (RR) comparing women in the highest quintile of total vitamin D intake at baseline with those in the lowest was 0.67 (95% CI 0.40 to 1.12; p for trend 0.03). Intake of vitamin D from supplements was also inversely associated with risk of MS; the RR comparing women with intake of 400 IU/day with women with no supplemental vitamin D intake was 0.59 (95% CI 0.38 to 0.91; p for trend 0.006). No association was found between vitamin D from food and MS incidence. Conclusion: These results support a protective effect of vitamin D intake on risk of developing MS. NEUROLOGY 2004;62:60–65 The incidence of multiple sclerosis (MS) is low in the tropics and increases with distance from the equator in both hemispheres.1 One hypothesis is that sunlight exposure and the resulting increase in vitamin D may exert a protective effect.2-5 During the winter at high latitudes, ultraviolet sunlight is too low to produce adequate amounts of vitamin D3, and vitamin D insufficiency lasting 4 to 6 months of the year at latitudes of 42° is common in individuals with low vitamin D intake.6,7 Vitamin D has strong immunoregulatory effects,8-10 and vitamin D supplementation prevents experimental autoimmune encephalomyelitis (EAE), an autoimmune disease in animals that is used as a model of MS.11 Studies on vitamin D and MS have found that individuals with MS tend to have insufficient vitamin D levels12,13 and that periods of low vitamin D precede the occurrence of high lesion activity, whereas periods of high vitamin D precede low lesion activity, as detected by MRI.14,15 There are, however, no prospective studies relating vitamin D to risk of developing MS. Therefore, we used the data from two large prospective cohorts to examine whether or not high vitamin D intake reduces the risk of MS. Methods. Study population. The study population comprised women participating in two prospective studies of female registered nurses living in the USA: the Nurses’ Health Study (NHS) and the Nurses’ Health Study II (NHS II). The NHS was established in 1976 and recruited 121,700 nurses aged 30 to 55 years; the NHS II was established in 1989 and recruited 116,671 nurses aged 25 to 42 years. As the first dietary assessment was conducted in 1980 in the NHS and in 1991 in the NHS II, the dates of return of the 1980 and 1991 questionnaires were chosen as the baseline (beginning of the follow-up). Women with incomplete baseline food frequency questionnaires or implausible caloric intakes ( 500 or 3,500 kcal/day in NHS, 800 or 4,200 kcal/day in NHS II) were excluded from the analyses. Further, as the occurrence of neurologic symptoms may have caused changes in diet or use of vitamin supplements, we excluded women whose symptoms of MS started before baseline. These exclusions left a total of 92,253 women for the analyses in the NHS and 95,310 in the NHS II. Ascertainment of MS. Newly diagnosed cases of MS were identified by self-report on biennial questionnaires sent to all participants and confirmed by asking the treating neurologist to complete a questionnaire on the certainty of the diagnosis (definite, probable, possible, not MS), clinical history (including date of MS diagnosis and date of the first symptoms of MS), and laboratory tests. If a neurologist was not involved or did not respond, we sent the questionnaire to the patient’s internist.16 In 90% of women with MS, the treating physician was a neurologist, and the diagnosis was supported by positive MRI findings in 76% (NHS) and 89% (NHS II) of the cases. The higher percentage of MRI in the NHS II reflects the higher proportion of cases with recent onset, as in both cohorts 89% of the cases diagnosed after 1990 had an MRI-supported diagnosis. For purposes of the investigation, we confirmed as cases women with a diagnosis of definite or probable MS according to their neurologist or physician; the validity of this approach has been previously reported.16 The sensitivity of the results to diagnostic errors was examined by restricting the analyses to definite MS cases. We documented 76 cases of MS (53 definite and 23 probable) in the NHS and 97 cases (76 definite and 21 probable) in the NHS II with onset of symptoms after baseline. Assessment of vitamin D intake. Participants in the NHS completed comprehensive semiquantitative food frequency questionnaires in 1980, 1984, 1986, 1990, and 1994 and those in the NHS II cohort in 1991 and 1995. The baseline 1980 questionnaire From the Departments of Nutrition (Drs. Willett and Ascherio, K.L. Munger and E. O’Reilly) and Epidemiology (Drs. Zhang, Hernán, Willett, and Ascherio), Harvard School of Public Health, and Division of Preventive Medicine (Dr. Zhang) and Channing Laboratory (Drs. Zhang, Willett, and Ascherio), Department of Medicine, Harvard Medical School and Brigham and Women’s Hospital, Boston, MA; and Multiple Sclerosis Center (Dr. Olek), Department of Neurology, University of California at Irvine. Supported by NIH grants CA87969, CA50385, and NS35624. Received July 22, 2003. Accepted in final form September 17, 2003. Address correspondence and reprint requests to Ms. K.L. Munger, Department of Nutrition, Harvard School of Public Health, 665 Huntington Ave., Boston, MA 02115; e-mail: kgorham@hsph.harvard.edu 60 Copyright © 2004 by AAN Enterprises, Inc. in the NHS cohort included 61 items; subsequent questionnaires were expanded to approximately 130 items. These questionnaires have been described in detail, and the validity and reproducibility of food and nutrient intakes have been previously documented.17,18 The food items that mostly contributed to vitamin D intake were skim/low-fat milk (38 to 40% of vitamin D from foods) and fish (10 to 12%). The correlations between intakes estimated from the food frequency questionnaire and those from four 1-week diet records were 0.81 for skim milk and 0.66 for fish.17,18 The information on current use, brand, and dosage of multivitamin supplements was collected on each biennial questionnaire. Current use of individual vitamin D supplements was also collected biennially; the dose of vitamin D supplements was not specified in the questionnaire and was assumed to be 400 IU (10 g). The validity of vitamin D intake was assessed by comparing it with the plasma concentrations of 25-hydroxyvitamin D (25[OH]D) among 323 healthy NHS women.19 The mean 25(OH)D was 22.0 ng/mL among women in the bottom quintile of vitamin D intake and 30.1 ng/mL among women in the top quintile; for plasma collected between January and April, the corresponding values were 15.9 and 27.9 ng/mL. Validity of estimated vitamin D intake in the NHS is further supported by its inverse association with risk of hip fractures.19 Statistical analyses. Each participant contributed persontime of follow-up from the date of return of the first food frequency questionnaire (1980 in NHS and 1991 in NHS II) to the date at onset of the first symptoms of MS, death from any cause, or end of follow-up, whichever came first. The end of follow-up was May 31, 1998, in NHS and May 31, 1999, in NHS II. Separate analyses were conducted within each cohort. For the main analyses, women were categorized by quintiles of intake of total (from foods and supplements) energy-adjusted vitamin D at baseline; adjustment for total energy intake was achieved using the residuals of the regression of vitamin D intake on total caloric intake.20 For each quintile of vitamin D intake, we estimated the incidence rate by dividing the number of MS cases by the number of person-years of follow-up. Relative risks (RR) were calculated by dividing the incidence rate in each quintile by the corresponding rate in the lowest quintile, which was used as the reference category. Similar analyses were conducted to examine the separate effects of vitamin D from foods or from supplements. In these analyses, intake of vitamin D from supplements (from either multivitamin or specific vitamin D supplements) was categorized in three groups: none, 400 IU/day, and 400 IU/day. In separate analyses that incorporated the repeated dietary measurements, the incidence of MS was related to the cumulative average of vitamin D intake from all the available dietary questionnaires up to the start of each 2-year follow-up interval.21 Cox proportional hazards models were used to adjust the RR estimates simultaneously for risk factors for MS, including pack-years of smoking22 and latitude at birth (north, middle, or south).16 The two cohorts were combined, and pooled RR were estimated using Cox’s proportional hazards models stratified by age (5-year age groups) and cohort. Tests for trend were conducted by using the median values of quintiles of vitamin D intake or of categories of vitamin D supplement intake as a continuous variable. Heterogeneity of RR estimates from the two cohorts was tested using a Wald test, where the squared difference between the log RR was divided by the sum of the variances of each of the log RR. All p values are two tailed. Results. Women in the top quintile of total vitamin D intake at baseline were less likely to have ever smoked and more likely to be current users at baseline of multivitamin supplements than women in the bottom quintile (table 1). Almost all the women in the top quintile of total vitamin D intake at baseline were multivitamin users as compared with 10% of women i