Kassandra LMunger, Department of Nutrition, Harvard School of Public Health
Published February 19, 2004
We thank Ebers et al for their interest in our study. We
excluded women with an incomplete baseline food frequency questionnaire.
This exclusion was made a priori following the same rules used in all
previous dietary analyses in these cohorts. Adjustment for multiple
analyses in our study was not necessary as only two measures of vitamin D
intake were considered: the combined amount of vitamin D intake from food
and from supplements. The P for trend was calculated using a proportional
hazards regression model using the median intake values for each category
of vitamin D from food or supplements as a continuous variable. This
method tests the overall null hypothesis that vitamin D intake is
unrelated to risk of MS without any specific assumptions.
In contrast with the results of Vieth et al [4] quoted by Ebers et
al, in a sub-study among 323 healthy women from the NHS cohort, we found
that vitamin D intake at levels below 400 IU does increase 25(OH)D levels.
Average winter plasma levels of 25(OH)D were positively correlated with
levels of vitamin D intake: 40 nmol/L in the lowest quintile of intake
(median=108 IU), 55 nmol/L in the third quintile (median=301 IU), and 70
nmol/L in the highest quintile (median=703 IU). Further, in our cohorts,
the association between vitamin D intake from supplements and risk of MS
was not materially altered by adjustment for physical activity
(unpublished data).
The women in the NHS cohort are older than those in the NHSII cohort
and because of modest overlap between the two cohorts, age-specific
incidence rates are difficult to compare. Further, the increasing use of
MRI may reduce the time between onset of MS and diagnosis, possibly
causing spurious changes in incidence rates.
The nurses’ intake of vitamin D is similar to that of women in other
US cohort studies. [6] Race was self-reported and “White” did not include
ethnic groups with low risk of MS.
The lack of significant interaction between latitude and vitamin D in
our study may be explained by the insufficient power to detect such an
interaction.
As this was a prospective study, recall bias was not of concern as
none of the women had MS or MS symptoms when completing the food frequency
questionnaires.
References
6. Merlino LA, Curtis J, Mikuls TR, et al. Vitamin D intake is
inversely associated with rheumatoid arthritis: results from the Iowa
Women’s Health Study. Arthritis and Rheumatism 2004; 50:72-77.
We thank Ebers et al for their interest in our study. We excluded women with an incomplete baseline food frequency questionnaire. This exclusion was made a priori following the same rules used in all previous dietary analyses in these cohorts. Adjustment for multiple analyses in our study was not necessary as only two measures of vitamin D intake were considered: the combined amount of vitamin D intake from food and from supplements. The P for trend was calculated using a proportional hazards regression model using the median intake values for each category of vitamin D from food or supplements as a continuous variable. This method tests the overall null hypothesis that vitamin D intake is unrelated to risk of MS without any specific assumptions.
In contrast with the results of Vieth et al [4] quoted by Ebers et al, in a sub-study among 323 healthy women from the NHS cohort, we found that vitamin D intake at levels below 400 IU does increase 25(OH)D levels. Average winter plasma levels of 25(OH)D were positively correlated with levels of vitamin D intake: 40 nmol/L in the lowest quintile of intake (median=108 IU), 55 nmol/L in the third quintile (median=301 IU), and 70 nmol/L in the highest quintile (median=703 IU). Further, in our cohorts, the association between vitamin D intake from supplements and risk of MS was not materially altered by adjustment for physical activity (unpublished data).
The women in the NHS cohort are older than those in the NHSII cohort and because of modest overlap between the two cohorts, age-specific incidence rates are difficult to compare. Further, the increasing use of MRI may reduce the time between onset of MS and diagnosis, possibly causing spurious changes in incidence rates.
The nurses’ intake of vitamin D is similar to that of women in other US cohort studies. [6] Race was self-reported and “White” did not include ethnic groups with low risk of MS.
The lack of significant interaction between latitude and vitamin D in our study may be explained by the insufficient power to detect such an interaction.
As this was a prospective study, recall bias was not of concern as none of the women had MS or MS symptoms when completing the food frequency questionnaires.
References
6. Merlino LA, Curtis J, Mikuls TR, et al. Vitamin D intake is inversely associated with rheumatoid arthritis: results from the Iowa Women’s Health Study. Arthritis and Rheumatism 2004; 50:72-77.