sunlight exposure during pregnancy and MS

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Low maternal sunlight exposure during pregnancy increased her offspring's

risk of multiple sclerosis by 50% or more, according to a study in

Australia.

The critical exposure period occurred during the first trimester and was

independent of birth month, a factor previously implicated in MS risk,

researchers reported in an article published online in *BMJ*.

Geographic region of birth also predicted a child's subsequent risk of MS,

Judith Staples, of Australian National University in Canberra, and

co-authors wrote.

* " Lower average daily levels of ambient ultraviolet radiation during the

first trimester predicted a higher subsequent risk of multiple sclerosis

independently of month of birth, " they wrote.*

*Several lines of evidence have supported a role for ultraviolet (UV)

radiation in the pathogenesis of MS, either directly or indirectly.

Increased UV radiation exposure, vitamin D intake, and serum vitamin D

levels have been associated with a reduced risk of MS.*

But UV exposure during early life, including maternal exposure during

pregnancy, had not been examined carefully, the researchers wrote. A study

of half siblings with MS showed that the risk of the condition can be

maternally mediated.

And experimental data on animal fetal development have shown several

associations involving vitamin D, the brain, and the spinal cord, the

authors continued. Maternal vitamin D depletion has been shown to alter

neurogenesis in the rat brain.

Data in humans have remained largely circumstantial. A recent genetic study

implicated vitamin D interaction with the major locus that determines MS

susceptibility (*PLoS Genet* 2009; 5: e1000369). Studies of perinatal

environmental exposures in the northern hemisphere have yielded mixed

results, the authors wrote.

A pooled analysis of births in the northern hemisphere showed an excess of

MS cases among people born in May and a relative deficit of cases among

those born in November. Moreover, the association was stronger in familial

MS, suggesting interaction between genetic and environmental factors,

possibly related to climate (*BMJ* 2005; 330: 120).

Extending the investigation to the Southern Hemisphere, Staples and

co-authors examined relationships between birth month and MS.

They analyzed data from a 1981 patient survey, involving individuals born

from 1920 to 1950 in Queensland, Western Australia, New South Wales

(reference region), South Australia, and Hobart, Tasmania. Diagnosis of MS

was verified by means of hospital records, treating physicians, MS

societies, and government sources.

They found a wide variation in average total daily ambient ultraviolet

radiation, ranging from 1.6 minimum erythemal dose units in Hobart in July

to 30.4 units in Perth, Western Australia in January.

The authors identified 1,524 cases of MS in a population of 2.5 million.

Consistent with previous Australian surveys, the MS incidence rate was

higher among women than men (incidence rate ratio 2.28). As compared with

the reference region, the MS incidence rate ratio was lower in Queensland

(0.59) and higher for people born in Tasmania (2.70).

Comparing MS cases by birth month, the authors found monthly incidence rate

ratios ranging from 1.23-fold to 1.34-fold greater compared with the

reference of May-June (*P*<0.05).

The highest incidence ratios were observed in the early summer months of

November-December compared with early winter months in May-June (1.10 to

1.63, *P*<0.01).

The incidence patterns remained significant in models that adjusted for age,

sex, and region of birth (1.10 to 1.58, *P*<0.01).

Daily ambient UV exposure at the time of birth or one to four months before

birth had no association with MS risk.

However, a significant inverse association -- ranging from 0.74 to

0.81 (*P*<0.01)

-- emerged from exposure in the five to nine months before birth (first to

early second trimester).

Limiting the analysis to the first trimester of pregnancy yielded an even

stronger inverse association association (0.72 to 0.84, *P*<0.001).

After adjustment for UV radiation in the first trimester, the authors found

no association between the period of birth and MS risk, but the region of

birth remained a significant predictor of MS risk.

The risk of MS varied in near-linear manner across the range of ambient UV

exposure during the first trimester of pregnancy. Using ≥25 erythemal dose

units as the reference, the the authors found the MS risk increased to 1.54

for 20 to 25 units to 1.67 for exposure <5 units.

" After adjustment for this exposure during early pregnancy, there was no

residual association between month of birth and multiple sclerosis, " the

authors wrote.

*Primary source: *BMJ

Source reference:

Staples J, et al " Low maternal exposure to ultraviolet radiation in

pregnancy, month of birth, and risk of multiple sclerosis in offspring:

longitudinal analysis " *BMJ* 2010; 340:

c1640.<http://www.bmj.com/cgi/content/full/340/apr29_1/c1640%3Cbr%20/%3E>

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--

Ortiz, MS, RD

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