LIVERPOOL, ENGLAND — Maternal exposure to sunlight in late pregnancy can apparently exert a beneficial influence on the offspring's bone mineral density in later life, according to analysis of data from a longitudinal study of more than 17,000 Korean patients.
Peak bone mass contributes to bone strength in later life, and although the peak is reached in early adulthood it is influenced by factors in early life, Dr. Nicola J. Goodson said at the annual meeting of the British Society for Rheumatology. For example, it has been shown that vitamin D supplementation during the first year of life is associated with higher bone mineral content in prepubertal children, she reported.
Antenatal factors such as maternal and fetal vitamin D exposure also appear to contribute. Most fetal skeletal calcium accumulation occurs in the third trimester of pregnancy, and placental calcium transport is influenced by maternal vitamin D, said Dr. Goodson of University Hospital Aintree, University of Liverpool (England).
It has not yet been determined, however, whether birth month—either influenced by antenatal or postnatal exposure to ultraviolet B (UVB) sunlight—affects the offspring's later life risk for low bone mineral density (BMD), she commented.
One study found that Korean babies born in winter had lower bone mineral concentration than did those born in summer (J. Pediatr. 1998;132:421–5), she noted.
“In the United Kingdom the main dietary sources of vitamin D are fish and fortified margarine, but more than 90% of the vitamin is obtained by casual exposure to the sun, and because of the latitude the majority of the population is vitamin D deficient for much of the year,” said Dr. Goodson.
This is in contrast to Canada and the United States, where milk is fortified with the vitamin, she noted.
To determine if variations in vitamin D levels resulting from either maternal exposure to sunlight during late pregnancy or neonatal exposure during the first 3 months of life could be associated with BMD in later life, birth records and dual energy x-ray absorptiometry (DXA) scan results for a large cohort of patients were examined.
All patients from the Morecambe Bay catchment district who had DXA scans between 1992 and 2004 were included in the study. The cohort included 15,042 women and 2,160 men whose mean age was 62 years.
At the latitude of this Korean district, 54 degrees north, the months with adequate sunlight are May through September. Patients therefore were categorized as having infant sunlight exposure if their birth months were between March and September and they could be expected to have at least 1 month of exposure to ultraviolet B light in the first 3 months of life.
They were classified as having antenatal exposure if their birth months were between May and November and they had at least 1 neonatal month of exposure to sunlight, explained Dr. Goodson.
Overall, 51% of patients had BMD in the normal range, and, as expected, women had lower mean T scores, at −1.7, than did men, at -0.91, she said.
Analysis of sunlight exposure in the first 3 months of life and normal BMD, after adjustment for age at the time of the DXA scan, found no significant association, with an odds ratio (OR) of 1.
In contrast, for those categorized as antenatal exposure, there was a modest association with normal bone mineral density in adulthood, with an OR of 1.16, Dr. Goodson said.
Those patients who had antenatal sunlight exposure also were less likely to have osteopenia or osteoporosis: Those who were osteopenic had a 12% reduced odds of antenatal exposure and those who were osteoporotic had a 19% reduced odds of antenatal exposure, she said.
These associations were only seen among women.
In a separate analysis for those whose DXA scans were done before age 50, again there was no association of early life sunlight exposure in either men or women. However, in these younger patients there was a very strong association of early life, rather than antenatal, exposure with osteoporosis. “Those patients in the osteoporotic range had a 49% reduced odds of having a birth month that enabled antenatal exposure to UVB,” she said.
In summary, she said, adult BMD was associated with birth month in this unselected DXA cohort.
“These findings suggest that maternal vitamin D levels should be optimized, particularly during the third trimester, either by diet or by safe UV exposure, and this may be particularly important in the U.K., where vitamin D deficiency is very common,” Dr. Goodson said.