There are limited data about vitamin D deficiency in healthy nonpregnant fertile women who will become pregnant in following 3–6 months. This study was focused on comparative affluent and healthy fertile women to receive preconception evaluation but their vitamin D levels were found to be low. Serum 25(OH)D levels reflect body stores of vitamin D . Nearly eighty-four percent of fertile nonpregnant women were 25(OH)D deficient in this study. Especially in young women aged 22–30 years, serum 25(OH)D level was lower in comparison with women between 31–44 years. These findings were nearly consistent with a population-based study by Fang et al  reported that serum 25(OH)D level was the lowest in the 18- 29-year-old group (46.66 ± 12.98 nmol/L) among all participant women aged 18–49 years in Tianjin, China. Our findings are also supported by a recent study from Pakistan on vitamin D deficiency in asymptomatic healthy young students. Almost eighty-nine percent of participants were vitamin D deficient (25(OH)D < 20ng/mL) .
This is alarming as these are pregnant in following 3–6 months. Several studies have described an association between insufficiency or deficiency in 25(OH)D levels and adverse pregnancy outcomes including gestational diabetes mellitus, preeclampsia, preterm delivery, recurrent abortion, and intrauterine growth restriction [24,25,26]. More recent data showed that preconception vitamin D status was associated with male live birth . Increased preconception vitamin D concentrations are associated with reduced pregnancy loss .
In this study, we also found that working women displayed a lower level of vitamin D relative to non-working individuals. Sowah, et al. also observed a higher prevalence of vitamin D deficiency in all occupational populations examined than the reported population burden of vitamin D deficiency in multiple populations, suggesting that workers may be particularly vulnerable to vitamin D deficiency . Among the occupations considered in the present study, the women almost were office-based workers who spend a considerable amount of time indoors without sunshine exposure. Women in an indoor setting would be expected to get their sunshine exposure during mornings and evenings, when sunlight intensity is relatively low. As we all know, vitamin D3 is needed from not only adequate intake from diet or supplements but also biosynthesis in the skin in response to sufficient exposure to sunlight. Several studies have concerned seasonal variation in serum 25(OH)D concentrations [29,30,31]. This study also found that serum 25(OH)D concentration was the lowest in January and the highest in July and August. Almost ninety-four percent of healthy preconception fertile women had serum 25(OH)D levels <20 ng/mL during winter in Xi’an, China. Similar to the results that traveling to a warmer climate during winter/spring was associated with higher serum 25OHD according to a prevalence of vitamin D deficiency in healthy women of reproductive age in Canada . Statistics Canada found that sunlight exposure more than 1 h per day and month of blood collection were positively associated with higher serum 25OHD levels .
It is interesting that when analyzed by season, non-working individuals and women between 31–44 years did not have significant winter decline in serum 25(OH)D levels seen in working individuals and young women aged 22–30 years. This altered pattern may be due to trends of increased multivitamin supplements among women aged 31–44 years. Mitchell et al. study suggests that increased oral intake of vitamin D compensated for decreased ultraviolet radiation B exposure . There may also have been differences in sun vacation or outdoor time with no sun protection (i.e. no protection from the sun using clothing or sunscreen) between working and non-working women.
In contrast to many studies [35,36], we found no association between BMI and serum 25OHD, which could be accounted for the normal and low BMI of our participates. Indeed, about 84% of women had BMI < 24 kg/m2, only one had BMI > 28 kg/m2 indicating obese.
Our study had some limitations. We did not collect information regarding dietary vitamin D intake, vitamin D/multivitamin supplements, or measures of plasma parathyroid hormone. Regarding occupation, we do not have data on natural ultraviolet B exposure time or amount, which may confound the effects of ultraviolet B in different subgroups. Finally, our data are cross-sectional, thus, we do not have insight on longitudinal changes in 25(OH)D, and we do not provide reproductive outcomes.