Our study on a large, nationally representative sample demonstrated that applying the new IOM definitions predicted a lower prevalence of vitamin D deficiency and inadequacy than that was reported previously [29
]. Our results showed that more than one-third of women of childbearing age had their vitamin D status below an optimal level (i.e., ≥20.0
]) in the United States. More importantly, we found that multiple factors were independently associated with suboptimal levels of vitamin D among these women.
Significant changes in maternal vitamin D and calcium metabolism occur during pregnancy. For example, a study has reported that serum concentrations of 1,25-dihydroxyvitamin D [1,25(OH)2
D], the active form of vitamin D, increased by 50–100% from the nonpregnant state to the second to third trimesters of the pregnancy [34
]. A longitudinal study conducted in Caucasian women of the United Kingdom showed that plasma 25(OH)D concentrations were lower among pregnant women than among nonpregnant controls [35
], a finding that may be largely explained by increased fetal demand for this essential nutrient which is almost entirely dependent on vitamin D from the mother [18
]. However, results from other cross-sectional studies have shown, similar to our findings, that pregnant women have a higher mean 25(OH)D concentration than nonpregnant women [29
]. The higher 25(OH)D concentrations among pregnant women are likely due to higher rates of dietary supplement use (i.e., 83.4% versus 47.3%) and milk consumption of at least 1 time/day (i.e., 63.8% versus 41.4%) among pregnant women than among nonpregnant women.
Our study showed that multiple factors were associated with suboptimal vitamin D status in women of childbearing age. The significant racial/ethnic disparities that we found in the prevalence of vitamin D deficiency/inadequacy were in agreement with those reported previously both in the general population as well as in pregnant women, showing non-Hispanic whites had the highest adjusted mean 25(OH)D concentration, followed by Mexican Americans, and non-Hispanic blacks had the lowest [30
]. Our results further demonstrated that, among US women of reproductive age, those who were non-Hispanic black, Mexican American, or of other racial/ethnic groups were all at a higher risk, to the similar magnitude, for vitamin D deficiency and inadequacy than non-Hispanic white women.
In the human body, about 50–90% of vitamin D comes from the biosynthesis of vitamin D3 (cholecalciferol) in the skin from 7-dehydrocholesterol that requires sunlight (ultraviolet radiation), and the remainder comes from a limited number of foods (mainly fatty fish, eggs, and liver as well as foods fortified with vitamin D such as margarine, cereals, and milk products) and from dietary supplements. A previous study based on data from the third NHANES (1988–1994) reported that seasonality, milk consumption, and dietary supplement use were significant and independent determinants of hypovitaminosis D in women of childbearing age [39
], which is consistent with the findings of the present study. Fish consumption can be a major source of vitamin D intake. Lym and Joh recently reported that frequent fish consumption was associated with a higher vitamin D level among Korean men [40
]. Brock et al. also reported that a high fish intake was associated with 25(OH)D ≥25
ng/mL) in Finland male smokers exposed to negligible solar UV light in winter [41
]. In the present study, however, this relationship was not significant in women of childbearing age. Thus, future research on the role of fish consumption in improving vitamin D nutrition is warranted.
Low vitamin D levels are linked to increased risk for high blood pressure, insulin resistance and diabetes, and cardiovascular disease in the general population [2
]. Similarly, among women of childbearing age, we found that having a history of diabetes or cardiovascular disease was significantly associated with vitamin D deficiency (but not vitamin D inadequacy) independent of other factors. Thus, women with these conditions should be assessed and counseled for their vitamin D status during their routine health care visits.
Multiple health-related behavioral factors have been shown to affect pregnancy outcomes. For example, physical inactivity in mothers is associated with increased risks for obesity and obesity-related chronic conditions such as diabetes mellitus and hypertension, and pregnant women who are obese are at increased risk for miscarriage, pregnancy-induced hypertension and preeclampsia, gestational diabetes, and thromboembolism, and at an increased risk of having children with macrosomia, spontaneous intrauterine demises, or delivered by Cesarean section which carries an increased risk for wound infection [42
]. Maternal alcohol use and cigarette smoking have been associated with a higher rate of infertility, spontaneous abortion or preterm birth, fetal alcohol syndrome (characterized by growth deficiencies, central nervous system impairment, behavioral disorders, and impaired intellectual devolvement with lifelong implication), and facial dysmorphia [45
]. Thus, maintaining healthy lifestyles is especially important for women of childbearing age. Previous studies have consistently reported low serum vitamin D concentrations in people with a high BMI [38
], results consistent with our finding that BMI of ≥ 30.0
was an independent predictor of vitamin D deficiency and inadequacy among women of childbearing age. Smoking has been associated with a low bone mass and an increased risk for osteoporotic fracture [51
]. Brot et al. further reported that middle-aged women who were current smokers had significantly reduced levels of 25(OH)D, 1,25(OH)2
D and parathyroid hormone [52
]. Our results also showed that smoking was independently associated with vitamin D deficiency in women of childbearing age. However, we found that other behavioral factors such as physical activity and alcohol drinking were not significant determinants of vitamin D status.
Nutritional assessment, education, and counseling are important components of periconception care. In the present study, although the prevalence of vitamin D deficiency and inadequacy decreased linearly across educational levels, education was not a significant contributor to the models after controlling for potential confounders. Also, we found that increasing use of health care services was not a significant determinant of vitamin D status either. However, these findings should be viewed with caution because the effectiveness of nutritional education and counseling was not assessed in this study.
This study has several limitations. First, the causal relationships between factors and vitamin D deficiency/inadequacy cannot be established from our cross-sectional study design. Second, the drift in serum 25(OH)D measurements over the time has been reported. However, we did not find significant differences in the prevalence of vitamin D deficiency and inadequacy between the NHANES 2003-2004 and 2005-2006 survey periods, suggesting that our results may not have been affected in a meaningful way by the variability of serum 25(OH)D assay [53
In conclusion, our results demonstrate that achieving an optimal vitamin D level remains a distant target for women of childbearing age in the United States. Given that adequate vitamin D concentrations in pregnant women are associated with healthy reproductive outcomes, periconceptional intervention programs may focus on raising vitamin D levels in these women and efforts to prevent vitamin D deficiency-linked health outcomes should begin during routine gynecologic care prior to conception and continue through the postpartum period. The multiple risk factors for vitamin D deficiency and inadequacy that we identified in this study should be useful in identifying at-risk groups of women who may be targeted by these programs.