The Association Between Active Transportation and Serum Total 25-Hydroxyvitamin D Levels Among US Childbearing-Aged Women

Abstract

Background:

Serum total 25-hydroxyvitamin D (25[OH]D) and physical activity (PA) both play important roles in maternal–fetal health. However, a high prevalence of vitamin D and PA insufficiency has been observed in women of childbearing age. Active transportation may increase overall PA levels and potentially boost serum 25(OH)D levels.

Methods:

Data from the National Health and Nutrition Examination Survey between 2007 and 2014 were used. A total of 5601 women aged 18–49 years were included. Transportation PA (TPA) was quantified as metabolic equivalents of task and serum 25(OH)D levels was measured. Multivariable logistic regression models adjusted for potential confounders were conducted.

Results:

The corresponding adjusted odds ratios associated with vitamin D insufficiency (<50 nmol/L) were 1.09 (95% confidence interval, 0.87–1.37) for 1 to 499 MET minutes per week of TPA, 0.69 (0.52–0.91) for 500 to 1000 MET minutes per week of TPA, and 0.95 (0.72–1.26) for >1000 MET minutes per week of TPA, respectively, compared with no TPA. Using vitamin D deficiency (<30 nmol/L) as the outcome led to similar results. The association between TPA and serum 25(OH)D levels was more robust in high sedentary time.

Conclusions:

A moderate level of TPA is related to lower odds of suboptimal vitamin D status among women of childbearing age.

Vitamin D has been shown to have beneficial effects on female fertility and maternal–fetal health.^1,2 Recent studies have revealed that low maternal vitamin D status is associated with an increased risk of adverse pregnancy outcomes and adverse fetal health.³ The risks of adverse effects of vitamin D insufficiency have been reported in the context of diseases, such as polycystic ovary syndrome, uterine leiomyomas, and endometriosis.⁴ The Institute of Medicine concluded that persons with 25-hydroxyvitamin D (25[OH]D) concentrations <30 nmol/L are at risk of vitamin D deficiency.⁵ The recommended level for serum 25(OH)D during pregnancy and lactation is 50 nmol/L or more.⁶ Nevertheless, a high prevalence of suboptimal vitamin D status among women was noted and has become a public health concern. Measuring circulating 25(OH)D levels is considered to be the best biomarker to examine vitamin D status.⁷

Physical activity (PA) has been observed to have a positive and direct association with serum vitamin D levels.⁸ While exposure to sunlight during the course of exercise might explain the increased production of vitamin D in the skin, there may be more factors involved because increased serum vitamin D levels were found not only in adults participating in outdoor activities but also in those participating in indoor physical activities.⁹ Another possible mechanism underlying this association is exercise-induced reductions in serum phosphorus and calcium which stimulate parathyroid hormone secretion and further activate the synthesis of renal calcitriol, a bioactive form of vitamin D. Despite the beneficial effect of PA, only approximately 50% of American women meet the recommended PA guidelines (PAGs) for aerobic PA.¹⁰ Participation in transportation PA (TPA), defined as any self-propelled human-powered mode of transportation including walking or bicycling, is recognized as a feasible strategy to meet the PA levels outlined in the PAGs.¹¹ In addition to engaging in more in PA, the 2018 PAGs recommends that Americans should reduce sedentary time (ST) as PA can ameliorate some of the health hazards of low PA. Studies consistently showed that the deleterious influence of high ST, including an increased risk of metabolic disease and all-cause mortality, was independent of PA levels.^12,13 Furthermore, the odds of having suboptimal vitamin D levels were found higher in sedentary women than those who were physically active.¹⁴ As such, ST may have a plausible effect modification on the association between TPA and vitamin D levels.

Although many studies have focused on the effects of recreational physical activities on serum vitamin D levels,¹⁵ to our knowledge, no study has explored the detailed association between TPA and serum 25(OH)D levels. Since studies about the impact of TPA on health revealed inconsistent findings,^16,17 the present study aims to examine the association between TPA and serum 25(OH)D levels. We hypothesize that TPA is positively associated with serum 25(OH)D levels and that ST may modify this association among women of childbearing age in a nationally representative population sample from the US National Health and Nutrition Examination Survey (NHANES).

Materials and Methods

This study used a cross-sectional study design and was based on data from continuous NHANES. No repeated observations on participants were included in the analysis. The NHANES program began in the 1970s and is conducted by the National Center for Health Statistics, a part of the Centers for Disease Control and Prevention. Since 1999, NHANES has continuously collected and publicly released data in 2-year cycles. The main purpose of NHANES is to assess the health and nutritional status of adults and children in the United States through its unique combination of in-home interviews as well as standardized physical examinations and laboratory measurements conducted in the Mobile Exam Center. The NHANES employs a complex survey methodology which creates a nationally representative random sample of the noninstitutionalized US population. All participants provided written informed consent and the National Center for Health Statistics obtained institutional review board approval to conduct the surveys. The present analyses utilize public domain data from NHANES and thus do not require additional institutional review board approval.¹⁸

We used NHANES data for women of childbearing age to examine relationships between TPA and serum vitamin D levels and differences in this association based on age, use of vitamin D supplements, and ST. We included women aged 18–49 years who did not report difficulties on walking in our study sample. While childbearing age has been defined in a variety of ways, we used ages 18–49 years to operationally define women of childbearing age.¹⁹

A total of four 2-year cycles of NHANES data (from 2007 to 2014) were included in this study because of the consistent availability and measurements of the included covariates. The exposure covariate was TPA, which was assessed by the NHANES PA questionnaire at a home interview, including a series of questions on participation in TPA. Survey participants were first asked, “In a typical week, do you walk or use a bicycle for at least 10 minutes continuously to get to and from places?” Participants who answered “yes” to this question were then asked to identify the frequency and duration of the TPA. To quantify the amount of PA reported by each participant, energy expenditure was calculated in metabolic equivalent of task (MET) minutes. MET represents the ratio between one’s metabolic rate while physically active and that at rest. In NHANES, a conventionally defined MET score of 4 was assigned to TPA. We calculated each participant’s energy expenditure associated with TPA by multiplying this MET score by the total duration (in minutes). The validity of self-reported PA used in NHANES may be indirectly inferred, as these measures show expected inverse associations with obesity and the risk of metabolic syndrome.^20,21 In the present analysis, TPA was categorized into 4 levels: 0, 1 to 499, 500 to 1000, and >1000 MET minutes per week, with categories selected based on the US PAGs.²² Current US PAGs recommend that adults perform 150 to 300 minutes of moderate-intensity PA or 75 to 150 minutes of vigorous-intensity PA per week, which are both approximately equivalent to 500 to 1000 MET minutes a week.

The outcome measure was serum 25(OH)D levels. Beginning in the 2007 to 2008 cycle, NHANES used a liquid chromatography tandem mass spectrometry (LC-MS/MS, CDC) method to measure serum 25(OH)D₃, 25(OH)D₂, and the C3 epimer of 25(OH)D₃.²³ The standardized 25(OH)D measurement allows laboratories and surveys to compare 25(OH)D measurements and avoid methodological bias. Total 25(OH)D (in SI units of nmol/L) was defined as the sum of 25(OH)D₃ and 25(OH)D₂ and excluded the C3 epimer of 25(OH)D₃. Serum 25(OH)D levels were categorized as deficient (<30 nmol/L) and insufficient (<50 nmol/L).⁹

Several demographic and lifestyle covariates were considered as potential confounders and effect modifiers. The demographic covariates included age, race/ethnicity, and poverty income ratio. According to the definition of advanced maternal age (≥35 y), age was treated as a categorical variable (<35 or ≥35 y) in the stratified analysis to test the potential effect modification of age on the prevalence of vitamin D deficiency.^24,25 Li et al²⁶ reported that women with advanced maternal age were more likely to have better vitamin D status compared with younger women. To categorize races and ethnicities, NHANES used non-Hispanic white, non-Hispanic black, Mexican American, and other races. Poverty income ratio was used to represent socioeconomic status. The ratio was the family income to the poverty level and then categorized as below poverty (<1.0), middle income (1–4.99), and higher income (>5).²⁷

Lifestyle covariates were waist circumference, body mass index (BMI), smoking status, occupational PA, recreational PA, ST, and use of vitamin D supplements. The body measures data including waist circumference and BMI were collected in the Mobile Exam Center by trained health technicians. Waist circumference was further dichotomized as <88 or ≥88 cm according to the definition of abdominal obesity in women.²⁸ The BMI was calculated using the standard formula: weight (in kilograms) divided by height (in meters squared). Categories based on standard cutoff points were used: underweight (<18.5), normal weight (≥18.5 and <25.0), overweight (≥25.0 and <30.0), or obese (≥30.0).²⁹ Smoking status was categorized as never, former, and current smoking according to the information obtained from the NHANES questionnaire. The use of vitamin D supplements was collected as part of the in-home interview. The NHANES participants were asked to provide information on the frequency, duration, and amount of vitamin D supplements used in the past 30 days, and categorized as consumption of ≥15 or <15 mcg/d based on the Recommended Dietary Allowance established by the Food and Nutrition Board.³⁰

Among PA-related covariates, recreational PA was defined as sports, fitness, and recreational activities performed at least 10 minutes continuously in a typical week. Occupational PA included work, such as paid or unpaid work, studying or training, household chores, and yard work that is performed at least 10 minutes continuously in a typical week. Information on participation in occupational and recreational physical activities was collected separately according to intensity. The NHANES PA questionnaire defined moderate intensity as “tasks that need moderate physical effort and cause small increases in breathing or heart rate” and vigorous intensity as activities that “require hard physical effort and cause large increases in breathing or heart rate.” A MET score of 4 was assigned to the moderate-intensity physical activities and a MET score of 8 was assigned to the vigorous-intensity physical activities, respectively. The total MET minutes of occupational and recreational moderate–vigorous physical activities (MVPAs) were estimated by summing the weekly MET minutes for both moderate and vigorous activity. In addition, ST was defined as the total time that participants reported sitting at work, at home, during transportation, or with friends, with the exclusion of sleeping on a typical day. Eight hours was defined as the cut point in analyses using daily ST in our study population.³¹

To assess the odds of serum 25(OH)D insufficiency/deficiency according to different TPA levels, crude and multivariable logistic regression were used. We performed logarithmic transformation of TPA to ensure normality of distribution and to meet the criteria for regression analysis. Multivariable models were adjusted for potential confounders determined a priori: age, race/ethnicity, poverty income ratio, smoking status, waist circumference, and recreational physical activities. Based on previous literature, we identified potential confounders and plotted possible pathways on a directed acyclic graph (Figure 1).^32-34 Because BMI was highly correlated with waist circumference in our study sample, we excluded BMI in multivariable models (which did include waist circumference) to avoid overadjustment. Moreover, waist circumference has been shown to be a better surrogate of abdominal adiposity, which was associated with serum vitamin D levels.³⁵ In addition, we did not include employment status as a confounder in the analyses since patterns of active transportation were similar between the employed and unemployed US adults.¹¹ The association between occupational PA with main exposure (TPA) and outcome of interest (vitamin D) was not demonstrated in previous studies. Therefore, we did not include occupational PA in the multivariable analysis.

Figure 1 —

A DAG showing the association between TPA and serum vitamin D level. BMI indicates body mass index; DAG, directed acyclic graph; PA, physical activity; TPA, transportation PA.

Interaction terms including age, use of vitamin D supplements, and ST were added to the multivariable model as binary variables adjusted for the same covariates to explore the potential effect modification in the association between TPA and serum 25(OH)D levels (aged <35 or ≥35 y, vitamin D supplements <15 or ≥15 mcg/d, and ST <8 or ≥8 h/d). Two sensitivity analyses were performed to test the robustness of the results. First, we excluded women who did not perform any TPA and repeated analyses only in women who performed TPA. Second, we used a more rigorous cutoff value of serum 25(OH)D levels (≥75 or <75 nmol/L) to repeat the analyses.³⁶

The NHANES implemented a complex cluster sample design to include adequate representation from various socioeconomic strata and minorities. To derive population level values from the study sample, we computed weighted estimates according to the NHANES Analytic and Reporting Guidelines including 8-year sampling weights. Each weight reflects the number of individuals in the United States represented by that NHANES participant. To produce weighted frequencies, all analyses were conducted using Stata (version 14.0; Stata Corp, College Station, TX) and adjusted for the clustered sampling design and the NHANES sample population weights. All tests were 2-tailed and a P value < .05 was considered statistically significant.

Results

In NHANES 2007 to 2014, there were a total of 6760 women aged 18–49 years eligible for our study. Those who reported having difficulty walking without special equipment were excluded (n = 206). We first removed individuals who had missing information for TPA (n = 3), BMI (n = 1), sedentary behavior (n = 7), and recreational (n = 2) and occupational physical activities (n = 3). We then removed those who did not provide information on their poverty income ratio (n = 525) or waist circumference (n = 412). To further explore the influence of missing values in our study sample, we examined the women who were excluded from our study due to missing values in accordance with the NHANES Analytic and Reporting Guidelines³⁷: compare participants included and excluded from the study sample due to missingness with respect to the main outcome variable. On average, vitamin D levels were 65.68 nmol/L among women excluded in the study and were 66.85 nmol/L among women included in the study (P = .486). The mean age of the excluded and included women was 33.06 and 33.72 years, respectively (P = .119). Similar results were found in BMI, ST, and TPA levels in both groups. The final study population was 5601 individuals, which represented 58 million women after weighted estimation. The demographic characteristics of the study population are presented in Table 1. All sample sizes shown in Table 1 are unweighted. Among our study population, approximately 73% of participants did not participate in any TPA. Active transport participation was more common in younger women with more occupational physical activities. Participating in the most TPA was also associated with being in a less affluent income level group and having the lowest amount of ST.

Table 1.

Descriptive Statistics by Transportation PA Level

	No transportation PA	Transportation PA 1–499 MET min/wk	Transportation PA 500–1000 MET min/wk	Transportation PA > 1000 MET min/wk
	Mean (SD) or n (%) 3906 (72.8)	Mean (SD) or n (%) 707 (11.7)	Mean (SD) or n (%) 433 (6.5)	Mean (SD) or n (%) 555 (9.0)
Age, y	34.3 (8.9)	32.8 (9.7)	31.7 (10.5)	31.3 (10.0)
Race
Non-Hispanic white	1633 (63.5)	261 (59.2)	124 (50.7)	186 (57.1)
Non-Hispanic black	786 (12.4)	178 (16.7)	111 (17.7)	102 (11.2)
Mexican American	630 (9.7)	115 (10.2)	79 (12.4)	125 (13.4)
Other race	857 (14.3)	153 (13.9)	119 (19.1)	142 (18.2)
Poverty income ratio
<1	987 (17.9)	235 (24.2)	155 (28.9)	221 (31.2)
1–4.99	2319 (60.3)	378 (56.3)	223 (50.0)	276 (51.6)
≥5	600 (21.7)	94 (19.5)	55 (21.1)	58 (17.3)
Waist circumferences, cm
<88	1610 (42.2)	302 (44.8)	206 (51.8)	274 (51.9)
≥88	2296 (57.8)	405 (55.2)	227 (48.2)	281 (48.1)
BMI, kg/m2
Underweight (<18.5)	113 (2.6)	23 (3.0)	13 (3.0)	17 (2.8)
Normal weight (18.5–24.9)	1334 (36.3)	243 (37.2)	185 (47.6)	227 (44.4)
Overweight (25.0–29.9)	1028 (27.0)	191 (27.2)	106 (23.3)	129 (23.4)
Obese (≥30)	1431 (34.1)	250 (32.6)	129 (26.1)	182 (29.4)
Smoking
Never smoker	2652 (64.5)	501 (66.8)	330 (75.2)	389 (68.7)
Current smoker	810 (21.6)	130 (18.0)	67 (13.7)	111 (20.0)
Former smoker	444 (14.0)	76 (15.2)	36 (11.1)	55 (11.4)
Total occupational MVPA, MET min/wk
0	2439 (62.42)	435 (61.49)	265 (61.31)	289 (52.09)
1–499	285 (7.29)	69 (9.77)	27 (6.26)	33 (5.98)
500–1000	187 (4.80)	36 (5.13)	21 (4.89)	25 (4.50)
>1000	995 (25.48)	167 (23.61)	121 (27.54)	208 (37.43)
Total recreational MVPA, MET min/wk
0	1724 (44.12)	285 (40.25)	157 (36.08)	241 (43.40)
1–499	698 (17.87)	169 (23.85)	55 (12.72)	97 (17.53)
500–1000	510 (13.08)	74 (10.52)	81 (18.66)	70 (12.65)
>1000	974 (24.93)	179 (25.37)	141 (32.53)	147 (26.42)
ST, h/d
<8	2452 (62.78)	430 (60.85)	269 (62.06)	413 (74.48)
≥8	1454 (37.22)	277 (39.15)	165 (37.94)	142 (25.52)

Abbreviations: BMI, body mass index; MET, metabolic equivalent; MVPA, moderate to vigorous physical activity; PA, physical activity; ST, sedentary time.

Vitamin D insufficiency (<50 nmol/L) and deficiency (<30 nmol/L) were observed in 25.4% and 10.4% of the participants, respectively. In addition, 73.5% of the participants who reported taking vitamin D supplements did not meet the daily recommended dose (≥15 mcg/d).³⁰ The crude analysis did not show statistically significant associations between TPA and vitamin D levels. After adjusting for age, race/ethnicity, poverty income ratio, smoking, recreational PA, and ST, the corresponding odds ratios (ORs) of vitamin D insufficiency were 1.09 (95% CI, 0.87–1.37) for 1 to 499 MET minutes per week of TPA, 0.69 (0.52–0.91) for 500 to 1000 MET minutes per week of TPA, and 0.95 (0.72–1.26) for >1000 MET minutes per week of TPA, respectively, compared with no transportation activity. Using vitamin D deficiency as the outcome of interest led to similar results; the corresponding ORs were 1.00 (reference; no transportation activity), 0.83 (95% CI, 0.60–1.13), 0.60 (0.38–0.93), and 0.65 (0.40–1.05), respectively (Table 2).

Table 2.

Crude and Multivariable Adjusted OR (95% CI) of Suboptimal Vitamin D Levels in Childbearing Age Women

	Crude OR (95% CI)	Adjusted OR (95% CI)	Crude OR (95% CI)	Adjusted OR (95% CI)
TPA level, MET min/wk
No TPA	Ref	Ref	Ref	Ref
1–499	1.20 (0.98–1.47)	1.09 (0.87–1.37)	0.93 (0.70–1.22)	0.83 (0.60–1.13)
500–1000	0.94 (0.73–1.21)	0.69 (0.52–0.91)	0.91 (0.60–1.39)	0.60 (0.38–0.93)
>1000	1.04 (0.81–1.34)	0.95 (0.72–1.26)	0.73 (0.49–1.09)	0.65 (0.40–1.05)
Age, y
18 to <25	—	Ref	—	Ref
25 to <35	—	0.87 (0.71–1.07)	—	0.71 (0.51–1.00)
35–49	—	0.75 (0.59–0.96)	—	0.63 (0.45–0.90)
Race
NH white	—	Ref	—	Ref
NH black	—	13.93 (11.44–16.96)	—	34.18 (23.34–50.04)
Mexican American	—	5.39 (4.20–6.92)	—	8.70 (5.21–14.55)
Other race	—	4.52 (3.58–5.70)	—	7.60 (4.92–11.78)
PIR
<1	—	Ref	—	Ref
1–4.9	—	0.98 (0.82–1.16)	—	1.15 (0.91–1.46)
≥5	—	0.87 (0.67–1.13)	—	0.81 (0.49–1.33)
Waist circumferences, cm
<88	—	Ref	—	Ref
≥88	—	1.77 (1.50–2.08)	—	1.91 (1.52–2.39)
Smoking
Current	—	Ref	—	Ref
Former	—	0.51 (0.39–0.66)	—	0.51 (0.31–0.84)
Never	—	0.90 (0.73–1.10)	—	0.77 (0.56–1.06)
Total recreational MVPA, MET min/wk
No recreational MVPA	—	Ref	—	Ref
1–499	—	0.79 (0.63–0.99)	—	0.53 (0.38–0.75)
500–1000	—	0.76 (0.56–1.03)	—	0.58 (0.40–0.86)
>1000	—	0.48 (0.39–0.60)	—	0.41 (0.30–0.57)

Abbreviations: CI, confidence interval; MET, metabolic equivalent; MVPA, moderate to vigorous physical activity; NH, non-Hispanic; OR, odds ratio; PA, physical activity; PIR, poverty income ratio; TPA, transportation PA of 10 minutes or longer. Note: Multivariable models were adjusted for age, race/ethnicity, poverty income ratio, waist circumference, smoking status, and total recreational PA level.

In the stratified analyses, ST was found to be a significant effect modifier, but not age or use of vitamin D supplements (P for interaction > .05). Figure 2 displays the associations of TPA with serum 25(OH)D levels stratified by daily ST. The association of TPA with serum vitamin D levels was stronger in participants who reported more than 8 hours of daily ST but it reached statistical significance only for vitamin D deficiency as the outcome (P for interaction = .19 for vitamin D insufficiency; P for interaction = .03 for vitamin D deficiency). Among those women with more than 8 hours of daily ST, the corresponding ORs for vitamin D insufficiency were 1.00 (reference; no transportation activity), 0.93 (95% CI, 0.64–1.38) for 1 to 499 MET minutes per week of TPA, 0.52 (0.34–0.79) for 500 to 1000 MET minutes per week of TPA, and 0.86 (0.49–1.50) for >1000 MET minutes per week of TPA (Figure 2A). For serum vitamin D deficiency (<30 nmol/L, Figure 2B), the corresponding ORs were 1.00 (reference; no transportation activity), 0.57 (95% CI, 0.38–0.84), 0.45 (0.24–0.86), and 0.30 (0.14–0.69), respectively.

Figure 2 —

The ORs and 95% CI of serum vitamin D insufficiency (<50 nmol/L) or deficiency (<30 nmol/L) according to TPA levels, by strata of ST. (A) Serum vitamin D insufficiency (<50 nmol/L). (B) Serum vitamin D deficiency (<30 nmol/L). The adjusted confounders included age, race/ethnicity, poverty income ratio, smoking status, waist circumference, and recreational physical activities. The error bars represent 95% CI. CI indicates confidence interval; OR, odds ratio; ST, sedentary time; TPA, transportation physical activity.

Sensitivity analyses using a serum vitamin D level of 75 nmol/L as the cutoff showed a consistent result. The corresponding ORs were 1.00 (reference; no transportation activity), 1.02 (95% CI, 0.77–1.34), 0.67 (0.48–0.95), and 1.16 (0.80–1.68), respectively. Next, we excluded women who did not perform any TPA and dichotomized TPA into 2 levels: the group performing at least some TPA (1–499 MET min/wk) versus the group fulfilling the PAGs (≥500 MET min/wk). Women who performed more than 500 MET minutes of TPA per week had significantly decreased odds of serum vitamin D insufficiency (<50 nmol/L) compared with women who performed less than 500 MET minutes of TPA per week (OR = 0.74, 95% CI, 0.57–0.96; P=.03).

Discussion

To the best of our knowledge, the detailed relationship between TPA and serum 25(OH)D levels has not been addressed in previous studies. From this large US representative sample, we found that among women aged 18–49 years, after accounting for recreational PA, participating in 500 to 1000 MET minutes of TPA per week was inversely associated with suboptimal vitamin D status. However, higher levels of TPA were not related to vitamin D status. The association of TPA with serum vitamin D levels was significant in participants who had more than 8 hours of daily ST, which suggest that for women who had sedentary lifestyles, incorporating any amount of TPA into daily life, even if it does not meet the PAGs, might be an important factor for the prevention of vitamin D deficiency. If the findings hold up in future studies of stronger design, women of childbearing age can be encouraged to participate in more TPA for optimal vitamin D status, which is likely to be beneficial for reproductive health.

Although the benefits of recreational PA have been documented, the impact of TPA on health was inconclusive.^16,17 Because of the distinctly different pattern compared with that of recreational physical activities,^38,39 TPA warrants further investigation regarding its health benefits. The American Heart Association has proposed participation in TPA to lower the risk of chronic diseases as a health promoting policy.⁴⁰ In the United States, several national initiatives in recent years have brought increased attention to TPA. For example, the Surgeon General’s report promotes walking across multiple PA domains including transportation, and encourages communities to improve access to safe and convenient places for walking and biking.⁴¹ The Federal Highway Administration developed a Strategic Agenda for Pedestrian and Bicycle Transportation, which aimed to guide federal pedestrian and bicycle activities.⁴² Despite these projects to promote TPA, data from various US surveillance systems have demonstrated a lower prevalence of participation in TPA among adults compared with that in Europe and Asia.^43,44 A cross-sectional study reported that a passive commute mode (driving and being a passenger) was associated with vitamin D deficiency,⁴⁵ but there was no study in the literature addressing the level of TPA and its relation to vitamin D status. Our study provides data showing a potential association of TPA with serum vitamin D levels. The quantified TPA used in our study is also helpful for making clinical recommendations.

Our study showed that women who had more TPA also had less ST. The finding may not only highlight the importance of TPA on its health impact but also on the behavior change point of view. Previous studies showed commensurate changes in total PA levels with changes in participation in active transportation.^46,47 In our study, we found approximately three-fourths of women of childbearing age did not participate in any active transportation. Participation in active transportation in the United States has been reported to be lower than that in many other developed nations. The reasons for the differences may be multifactorial, including residential density, land use planning, cultural norms, and cost of automobile ownership.⁴³ Furthermore, 50% of the US population did not meet the overall PAGs for aerobic PA, and total daily ST has increased by nearly an hour over the last decade. Including TPA in routine daily living could help individuals overcome barriers to engaging in PA. These findings could have noteworthy implications for prioritizing the bicycle and pedestrian infrastructure.

In line with previous surveys, over one-fourth of women in our study had vitamin D insufficiency.⁴⁸ The high prevalence of suboptimal vitamin D status worldwide, even in countries with low latitudes, has greatly impacted women’s health. Taking a vitamin D supplement has been shown to have a positive correlation with an adequate serum vitamin D level, but vitamin D insufficiency was still common in pregnant women who took prenatal vitamins as well as their breastfed infants.⁴⁹ On the basis of minimal sun exposure, a 15 mcg Recommended Dietary Allowance for vitamin D has been suggested for healthy women, including women during pregnancy and lactation.³⁰ However, over two-thirds of women in our study did not take vitamin D supplements with the recommended dose. Given the disparate findings regarding vitamin D supplementation and serum 25(OH)D levels, women may consider not only relying on taking vitamins but also increasing active transportation to maintain an optimal vitamin D status.

According to a recent clinical trial, a target serum 25(OH)D level for preconception, compared with that for gestation, was suggested to be 75 nmol/L or more to increase rates of pregnancy and livebirth.⁵⁰ Our sensitivity analysis found that performing 500 to 1000 MET minutes of TPA per week was significantly associated with lower odds of having a serum 25(OH)D level <75 nmol/L. Therefore, if our findings are replicated in studies with stronger design, TPA may be suggested among women of childbearing age to ensure the likelihood of optimal vitamin D status.

A previous study focused on recreational physical activities revealed a linear relationship between PA and serum 25(OH)D levels, in which higher PA was associated with greater odds of clinically sufficient serum vitamin D levels.⁵¹ Inconsistent with this previous study, we found that women with the highest level of TPA (>1000 MET min/wk) did not have lower odds of vitamin D insufficiency. One of the potential explanations may be that women with this level of TPA might expect more sun exposure during commute transportation and thus be more inclined to use sunscreen, which decreases sun absorption and serum vitamin D levels.⁵² Another reason might be related to exposure to more air pollution, as exposure to air pollution has been associated with serum vitamin D insufficiency.⁵³

To apply our findings to practice, 500 to 1000 MET minutes per week of TPA can be translated into approximately 150 to 300 minutes of TPA per week, which equals 30 to 60 minutes of brisk walking (4 miles per hour [mph]) or light effort of bicycling (10–12 mph) 5 days per week. Congruent with the present findings, a previous study of recreational PA suggested that at least 30 minutes per day of MVPA was the minimum amount of activity associated with a lower risk of vitamin D insufficiency.¹⁵ Our study was also consistent with the latest PAGs suggesting that beneficial health effects of PA can be accumulated through small bouts of activity and that at least 500 MET minutes per week of PA should be performed to obtain the greatest health benefits.

A strength of our study is the large sample size representative of the US population. The consistent and comprehensive collection of PA data allows for the quantification of the TPA level, which we categorized according to the current PAGs. Furthermore, a variety of sociodemographic and behavior- and health-related variables were included as potential confounders that increase the internal validity of our study. Sensitivity analyses also supported our main results. However, the study has some limitations. First, the design was cross-sectional, and thus, no causal effect can be inferred. Second, the self-reported PA might be subject to the risk of measurement error although this is the most common method in large-scale studies assessing PA. However, objective, device-assessed measurements of TPA are not likely practical for large studies; additionally, they cannot provide the context of the activity carried out (eg, recreational vs transportation). Third, the study only included TPA for more than 10 minutes in the analysis, which might underestimate the true amount of TPA carried out. While previous PAGs stipulated at least a 10-minute bout duration to “count” for heath, the latest PAGs no longer require a minimum duration. Finally, we did not take into account the change of waist circumference and PA levels in pregnant women; however, this is unlikely to have a large impact since only 3.76% of women in our study sample reported being pregnant. In addition, we acknowledge the potential role of additional factors, such as social environment, that may change over time and their influence on the association between active transportation and total serum 25(OH)D levels. Further studies will address this through the use of more advanced analytic approaches (eg, mixed effects models) in other population-based data sets. Also, future research that incorporates a longitudinal design and addresses residential region and sunlight exposure may decrease residual confounding.

Conclusions

A moderate level of TPA—equivalent to 30 to 60 minutes of brisk walking (4 mph) or light bicycling (10–12 mph) 5 days per week, but not more, may be related to lower odds of suboptimal vitamin D status among women of childbearing age. This level of TPA corresponds well with the amount of PA currently recommended by the PAGs. The association of TPA with serum vitamin D appeared stronger in those with more sitting time. Further studies with longitudinal design are needed to confirm these results.