The relationship between physical activity and vitamin D status in postpartum lactating and formula-feeding women

Existing research shows an association between physical activity levels and vitamin D status in the elderly, men, women, children, and adolescent populations. This association has not yet been investigated in postpartum women. We hypothesized that based on the relationship between vitamin D and physical activity found in other populations, greater physical activity levels in postpartum women will be associated with higher serum 25(OH)D levels. A post hoc analysis of 286 postpartum women with self reported physical activity data from the America on the Move survey, and measured circulating serum 25(OH)D (measured by RIA) as an indicator of vitamin D status, was gathered at baseline (4-6 weeks postpartum), 4 months, and 7 months postpartum. The data were analyzed using SAS 9.4 (Cary, NC). 39.9% of women at visit 1 (baseline), 52.8% of women at visit 4 (month 4), and 55.9% of women at visit 7 (month 7) were meeting the NIH recommendation of 150min of moderate intensity (3-6 METs) physical activity per week. Significant differences were seen in physical activity by race (p=0.007). Caucasians were more likely to meet the standard recommendation than African Americans or Hispanics. Using multiple regression models to examine associations between duration of physical activity and 25(OH)D concentration, controlling for race, BMI, feeding type, and METs, it was found that at visit 1, an increase in physical activity was associated with an increase in 25(OH)D of 1.3nmol/L (p=0.03) and achieving at least 2.5h/wk of physical activity had a trending association with an increase in 25(OH)D of 7.23nmol/L (p=.05). At visit 4 (also controlling for treatment group and sun exposure) achieving at least 1.5h/wk of physical activity was associated with an increase in 25(OH)D of 11.73nmol/L (p=.04). By visit 7, no association between physical activity and maternal 25(OH)D was observed. In a repeated measures, mixed model analysis predicting maternal 25(OH)D during the study, achieving at least the recommended 150min per week of physical activity (>2.5h) was not significantly associated with vitamin D status (pNS). While no definitive conclusions can be drawn regarding precise levels of physical activity influencing 25(OH)D levels in postpartum women, the data suggest that increased activity during the first 4 months after birth is associated with improved vitamin D status. Additional research is needed because of the inconsistency seen at visit 7.