Association Between Vitamin D Status and Physical Function in the Severely Obese

Context: Mortality is 85% higher in severely obese subjects (body mass index [BMI] >40 kg/m(2)) than in subjects with a healthy BMI; poor physical function may be contributory. Hypovitaminosis D is common in obese subjects and is associated with physical dysfunction in the elderly. Objective: We determined the relationship between vitamin D status and physical function in severely obese subjects. Design, Setting, and Patients: We conducted a clinic-based, cross-sectional study of severely obese subjects. Participants were stratified into three groups according to the Institute of Medicine (IOM) vitamin D status categorization. Main Outcome Measures: We compared levels of self-reported activity and times taken to walk 500 m and to ascend and descend a 17-cm step 50 times. Results: We recruited 252 subjects (age, 43.7 +/- 11.2 y; BMI, 50.7 +/- 9.7 kg/m(2)); 25-hydroxyvitamin D (25OHD) concentrations were less than 30 nmol/L in 109 participants. Participants with a 25OHD >50 nmol/L, compared to those with a 25OHD <30 nmol/L, had the highest activity levels (3.1 +/- 3.4 h/wk versus 1.5 +/- 2.5 h/wk; P = .015) and the shortest 500-m walk times (6.2 +/- 1.1 min versus 7.4 +/- 1.5 min; P = .003). Serum 25OHD concentrations had a weakly positive association with activity level (r = 0.19; P = .008) and a moderately negative association with 500-m walk time (r = -0.343; P < .001). Conclusions: Vitamin D status had a significant relationship with physical activity and physical function in this cohort of severely obese subjects. Low activity levels are likely to perpetuate the problem of hypovitaminosis D due to less time spent outdoors. Studies exploring the effects of vitamin D supplementation in this population are warranted.