Effects of sunlight exposure and vitamin D supplementation on vitamin D levels in postmenopausal women in rural Thailand: A randomized controlled trial

Background: Despite the abundant sunlight in Thailand, vitamin D deficiency is common in premenopausal and postmenopausal Thai women. Sunlight exposure is a natural way to increase one's intake of vitamin D. However, limited research has been conducted regarding natural exposure to sunlight as a strategy to improve vitamin D status in postmenopausal women. Objective: This study aimed to determine the effects of sunlight exposure compared with oral supplementation with vitamin D2 (weekly 20,000 IU) in combination with sunlight exposure on 25(OH)D levels. Methods: A 12-week randomized controlled trial was conducted in 52 postmenopausal women, age 50-70 years. The participants were randomized to either the sunlight exposure group or the sunlight exposure with vitamin D supplementation group. Serum 25(OH)D concentration and parathyroid hormone (PTH) were measured using standard assays at baseline and 12 weeks. Results: After 12 weeks, mean serum 25(OH)D had decreased from 32.3 to 29.7 ng/ml in the sunlight exposure group, but significantly increased in the combination group (from 29.9 to 32.4 ng/ml). At the end of the study, 25(OH)D levels were significantly higher in the sunlight exposure with vitamin D supplementation group compared with the sunlight exposure group. However, this difference was not observed in women aged > 60 years. Serum PTH had decreased in both groups, but not to a significant extent. Conclusion: A combination of weekly vitamin D2 supplementation at a dose of 20,000 IU with sunlight exposure is more effective than sunlight exposure alone in postmenopausal Thai women. Sunlight exposure alone is not sufficient to maintain 25-hydroxyvitamin D levels in this setting.