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

•Serum 25(OH)D had decreased in the sunlight exposure group, but significantly increased in the combination group.•25(OH)D levels were significantly higher in the sunlight exposure with vitamin D supplementation group compared with the sunlight exposure group.•Serum PTH had decreased in both groups. 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. 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. 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. 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. 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.