Can Current Recommendations on Sun Exposure Sufficiently Increase Serum Vitamin D Level?: One-Month Randomized Clinical Trial

J Korean Med Sci. 2020 Mar 2;35(8):e50. doi: 10.3346/jkms.2020.35.e50.


Background: Lack of sunlight exposure is the primary reason for the worldwide epidemic of vitamin D deficiency. Although recommended sunlight exposure guidelines exist, there is no evidence regarding whether current guidelines are optimal for increasing vitamin D levels among individuals with vitamin D deficiency.

Methods: Sixty Korean adults aged 20-49 years with serum 25-hydroxyvitamin D (25[OH]D) levels of < 20 ng/mL were randomly assigned to three groups: sunlight exposure (n = 20), vitamin D supplementation groups (n = 20), and daily living (n = 20) for 1 month. The sunlight exposure group had sunlight exposure on 20% to 30% of their body surface areas for 30-60 minutes per day, 3 times a week during the summer season. Vitamin D supplementation was prescribed with 800 IU/day of vitamin D. The serum levels of 25(OH)D were measured at baseline and at 1-month follow-up examinations.

Results: The largest change in serum 25(OH)D was observed among the vitamin D supplementation group (+3.5 ng/mL, P < 0.001). The sunlight exposure group showed a slight increase in serum 25(OH)D level, but the absolute increase was less than one-third that of the vitamin D supplementation group (+0.9 ng/mL, P = 0.043). Only two participants in the sunlight exposure reached serum concentrations of 25(OH)D ≥ 20 ng/mL at follow-up. The daily living group showed no difference in vitamin D levels (-0.7 ng/mL, P = 0.516).

Conclusion: Sunlight exposure was not sufficient to overcome vitamin D insufficiency or deficiency in the current study subjects. Effectiveness of current sunlight exposure guidelines among various populations should be reassessed in larger clinical studies.

Trial registration: Clinical Research Information Service Identifier: KCT0002671.

Keywords: 1,25(OH)₂D; 25(OH)D; Randomized Clinical Trial; Sunlight Exposure; Vitamin D Deficiency; Vitamin D Supplementation.

Publication types

  • Randomized Controlled Trial

MeSH terms

  • Adult
  • Dietary Supplements
  • Female
  • Guidelines as Topic
  • Humans
  • Male
  • Middle Aged
  • Seasons
  • Sunlight*
  • Vitamin D / administration & dosage
  • Vitamin D / analogs & derivatives*
  • Vitamin D / blood
  • Young Adult


  • Vitamin D
  • 25-hydroxyvitamin D

Associated data

  • CRiS/KCT0002671