Human Nutrition: Letter Ann Nutr Metab 2020;76:86 Concerns with Vitamin D Supplementation and Synthesis for Cardiorespiratory Function Masahiro Ichikawa a Shunsuke Taito b Masahiro Banno c a Department of Endocrinology and Diabetes, Yokohama City University Medical Center, Yokohama, Japan; b Division of Rehabilitation, Department of Clinical Practice and Support, Hiroshima University Hospital, Hiroshima, Japan; c Department of Psychiatry, Seichiryo Hospital, Nagoya, Japan Received: February 8, 2020 Accepted: February 16, 2020 Published online: March 20, 2020 Masahiro Ichikawa Department of Endocrinology and Diabetes Yokohama City University Medical Center Urafunecho 4-57, Minami-ku, Yokohama 232-0024 (Japan) E-Mail masahiroichikawa1212 @gmail.com © 2020 S. Karger AG, Basel karger@karger.com www.karger.com/anm DOI: 10.1159/000507107 Dear Editor We read with great interest the arti- cle published by Sun et al. [1], in which the authors examined the effects of vita- min D 3 supplements on body composi- tion and physical fitness, including hand grip strength, leg extension power, and cardiorespiratory fitness in healthy adults. The authors reported that vita- min D 3 supplementation improved lean body mass but not physical fitness. How- ever, we have 2 concerns regarding this paper, which we would like to point out. First, higher doses of vitamin D 3 sup- plementation could have an effect on phys- ical fitness. A previous meta-analysis [2] demonstrated that vitamin D 3 supplemen- tation improved both upper and lower limb muscle strength in healthy individuals aged 18–40 years. The doses of vitamin D 3 reported in the previous meta-analysis [2] were between 14,000 and 60,000 IU/week, whereas the dose used in the study con- ducted by Sun et al. [1] was 2,940 IU/week. Therefore, increasing the vitamin D 3 dose could have resulted in improved muscle strength and cardiorespiratory fitness in their study. Second, information about the dura- tion of sun exposure, use of protective clothing such as long sleeves, and skin phototype (SPT) was not provided by the authors. Vitamin D 3 is synthesized through the action of ultraviolet radiation in the epidermis, and the effective sun- exposed skin area and SPT are also known to influence vitamin D 3 synthesis [3]. Ta- ble 1 in their study [1] shows that there were many female participants; as women often use protective clothing for cosmetic reasons, this could have affected the effec- tive sun-exposed skin area, possibly re- ducing vitamin D 3 synthesis. Moreover, Webb and Engelsen [4] reported that the duration of sun exposure required for vitamin D 3 production varies depend- ing on SPT. Therefore, reporting infor- mation about SPT would help estimate vitamin D 3 synthesis because although most Japanese individuals have SPT III, the SPT ranges from II to IV [3]. To clar- ify this, additional information, indicat- ing duration of sun exposure, use of pro- tective clothing, and SPT, should be re- ported. Acknowledgment We gratefully acknowledge Systematic Review Workshop Peer Support Group. Disclosure Statement The authors have no conflicts of inter- est to declare. Funding Sources The authors have no funding sources. Author Contributions M.I. wrote the manuscript. S.T. and M.B. revised the manuscript. All authors read and approved the manuscript. References 1 Sun X, Tanisawa K, Zhang Y, Ito T, Oshima S, Higuchi M, et al. Effect of Vitamin D Sup- plementation on Body Composition and Physical Fitness in Healthy Adults: A Double- Blind, Randomized Controlled Trial. Ann Nutr Metab. 2019;75(4):231–7. 2 Tomlinson PB, Joseph C, Angioi M. Effects of vitamin D supplementation on upper and low- er body muscle strength levels in healthy indi- viduals. A systematic review with meta-analy- sis. J Sci Med Sport. 2015 Sep;18(5):575–80. 3 Miyauchi M, Nakajima H. Determining an Effective UV Radiation Exposure Time for Vitamin D Synthesis in the Skin Without Risk to Health: Simplified Estimations from UV Observations. Photochem Photobiol. 2016 Nov; 92(6):863–9. 4 Webb AR, Engelsen O. Calculated ultraviolet exposure levels for a healthy vitamin D status. Photochem Photobiol. 2006 Nov-Dec; 82(6): 1697–703.