{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,2]],"date-time":"2026-10-02T04:17:38Z","timestamp":1790914658642,"version":"4.1.0"},"reference-count":42,"publisher":"MDPI AG","issue":"15","license":[{"start":{"date-parts":[[2026,7,28]],"date-time":"2026-07-28T00:00:00Z","timestamp":1785196800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Nutrients"],"abstract":"<jats:p>Background: Vitamin D is involved in musculoskeletal function, but evidence that supplementation improves athletic performance remains inconsistent, particularly in athletes with sufficient baseline vitamin D status. Methods: In this double-blind randomized controlled trial, 18 professional female soccer players were randomized during the autumn preparatory period (August\u2013September) to receive vitamin D3 (4000 IU\/day; n = 9) or placebo (n = 9) for eight weeks. Outcomes included total serum 25-hydroxyvitamin D [25(OH)D] and 1,25-dihydroxyvitamin D [1,25(OH)2D] concentrations, hematological variables, RAST total sprint time, 5- and 30-m sprint performance, and countermovement-jump outcomes. Results: At baseline, after summer exposure, 25% of participants had insufficient or deficient 25(OH)D concentrations (\u226430 ng\/mL). The remaining cohort (75%) had sufficient 25(OH)D concentrations (&gt;30 ng\/mL). After eight weeks, no statistically significant between-group differences were observed in vitamin D metabolites, hematological variables, or performance outcomes (\u039425(OH)D: SG +12.4 \u00b1 8.2 ng\/mL vs. PG +3.1 \u00b1 6.5 ng\/mL; p = 0.12). RAST total sprint time (p = 0.001) and 30-m sprint performance (p = 0.005) improved over time. Conclusions: Vitamin D3 supplementation at 4000 IU\/day for eight weeks was not associated with additional improvements in muscle strength, sprint performance, or countermovement-jump outcomes compared with placebo. Because most participants had sufficient baseline 25(OH)D concentrations, larger trials in female athletes with confirmed vitamin D insufficiency or deficiency are needed to determine whether individualized supplementation or longer intervention periods provide additional physiological or performance-related benefits. The trial was retrospectively registered at ClinicalTrials.gov (NCT07641075) on 8 June 2026.<\/jats:p>","DOI":"10.3390\/nu18152460","type":"journal-article","created":{"date-parts":[[2026,7,28]],"date-time":"2026-07-28T07:55:41Z","timestamp":1785225341000},"page":"2460","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["Eight-Week Vitamin D3 Supplementation at 4000 IU\/Day Was Not Associated with Further Improvements in Speed or Power Performance in Professional Female Soccer Players: A Randomized Controlled Trial"],"prefix":"10.3390","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-1590-3002","authenticated-orcid":false,"given":"Ma\u0142gorzata Magdalena","family":"Michalczyk","sequence":"first","affiliation":[{"name":"Institute of Sport Sciences, The Jerzy Kukuczka Academy of Physical Education in Katowice, Mikolowska 72a, 40-065 Katowice, Poland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9749-5278","authenticated-orcid":false,"given":"Mariola","family":"Gepfert","sequence":"additional","affiliation":[{"name":"Institute of Sport Sciences, The Jerzy Kukuczka Academy of Physical Education in Katowice, Mikolowska 72a, 40-065 Katowice, Poland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6551-9151","authenticated-orcid":false,"given":"Robert","family":"Roczniok","sequence":"additional","affiliation":[{"name":"Institute of Sport Sciences, The Jerzy Kukuczka Academy of Physical Education in Katowice, Mikolowska 72a, 40-065 Katowice, Poland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0153-4373","authenticated-orcid":false,"given":"Grzegorz","family":"Zydek","sequence":"additional","affiliation":[{"name":"Institute of Sport Sciences, The Jerzy Kukuczka Academy of Physical Education in Katowice, Mikolowska 72a, 40-065 Katowice, Poland"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,7,28]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"3","DOI":"10.1007\/s40279-017-0841-9","article-title":"Vitamin D and the Athlete: Current Perspectives and New Challenges","volume":"48","author":"Owens","year":"2018","journal-title":"Sports Med."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"249","DOI":"10.1080\/17461391.2016.1225823","article-title":"Vitamin D in Relation to Bone Health and Muscle Function in Young Female Soccer Players","volume":"17","author":"Yu","year":"2017","journal-title":"Eur. 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