Vitamin D insufficiency has been linked to a higher risk of URTI, the most common non-injury-related illness among athletes, "accounting for a 35% to 65% incidence rate during high-intensity training or in competition".
This led researchers at the University of Louisiana at Monroe, Kyung Hee University and Texas A&M University to conduct a study to assess the impact of vitamin D3 supplementation on salivary immune functions and symptoms of URTI in taekwondo athletes lacking in vitamin D.
Taekwondon't have enough D
They recruited 25 vitamin D-insufficient male taekwondo athletes between 19 and 22 years old and randomly placed them in two groups: 13 received 5,000IU of vitamin D3 each a day, while each of the remaining 12 received a placebo capsule daily, both during four weeks of winter training.
Pre- and post-tests were conducted three days before and after the intervention period, and blood samples were collected twice (during the pre- and post-tests) so the athletes' serum vitamin D concentration could be analysed.
At the same time, salivary samples were obtained from them pre-, mid- and post-tests for secretory immunoglobulin A (SIgA) and lactoferrin analyses, while symptoms of URTI were reported daily via questionnaire during the intervention period.
The researchers reported that in the supplemented group, serum vitamin D concentration rose by 255.6%, but remained unchanged in the placebo group; this change was linked to diminished URTI symptoms, including runny nose, sneezing, and coughing.
Both groups experienced a marked increase in SIgA — an immunological barrier against viral pathogens — but interestingly, only the placebo group exhibited an elevated salivary lactoferrin level in response to winter training.
Lactoferrin being one of the antimicrobial proteins that protect against respiratory infection, this may have been a sign of higher URTI risk, which led the bodies of the athletes in the placebo group to use it as a first line of defence against URTI.
The researchers also listed several limitations of the study, saying that despite its RCT design, its small sample size may have hampered the effectiveness of vitamin D3 supplementation against URTI in the athletes.
Recruiting only male athletes was another factor that may have limited the confirmation of vitamin D3's benefits.
Additionally, despite the study using a smaller number of questions (11, compared to other studies that had used 21 to 44 questions) to increase the completion rate, only 17 of the 25 participants completed the questionnaire. This may, in turn, have decreased the returning rate.
Previous studies have also reported low returning rates, and the definition of URTI symptoms could vary, according to lab measurements, self-reported colds, and differences in questionnaires.
However, the researchers added that though "clinically evaluated URTI episodes provide accurate information of true infections, the URTI symptoms evaluated by questionnaire should not be dismissed to identify the perceived conditions".
They concluded: "The present randomised controlled trial revealed that a daily dose of 5000IU of vitamin D3 increases serum 25(OH)D concentration to a sufficient level, and the increased serum 25(OH)D level reduces the URTI symptoms following four weeks of winter training in vitamin D-insufficient male taekwondo athletes.
"Vitamin D3 supplementation may be effective in reducing the symptoms of URTI during winter training in vitamin D-insufficient taekwondo athletes."
Source: International Journal of Environmental Research and Public Health
"Vitamin D3 Supplementation Reduces the Symptoms of Upper Respiratory Tract Infection during Winter Training in Vitamin D-Insufficient Taekwondo Athletes: A Randomized Controlled Trial"
Authors: Hyun Chul Jung, et al.