This study aims to evaluate the effects of high-dose vitamin D3 supplementation on selected physiological, neuromuscular, and cognitive parameters in athletes. At the baseline visit, participants will undergo a series of assessments at the Functional Research Laboratory at AWF Katowice, including a 10 mL venous blood sample collection, body composition analysis (via InBody and DEXA scans), neuromuscular function evaluation using tensiomyography (TMG), and computerized cognitive tests via the Vienna Test System.

Following baseline testing, participants will be randomly assigned to either the vitamin D3 group or the placebo group. The active group will receive a daily dose of 4,000 IU of vitamin D3 (1 capsule with dinner), while the control group will receive an identical placebo. The intervention period will last for 4 or 8 weeks. Upon completion of the supplementation period, all baseline assessments will be repeated to evaluate the post-intervention changes.

For the first visit, the participant is invited to the Functional Research Laboratory at the Academy of Physical Education (AWF) in Katowice. The visit will be initiated with a blood sample collection (10 mL) from the cubital vein, which will be followed by body mass and body composition measurements. These measurements will be performed using the InBody device (in a standing position, in light clothing or underwear) and a DEXA scan (in a supine position, in light clothing), which is utilized to evaluate bone and fat tissue using a minimal dose of X-ray radiation.

As the next stage, a tensiomyographic (TMG) assessment will be conducted in a supine position and in underwear. Single-use electrodes will be attached to the biceps brachii (long head) of both upper limbs and the gastrocnemius (lateral head) of both lower limbs. A completely non-invasive, very low-voltage and low-intensity electrical current will be emitted by an electrostimulator in order to induce muscle contraction and measure its response speed. This assessment will be performed at rest, without a warm-up, is entirely harmless, and can be repeated.

Subsequently, cognitive function tests will be performed using the computerized Vienna Test System (SCHUHFRIED GmbH, Austria). The first test involves reading color-words and naming colors, while the second test involves reading words in an incongruent color and naming colors with an incongruent meaning. Responses will be provided by pressing one of four buttons on a specialized keyboard (time limit: 2500 ms per trial). Each diagnostic block (60 trials) will be preceded by a practice block (10 trials).

Following the completion of all assessments, the participant will be randomly assigned to one of two groups: the supplemented group (vitamin D3) or the placebo group. The randomization into groups will be carried out using balls placed in a black bag. Depending on the assigned group, and once the 25(OH)D3 level results are obtained by the research team (the next business day), the supplement in an appropriate dose or a placebo product will be allocated. The collection of the products will take place on the 3rd day from the start of the project. The dose of vitamin D3 will be of 4,000 IU/day. If assignment to the placebo group occurs during randomization, a placebo supplement will be provided. The supplements will be consumed 1 capsule in the evening (with dinner). The supplementation period will last for 4 or 8 weeks.

After 4 or 8 weeks of supplementation, the study will be concluded, at which point the participant will be invited to AWF again, where all assessments from stage 1 will be repeated.

Inclusion Criteria:

• a minimum of 8 years of training experience,
• absence of injuries in the 6 months leading up to the assessments,
• consistent engagement in training sessions a minimum of 5 times per week over the last 6 months.

Exclusion Criteria:

• vitamin D supplementation within one month prior to the study,
• use of multivitamin supplements,
• intestinal disorders.

Start data- 20.05.2026 End- 31.12.2028