The active form of vitamin D (calcitriol) exerts its biological effects by binding to nuclear vitamin D receptors (VDRs), which are found in most human extraskeletal cells, including skeletal muscles. Vitamin D deficiency may cause deficits in strength, and lead to fatty degeneration of type II muscle fibers, which has been found to negatively correlate with physical performance. Vitamin D supplementation has been shown to improve vitamin D status and can positively affect skeletal muscles. The purpose of this study is to summarize the current evidence of the relationship between vitamin D, skeletal muscle function and physical performance in athletes. Additionally, we will discuss the effect of vitamin D supplementation on athletic performance in players. Further studies are necessary to fully characterize the underlying mechanisms of calcitriol action in the human skeletal muscle tissue, and to understand how these actions impact the athletic performance in athletes.

Vitamin D is synthesised in the skin during exposure to sunlight. The fundamental roles of vitamin D are the regulation of calcium and phosphate metabolism and bone mineralisation. Low vitamin D levels in athletes may adversely affect their exercise capabilities. The aim of our study was to investigate changes in serum levels of 25(OH)D3, calcium and bone turnover markers in football players in two training periods differing in the exposure to sunlight (after the summer period and after the winter period). We investigated 24 Polish professional soccer players. Serum levels of the following parameters were determined: 25(OH)D3, calcium, osteocalcin (OC), parathormone (PTH), procollagen type I N - terminal peptide (P1NP), and beta - CrossLaps (beta - CTx). We showed significantly higher levels of 25(OH)D3 and calcium and lower levels of PTH after the summer period versus the winter period. No significant differences in the levels of bone turnover markers were found. Furthermore, we did not observe any significant correlations between the levels of 25(OH)D3 and other parameters. Normal levels of 25(OH)D3 were observed in 50% of the players after the summer period and only in 16.7% of the players after the winter period. It is justified to measure the levels of 25(OH)D3, calcium and PTH in soccer players, especially after the winter period, when the exposure to sunlight is limited.

A well-balanced diet is one of the main factors that may play a supportive role in enhancing acute training stimuli in optimal training adaptation. The aim of the present study was to examine the energy and macro- and micronutrient intake including and excluding supplements among top-level Polish football players during one week of the general preparatory period. In addition, the study looked at whether athletes consume carbohydrates in recommended amounts, depending on the completed training sessions. A total of 26 professional football players were included in the study. The preseason dietary intake was assessed using a 7-day estimated food record. The energy value of the diet and the amounts of the dietary ingredients were assessed using the software Dieta 6.0. The average consumption of energy, vitamin B2, vitamin C, vitamin E, folate, and calcium was lower than recommendations, and average intake of sodium and potassium was higher than the norm in the diets of the athletes. The results of this study do not confirm the justification for adding protein preparations to diets of the studied players. Furthermore, football players dietary carbohydrate intake was relatively low in comparison to requirements based on training loads. Based on our results we conclude that further work is necessary to reinforce education about nutritional habits and adjust nutritional strategies to individual needs to enhance athletic performance.

Vitamin D is mainly known for its effects on the bone and calcium metabolism. The discovery of Vitamin D receptors in many extraskeletal cells suggests that it may also play a significant role in other organs and systems. The aim of our study was to assess the relationship between 25(OH)D3 levels, lower limb isokinetic strength and maximum oxygen uptake in well-trained professional football players. We enrolled 43 Polish premier league soccer players. The mean age was 22.7±5.3 years. Our study showed decreased serum 25(OH)D3 levels in 74.4% of the professional players. The results also demonstrated a lack of statistically significant correlation between 25(OH)D3 levels and lower limb muscle strength with the exception of peak torque of the left knee extensors at an angular velocity of 150°/s (r=0.41). No significant correlations were found between hand grip strength and maximum oxygen uptake. Based on our study we concluded that in well-trained professional soccer players, there was no correlation between serum levels of 25(OH)D3 and muscle strength or maximum oxygen uptake.

Carnosine is a dipeptide formed from the amino acids β-alanine and histidine and found in large amounts in the brain and muscle, especially fast twitch muscle. Carnosine has an antioxidant role and accounts for about 10% of the muscle's ability to buffer the H+ ions produced by high intensity exercise. Due to the interesting role of carnosine, the aim of the study was observe the effects of carnosine intake on pro-antioxidant status in highly trained athletes exposed to intense exercise.Fourteen male athletes from the Polish national kayak and canoe teams participated in placebo-controlled and cross-over study. The athletes were supplemented with 4 g/d carnosine for 14 days. Blood samples were collected before and 30 min, 24 h and 48 h after 2000 m exercise trial. In blood, hydrogen peroxide (H2O2), nitric oxide (NO), markers of RO/NS activity 8-isoprostanes and 3-nitrotyrosine, total (GSHt) and oxidised glutathione (GSSG), antioxidant status (APO) and superoxide dismutase (SOD) were determined. There were not observed statistically significant differences in exercise-induced changes in H2O2 and NO concentrations and SOD activity after carnosine intake. However, carnosine prevented an increase in 8-isoprostanes, 3-nitrotyrosine and GSSG concentrations as well as elevated redox status (GSHt-2GSSG)/GSSG at post-exercise period.Although, oral supplementation with 4 g carnosine did not affect RO/NS generation, it significantly attenuated exercise-induced glutathione loss, reduced oxidation/nitration markers concentration and SOD activity. These results suggest that carnosine could provide antioxidative protection for highly trained athletes.