4. Discussion
insufficiency, Denniff, illness or injuries, ULs
VITAMIN D DEFICIENCY
The objective of this study was to compare the efficacy of individualized and standardized vitamin D supplement regimens for correcting insufficient 25(OH)D levels, as well as to determine the applicability of Groningen’s formula to athletes with possibly elevated requirements for vitamin D.The current supplementation study involved a cohort of young, healthy German athletes who were at an elevated risk of vitamin D deficiency during the winter months, which may negatively impact their athletic performance and health status. This study’s findings reiterate the limited impact of nutritional vitamin D intake on 25(OH)D levels, underscoring the challenges associated with achieving a sufficient vitamin D status through dietary means. Instead, this study suggests that vitamin D supplementation may be necessary during periods when vitamin D synthesis is limited.During the middle of winter, only 26% of the examined athletes had a sufficient vitamin D status (25(OH)D > 30 ng/mL). This observation is consistent with other studies conducted at similar latitudes that reported vitamin D insufficiency rates ranging from 40 to 60% [The results provide evidence that the formula established by Groningen et al. [Although the calculated loading dose using Groningen’s formula delivered good results, when split up into daily doses of 4000 IU, the mean 25(OH)D concentration decreased afterward, during the maintenance phase (37.9 ± 10.3 ng/mL). This indicated that the maintenance dose of 1000 IU/day was not high enough to ensure the target concentration of 40 ng/mL in athletes.As shown in This result is consistent with the findings of Backx et al. [In the INDV group, the mean 25(OH)D levels reached 41.1 ± 10.9 ng/mL at the calculated time point; on an individual basis, only 50% reached the target concentration of 40 ng/mL or more. This may be due to inter-individual differences. A closer look at the differences in the characteristics of those who reached 40 ng/mL and those who did not shows that the latter had a significantly higher weight than those who reached 40 ng/mL or more at the calculated time point (95%CI [−0.21, 18.66]), 68.5 vs. 77.7 kg, The findings of this study are consistent with those of previous research, which has demonstrated an inverse relationship between the serum 25(OH)D levels achieved using a fixed dose of oral cholecalciferol and both body weight and BMI. The mechanism underlying this relationship is believed to be a greater distribution volume resulting from increased body fat content [To date, there has not been a consensus on tolerable upper intake level (UL): actual guidelines vary between 4000 [Although supplementing with higher doses may expedite the correction of an inadequate vitamin D status, the administration of doses exceeding these ULs should be carefully considered as higher doses of supplementation may pose a heightened risk of overdosing, especially when athletes are not supervised individually. According to a study conducted by Arabi et al. [While the requirement for vitamin D in athletes has not been definitively established, several investigations have indicated a significant correlation between vitamin D insufficiency and athletic performance. These include power and strength parameters, such as vertical jump, muscle power, and mean power output in combat sports [Regular assessments of vitamin D status throughout the year are necessary to address the high prevalence of vitamin D insufficiency in athletes. In sports practice, the feasibility of such regular measurements can be increased by implementing volumetric absorptive microsampling technology (VAMSOne of the strengths of this study is the high individuality of the supplementation strategies analyzed. Each athlete in the individualized supplementation group was measured at the individually calculated time point of reaching a 25(OH)D concentration of 40 ng/mL. This was mainly achieved using a microsampling technique for blood collection, such as VAMSMoreover, only a few microliters of capillary blood are needed instead of needing to collect a few milliliters. Therefore, in practice, the frequency of performing venipuncture is a limiting factor and, thus, favors the use of microsampling in frequent sample collection. Compared to dried blood spots (DBS), autonomous sample collection is facilitated and accelerated in VAMSWhen analyzing whole blood samples, it is important to consider the influence of the hematocrit. Denniff and Spooner [Finally, it is imperative to acknowledge the limitations of this study. The generalizability of these results is subject to certain limitations. For instance, athletes with an average training volume of 7.4 ± 4.1 h per week were included in this study. Elite athletes with considerably higher training volumes may have produced slightly different results. Moreover, the current research was not specifically designed for specific sports disciplines; rather, it included athletes from different kinds of indoor and outdoor sports. Another limitation is the exact calculation of individualized supplementation. The baseline 25(OH)D concentration was assessed within a period of one week before the start of the 10-week study period. Therefore, the duration of the loading dose did not consider this one-week period in which vitamin D status could have declined further until supplementation started. This discrepancy may have influenced the efficiency of the application of the formula and may explain why not all athletes reached the goal status at the pre-calculated time points. It is also important to note that the samples were collected within 24 h and, in some cases, up to 48 h, after the pre-calculated threshold of 40 ng/mL was reached. Participants who received their final loading dose on the day of the second measurement may not have fully absorbed and metabolized it at the time of testing. It is worth mentioning that Chen et al. [Nonetheless, these issues need to be considered in the practical application of the formula because, in practice, it will be rare for the results of vitamin D status measurements to be available on the same day.Finally, this study did not consider the severity of vitamin D insufficiency. The supplementation provided to the INDV group was standardized to 4000 IU daily, which meant that athletes with severe deficiency did not receive a higher daily loading dose than athletes with mild insufficiency. As a result, these athletes had a much longer period with a deficient vitamin D status. It is appropriate to consider the potential utility of customizing the initial loading dose based on the degree of vitamin D insufficiency, such as increasing the daily dose up to 10,000 IU in a closely monitored environment.Although this study has some limitations, it emphasizes the importance of vitamin D supplementation strategies for athletes, especially when endogenous synthesis is impaired, as these requirements cannot be provided solely from nutrition. To prevent a possible decline in athletic performance and long failure times due to illness or injuries, athletes need to be informed and adequately supported regarding vitamin D. It has been previously stated that an individualized approach is warranted when correcting vitamin D deficiency [A potential strategy for incorporating vitamin D measurement and supplementation in sports practice could involve athletes measuring their 25(OH)D levels at the end of summer and during the winter months using a VAMS