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Dear editorWe read the article by Al-Eisa et al1 entitled “Correlation between vitamin D levels and muscle fatigue risk factors based on physical activity in healthy older adults,” regarding an important subject – the association among vitamin D deficiency, physical activity, and muscle fatigue; the findings of this study indicate that increased physical activity is associated with higher vitamin D levels and less fatigue. We would like to comment on this well-designed and performed study.In this study, significantly higher levels of vitamin D concentrations were reported in physically active participants compared with those with lower physical activity. However, there were no data on sunlight exposure or the seasons in which the study was conducted. Physically active participants may have higher exposure to sunlight compared with those with lower physical activity. Furthermore, subjects with higher physical activity may have a healthier diet behavior, which means they may have higher vitamin D intake by diet. If these data are not available, it is better to mention this as a limitation of the study.Second, the authors stated that they evaluated fatigue using the visual analog scale (VAS) as well. However, the “Methods” section indicates that VAS was used to assess pain. The authors should clearly mention if VAS was used to assess pain, fatigue, or both.Finally, the mean total and free calcium levels seem to be erroneously reported. In particular, physically active subjects seem to have severe hypercalcemia when their total and free calcium levels in Table 2 are taken into account.

Higher vitamin D levels may be associated with higher levels of sunlight exposure and higher intake of vitamin D by diet