PTH and chronic kidney disease: Moreover, the KNHANES IX-1 dataset did not include a number of variables that could influence both vitamin D status and muscle health, such as duration of sunlight exposure, seasonal variation, ultraviolet index, season of blood sampling, geographic region, resistance exercise frequency, vitamin D supplementation, chronic diseases (e.g., chronic kidney disease, osteoarthritis, diabetes), medications (e.g., steroids, statins), parathyroid hormone and calcium levels, or endogenous hormone concentrations (e.g., testosterone, estradiol).