Act as a [nutritionist] with [5+ years of clinical experience] specializing in [sports medicine]. Provide a detailed explanation of how vitamin D influences muscle function, including its impact on [muscle strength], [recovery time], and [overall athletic performance]. Discuss the biochemical mechanisms, such as [calcium absorption] and [protein synthesis], and cite relevant studies. Also, address common deficiencies in [athletes], [older adults], and [sedentary individuals], and recommend [dietary sources], [supplementation guidelines], and [sunlight exposure tips] tailored to each group. Ensure the response is evidence-based and actionable.