One reason it is difficult to establish recommended requirements for vitamin D is because:

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Individual exposure to sunlight varies greatly, making it difficult to establish a recommended requirement for vitamin D. Sunlight is the primary source of vitamin D, and the amount of vitamin D produced in the body from sunlight exposure depends on many factors such as location, time of day, skin color, and use of sunscreen. As a result, it is difficult to determine a specific amount of vitamin D that would be beneficial for everyone.

One Reason It Is Difficult to Establish Recommended Requirements for Vitamin