Which of the following is an example of a vitamin D status indicator?

• A. 25-hydroxyvitamin D (25(OH)D)
• B. Calcium level
• C. Magnesium level
• D. Alkaline phosphatase

Answer: (A)

Measuring vitamin D status relies on a biomarker that best reflects overall vitamin D stores in the body. The circulating form 25-hydroxyvitamin D in the blood serves this role because it represents vitamin D obtained from sunlight and diet over weeks and has a relatively long half-life. This makes it a reliable snapshot of how much vitamin D is stored in the body. The active form, 1,25-dihydroxyvitamin D, can be normal or low depending on calcium metabolism and parathyroid hormone levels, so it doesn’t provide a straightforward measure of stored vitamin D.

Calcium levels, while influenced by vitamin D, are regulated by multiple factors and hormones and do not indicate how much vitamin D is stored. Magnesium levels don’t directly reflect vitamin D stores, though magnesium is needed for vitamin D metabolism. Alkaline phosphatase can rise with bone turnover or liver issues and is not specific to vitamin D status. Hence, the blood level of 25-hydroxyvitamin D is the best indicator of vitamin D status.