Vitamin D is both a nutrient you eat as well as a hormone that is produced in your skin in response to sunlight; it plays a significant role in bone health because it helps regulate calcium and phosphorus— two essential minerals that work together to build strong bones.

This test evaluates 25-hydroxyvitamin D (25-D): the best measure of how much vitamin D is stored in your body. A prolonged deficiency in vitamin D can pose health risks. Vitamin D occurs naturally in very few foods so most people cannot get sufficient vitamin D through nutrition alone. If vitamin D levels are low, vitamin supplementation may be helpful, especially when access to natural sunlight is limited.