Calcitriol is a man-made active form of vitamin D. Most people get enough vitamin D from exposure to the sun and from fortified food products (e.g., dairy products, vitamins). Vitamin D helps control parathyroid hormone and the levels of certain minerals (e.g., calcium, phosphorus) that are needed for building and keeping strong bones. Calcium citrate is used to prevent or treat low blood calcium levels in people who do not get enough calcium from their diets. vitamin K2, calcium regulation is disrupted. In fact, low levels of vitamin K2 are associated with an increased risk of heart disease and atherosclerosis.