Vitamin D and Calcium—An Overview, Review of Metabolism, and the Importance of Co-Supplementation

Vitamin D is a conditionally essential fat-soluble vitamin found in foods such as fish; fish oil; egg yolks; animal fats; some mushroom varieties; and fortified foods such as cheese, margarine, milk, infant formula, and some ready-to-eat cereals. Calcium (Ca) is found in milk, cheese, canned fish, ready-to-eat cereals, milk substitutes, dark green leafy vegetables, and sports drinks. There are more than fifty metabolites of vitamin D. Vitamin D participates in immune regulation, apoptosis induction, insulin secretion, inflammation, cell differentiation, calcium balance and regulation, bone mineralization, and phosphorus homeostasis. Ca is an essential macro-mineral involved in bone and teeth matrices, strength, and hardness; muscle and cardiovascular movement; neurological messaging; and in the release of hormones. Peer-reviewed journal articles were accessed from the search engine PubMed. The authors reviewed the references in the peer-reviewed journal articles, websites, and review articles if the authors proposed a new theory or mechanism. Vitamin D and Ca have important relationships; therefore, many factors may impede or interfere with the body’s ability to absorb or utilize vitamin D and or Ca and may result in low or excessive levels of each. Additionally, genetic/medically related absorption issues and low intake may also result in deficiencies. This review discusses the introduction of each, their functions, absorption, somatic transportation, the relationship between vitamin D and Ca, and recommendations and supplementation strategies if available.