Preface

The health of the musculoskeletal system depends on vitamin D since it controls the metabolism of calcium and phosphorus. For most people, sunlight with enough ultraviolet B (UVB) radiation is the primary source of it, as it is synthesized in the skin upon exposure. Foods and dietary supplements can also provide it. When exposure to sunlight containing UVB radiation is restricted or limited (as in the winter months), dietary sources become crucial (e.g., due to lack of time spent outdoors or little skin exposure).

Vitamin D is acquired by humans through their food and exposure to sunshine. Vitamin D is found naturally in very few foods. Vitamin D3 is abundant in oily fish, including sardines, salmon, and mackerel. Vitamin D is said to be present in egg yolks, yet the concentrations are somewhat varied. Moreover, egg yolks are a poor source of vitamin D due to their high cholesterol content. Additionally, a few foods—like milk, orange juice, and some bread and cereals—are fortified with vitamin D.

In two hydroxylation steps, vitamin D is transformed into its active metabolite, 1,25-dihydroxyvitamin D (1,25(OH)2D). The primary circulating metabolite of vitamin D, 25(OH)D, is produced in the liver during the first hydroxylation of vitamin D. It is frequently utilized as a biomarker of vitamin D status. In the kidney, 25(OH)D is converted into 1,25(OH)2D during the second hydroxylation. A deficiency of vitamin D is characterized by most specialists as a level of under 20 ng for each milliliter. In 1997, the Institute of Medicine of the US National Academy of Sciences recommended new adequate intakes for vitamin D as 200 IU for children and adults up to 50 years of age, 400 IU for adults 51 to 70 years of age, and 600 IU for adults 71 years of age or older. Vitamin D deficiency can be divided based on UBV, dark skin, being old, and latitude, season, and time of the day of UBV. The other category includes medical/physical conditions or any deficiency, such as fat malabsorption, obesity, chronic kidney disease, and use of medication (e.g. anticonvulsant).

The chapters below discuss the most updated research data available on Vitamin D and its relation to several chronic diseases.