Vitamins are a large and varied class of chemical compounds that are essential for human health but which the body cannot produce on its own. Vitamin D, a lipid-soluble prohormone, plays a crucial role in maintaining healthy bones and muscles by facilitating the body’s use of calcium and phosphate. We may get it via the sun’s UVB rays and can get it from fish, eggs, and cod liver oil. Vitamin D comes in two forms—D2 (ergocalciferol) and D3 (cholecalciferol)—the difference between them being in the structure of their side chains. The Institute of Medicine (previously the National Academy of Medicine) recommends that adults consume 600–800 IU of vitamin D daily. This dosage is often considered adequate for overall health maintenance and deficiency avoidance. Children aged 0–12 months have a daily RDA of 10 mcg (10 mcg = 400 IU), whereas those aged 1 year and above have an RDA of 15 mcg (15 mcg = 600 IU). Vitamin D levels in breast milk are notoriously low. Vitamin D deficiency and/or rickets are more common in infants who are nursed alone and who do not get enough time in the sun or other vitamin D-rich foods. Therefore, for healthy development and growth, infants need enough vitamin D. Overdosing on vitamin D supplements may lead to hypervitaminosis D, which has a low prevalence but a high mortality rate.