Vitamin D is a fat-soluble vitamin critical for maintaining optimal health. Here are some key points about it:

1. Forms and Sources: Vitamin D comes in two main forms: Vitamin D2 (ergocalciferol) is found in plants and fungi, while Vitamin D3 (cholecalciferol) is synthesized by the skin upon exposure to sunlight and is present in animal-based foods^1^. Vitamin D can also be consumed through supplements and fortified foods.

2. Functions: Vitamin D helps the body absorb calcium and phosphorus, essential for forming and maintaining strong bones^2^. It also supports muscle, nerve, and immune system health^3^.

3. Sources: While sunlight is a significant source, only a few foods naturally contain Vitamin D. These include fatty fish (like salmon and mackerel), liver, egg yolks, and fortified foods such as some dairy products, cereals, and juices^4^.

4. Deficiency and Health Risks: Insufficient Vitamin D can lead to bone disorders such as rickets in children and osteomalacia in adults^4^. Additionally, low levels have been associated with an increased risk of certain chronic conditions, including osteoporosis, multiple sclerosis, cardiovascular diseases, and some cancers^3^.

5. Recommended Intake: The Recommended Dietary Allowance (RDA) for most adults (19-70 years) is 600 IU (15 mcg) per day, increasing to 800 IU (20 mcg) for those over 70^1^. The Tolerable Upper Intake Level for adults is 4,000 IU (100 mcg) daily^1^.

Summary: Vitamin D plays a vital role in bone health and overall physiological functions. It can be obtained from sunlight, diet, and supplements, and adequate intake is crucial to prevent deficiencies and related health issues.