Vitamin D is a fat-soluble vitamin that helps the body absorb and retain calcium and phosphorous, both the nutrients are very essential for the building of bones. Vitamin D is a nutrient that we eat and also a hormone that our bodies make. It is naturally present in few foods, and also available as a dietary supplement. It is also produced through a process of vitamin D synthesis when Ultraviolet (UV) rays from sunlight comes in contact with our skin.
Vitamin D obtained from sun exposure, foods, and supplements are biologically inert and must undergo two hydroxylations in the body for activation. The first hydroxylation, which occurs in the liver, converts vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as “calcidiol.” The second hydroxylation occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxy vitamin D [1,25(OH)2D], also known as “calcitriol”. 
Vitamin D supplements are available in two forms: vitamin D2 (“ergocalciferol” or pre-vitamin D) and vitamin D3 (“cholecalciferol”). Both these are naturally occurring forms and are produced in the presence of sun’s ultraviolet-b (UVB) rays. D2 is produced in plants and fungi and D3 in animals, including humans. Both forms are well absorbed in the small intestine.
Vitamin D Benefits
Studies have shown that Vitamin D can reduce cancer cell growth, reduce inflammation and also help control infections. It also helps in processes such as neuromuscular, immune function and glucose metabolism. Many of the body organs and tissues have receptors for Vitamin D, that can do major functions apart from bone development.
Vitamin D Recommended Daily Intake
The Recommended Dietary Allowance for adults 19 years and older is 600 IU daily for men and women, and for adults >70 years it is 800 IU daily.  For infants 0-12 months it is (400 IU) both male and female. 1-13 years is (600 IU) both male and female. Also during pregnancy and lactation it is (600 IU) 
The Tolerable Upper Intake Level is the maximum daily intake unlikely to cause harmful effects on health. The UL for vitamin D for adults and children ages 9+ is 4,000 IU. 
Vitamin D Deficiency
Many people have insufficient levels of Vitamin D, because they live in places where there is limited sunlight in winter, or also because they have limited exposure to sun as they stay inside most of the time. There are various reasons of deficiency that include lack in the diet, poor absorption, or having a metabolic need for higher amounts. Rickets, a condition in infants and small children caused by prolonged deficiency of Vitamin D. It leads to soft bones and skeletal deformities caused by failure of bone tissues to harden. There is also a condition called Osteomalacia in adults of weak and softened bones that can be reversed with supplementation.
Foods Rich in Vitamin D
Cremini mushrooms are commonly known as “baby bella” or “baby portobello” mushrooms. These mushrooms are dark brown and firmer compared to the white button mushrooms. Cremini mushrooms contain potassium, amino acids, vitamin D, selenium, riboflavin, phosphorus, zinc, folate, and manganese. They are considered very healthy. And if you are not a meat lover than including these in your meals would be a great idea. There are other mushrooms that are a rich source of vitamin D like maitake and shiitake mushrooms.
- Per 100g of raw cremini mushrooms has 31.9 µg of Vitamin D.