Vitamin D deficiency is also more common after surgical procedures that reduce absorption from the intestine, including weight loss procedures. Critical illness
Vitamin D deficiency is associated with increased mortality in critical illness. People who take vitamin D supplements before being admitted for intensive care are less likely to die than those who do not take vitamin D supplements. Additionally, vitamin D levels decline during stays in intensive care. Vitamin D3 (cholecalciferol) or calcitriol given orally may reduce the mortality rate without significant adverse effects. Breastfeeding
Infants who exclusively breastfeed need a vitamin D supplement, especially if they have dark skin or have minimal sun exposure. The American Academy of Pediatrics recommends that all breastfed infants receive 400 international units (IU) per day of oral vitamin D.

Pathophysiology
Decreased exposure of the skin to sunlight is a common cause of vitamin D deficiency. People with a darker skin pigment with increased amounts of melanin may have decreased production of vitamin D. Melanin absorbs ultraviolet B radiation from the sun and reduces vitamin D production. Sunscreen can also reduce vitamin D production. Medications may speed up the metabolism of vitamin D, causing a deficiency.The liver is required to transform vitamin D into 25-hydroxyvitamin D. This is an inactive metabolite of vitamin D but is a necessary precursor (building block) to create the active form of vitamin D.The kidneys are responsible for converting 25-hydroxyvitamin D to 1,25-hydroxyvitamin D. This is the active form of vitamin D in the body.
The Canadian Paediatric Society recommends that pregnant or breastfeeding women consider taking 2000 IU/day, that all babies who are exclusively breastfed receive a supplement of 400 IU/day, and that babies living north of 55°N get 800 IU/day from October to April.Treating vitamin D deficiency depends on the severity of the deficit. Treatment involves an initial high-dosage treatment phase until the required serum levels are reached, followed by the maintenance of the acquired levels. The lower the 25(OH)D serum concentration is before treatment, the higher is the dosage that is needed in order to quickly reach an acceptable serum level.The initial high-dosage treatment can be given on a daily or weekly basis or can be given in form of one or several single doses (also known as stoss therapy, from the German word Stoß, meaning "push").Therapy prescriptions vary, and there is no consensus yet on how best to arrive at an optimum serum level. While there is evidence that  vitamin D3 raises 25(OH)D blood levels more effectively than vitamin D2, other evidence indicates that D2 and D3 are equal for maintaining 25(OH)D status. Initial phase
Daily or weekly or monthly dose
For treating rickets, the American Academy of Pediatrics (AAP) has recommended that pediatric patients receive an initial two- to three-month treatment of "high-dose" vitamin D therapy. In this regime, the daily dose of cholecalciferol is 1,000 IU for newborns, 1,000 to 5,000 IU for 1- to 12-month-old infants, and 5,000 IU for patients over 1 year of age.For adults, other dosages have been called for.