[1][4] Muscle weakness is also a common symptom of vitamin D deficiency, further increasing the risk of falls and bone fractures in adults.

[8] It may only be detected on blood tests but is the cause of some bone diseases and is associated with other conditions:[1] Those most likely to be affected by vitamin D deficiency are people with little exposure to sunlight.

[37] There is an increased risk of vitamin D deficiency in people who are considered overweight or obese based on their body mass index (BMI) measurement.

[38] Alternatively, vitamin D is fat-soluble, so excess amounts can be stored in fat tissue and used during winter when sun exposure is limited.

In the UK, the prevalence of low vitamin D status in children and adolescents is found to be higher in winter than in summer.

Additionally, vitamin D deficiency has been associated with urbanisation in terms of both air pollution, which blocks UV light, and an increase in the number of people working indoors.

The elderly are generally exposed to less UV light due to hospitalisation, immobility, institutionalisation, and being housebound, leading to decreased levels of vitamin D.[44] Because of melanin which enables natural sun protection, dark-skinned people are susceptible to vitamin D deficiency.

[45] Rates of vitamin D deficiency are higher among people with untreated celiac disease,[46][47] inflammatory bowel disease, exocrine pancreatic insufficiency from cystic fibrosis, and short bowel syndrome,[47] which can all produce problems of malabsorption.

[50] Vitamin D3 (cholecalciferol) or calcitriol given orally may reduce the mortality rate without significant adverse effects.

[50] Infants who exclusively breastfeed need a vitamin D supplement, especially if they have dark skin or have minimal sun exposure.

[1] In addition, a vitamin D deficiency may lead to decreased absorption of calcium by the intestines, resulting in increased production of osteoclasts that may break down a person's bone matrix.

Periodic measurement of serum calcium in individuals receiving large doses of vitamin D is recommended.

[4] The official recommendation from the United States Preventive Services Task Force is that for persons that do not fall within an at-risk population and are asymptomatic, there is not enough evidence to prove that there is any benefit in screening for vitamin D deficiency.

[63] 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").

A review of 2008/2009 recommended dosages of 1000 IU cholecalciferol per 10 ng/mL required serum increase, to be given daily over two to three months.

[66] In another proposed cholecalciferol loading dose guideline for vitamin D-deficient adults, a weekly dosage is given, up to a total amount that is proportional to the required serum increase (up to the level of 75 nmol/L) and within certain bodyweight limits, to body weight.

They concluded that supplementation with vitamin D can be equally achieved with daily, weekly, or monthly dosing frequencies.

Because vitamin D is fat-soluble, it is hypothesized that absorption would be improved if patients are instructed to take their supplement with a meal.

Raimundo et al.[73][74] performed different studies confirming that a high-fat meal increased the absorption of vitamin D3 as measured by serum 25(OH) D. A clinical report indicated that serum 25(OH) D levels increased by an average of 57% over a 2-month to 3-month period in 17 clinic patients after they were instructed to take their usual dose of vitamin D with the largest meal of the day.

Once the desired serum level has been achieved, be it by a high daily or weekly or monthly dose or by a single-dose therapy, the AAP recommendation calls for a maintenance supplementation of 400 IU for all age groups, with this dosage being doubled for premature infants, dark-skinned infants and children, children who reside in areas of limited sun exposure (>37.5° latitude), obese patients, and those on certain medications.

[78] Patients with chronic liver disease or intestinal malabsorption disorders may also require larger doses of vitamin D (up to 40000 IU, or 1 mg, daily).

Older people or those who have fatty liver or metabolic syndrome have a reduced ability to absorb vitamin D3.

[82][83] Since Vitamin D is fat-soluble, it's advised to be taken with a meal high in fat since it significantly increase its uptake in healthy individuals.

In the United States, milk has been fortified with 10 micrograms (400 IU) of vitamin D per quart since the 1930s, leading to a dramatic decline in the number of rickets cases.

[98] Vitamin D3, however, appears to decrease the risk of death from cancer but concerns with the quality of the data exist.

[100] Low levels of 25-hydroxyvitamin D, a routinely used marker for vitamin D, have been suggested as a contributing factor in increasing the risk the development and progression of various types of cancer.

This overexpression could lead to lower levels of active vitamin D in tissues, potentially promoting the development and progression of melanoma.

[100] Vitamin D deficiency is thought to play a role in the pathogenesis of non-alcoholic fatty liver disease.