Vitamin D deficiency

Vitamin D deficiency, or hypovitaminosis D, most commonly results from inadequate sunlight exposure (in particular sunlight with adequate ultraviolet B rays).[1] Vitamin D deficiency can also be caused by inadequate nutritional intake of vitamin D, disorders limiting vitamin D absorption, and conditions impairing vitamin D conversion into active metabolites—including certain liver, kidney, and hereditary disorders.[2] Deficiency impairs bone mineralization, leading to bone softening diseases such as rickets in children. It can also worsen osteomalacia and osteoporosis in adults, leading to an increased risk of bone fractures.[1][2] Muscle weakness is also a common symptom of vitamin D deficiency, further increasing the risk of fall and bone fractures in adults.[1]

Ultraviolet B rays from sunlight is a large source of vitamin D. Fatty fish such as salmon, herring, and mackerel are also sources of vitamin D. Milk is often fortified with vitamin D and sometimes bread, juices, and other dairy products are also fortified with vitamin D as well. Many multivitamins now contain vitamin D in different amounts.[1]

Classifications

Mapping of several bone diseases onto levels of vitamin D (calcidiol) in the blood[3]
Normal bone vs. Osteoporosis

Vitamin D deficiency is typically diagnosed by measuring the concentration of the 25-hydroxyvitamin D in the blood, which is the most accurate measure of stores of vitamin D in the body.[4]

  • Deficiency: <20 ng/mL
  • Insufficient: 20–29 ng/mL
  • Normal: 30–100 ng/mL

Vitamin D levels falling within this normal range prevent clinical manifestations of vitamin D insufficiency as well as vitamin D toxicity from taking in too much vitamin D.[4]

Signs and symptoms

Child with rickets

Vitamin D deficiency can be asymptomatic, but may also cause several problems including:

  • Osteomalacia, a bone-thinning disorder that occurs exclusively in adults and is characterized by proximal muscle weakness and bone fragility.
  • Osteoporosis, a condition characterized by reduced bone mineral density and increased bone fragility.
  • Increased risk of fracture[5][6]
  • Rickets, a childhood disease characterized by impeded growth and deformity of the long bones.[7] The earliest sign of subclinical vitamin D deficiency is craniotabes, abnormal softening or thinning of the skull.[8]
  • Muscle aches and weakness [1]
  • Muscle twitching (fasciculations) is commonly seen due to reduced ionised calcium, arising from a low vitamin D.[9]
  • Light-headedness
  • Periodontitis, local inflammatory bone loss that can result in tooth loss.[10]
  • Pre-eclampsia: There has been an association of vitamin D deficiency and women who develop pre-eclampsia in pregnancy. The exact relationship of these conditions is not well understood.[11] Maternal vitamin D deficiency may affect the baby, causing overt bone disease from before birth and impairment of bone quality after birth.[7][12]
  • Depression: Hypovitaminosis D is a risk factor for depression. Some studies have found that low levels of vitamin D are correlated with depressed feelings and are found in patients who have been diagnosed with depression.[13]

Pathophysiology

Normal Synthesis of Vitamin D

Vitamin D deficiencies are often caused by decreased exposure of the skin to sunlight. People with a darker pigment of skin or increased amounts of melanin in their skin may have decreased production of Vitamin D.[1] Melanin absorbs ultraviolet B radiation from the sun and reduces vitamin D production.[1] Sunscreen can also reduce vitamin D production.[1] Medications may speed up the metabolism of vitamin D, causing a deficiency.[1] Some types of liver diseases and kidney diseases can decrease vitamin D production leading to a deficiency.[1] 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.[14]

In liver disease, the 25-hydroxyvitamin D may not be formed, leading to a vitamin D deficiency.[14] The kidneys are responsible for converting 25-hydroxyvitamin D to 1,25-hydroxyvitamin D.[14] This is the active form of vitamin D in the body. Kidney disease often prevents 1,25-hydroxyvitamin D from being formed, leading to a vitamin D deficiency.[14] Intestinal conditions that result in malabsorption of nutrients may also contribute to vitamin D deficiency by decreasing the amount of vitamin D absorbed via diet.[14] 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.[15] In states of hypocalcemia, calcium will leave the bones and may give rise to secondary hyperparathyroidism, which is a response by the body to increase serum calcium levels.[15] The body does this by increasing uptake of calcium by the kidneys and continuing to take calcium away from the bones.[15] If prolonged, this may lead to osteoporosis in adults and rickets in children.[15]

Risk factors

Those most likely to be affected by vitamin D deficiency are people with little exposure to sunlight.[16] Certain climates, dress habits, the avoidance of sun exposure and the use of too much sunscreen protection can all limit the production of vitamin D.[16]

Age

Elderly people have a higher risk of having a vitamin D deficiency due to a combination of several risk factors, including: decreased sunlight exposure, decreased intake of vitamin D in the diet, and decreased skin thickness which leads to further decreased absorption of vitamin D from sunlight.[17]

Malnutrition

Although rickets and osteomalacia are now rare in Britain, osteomalacia outbreaks in some immigrant communities included women with seemingly adequate daylight outdoor exposure wearing typical Western clothing.[18] Having darker skin and reduced exposure to sunshine did not produce rickets unless the diet deviated from a Western omnivore pattern characterized by high intakes of meat, fish, and eggs, and low intakes of high-extraction cereals.[19][20][21] In sunny countries where rickets occurs among older toddlers and children, vitamin D deficiency has been attributed to low dietary calcium intakes. This is characteristic of cereal-based diets with limited access to dairy products.[21] Rickets was formerly a major public health problem among the US population; in Denver, where ultraviolet rays are about 20% stronger than at sea level on the same latitude,[22] almost two-thirds of 500 children had mild rickets in the late 1920s.[23] An increase in the proportion of animal protein in the 20th-century American diet coupled with increased consumption of milk fortified with relatively small quantities of vitamin D coincided with a dramatic decline in the number of rickets cases.[24][25][26] One study of children in a hospital in Uganda however showed no significant difference in vitamin D levels of malnourished children compared to non-malnourished children. Because both groups were at risk due to darker skin pigmentation, both groups had vitamin D deficiency. Nutritional status did not appear to play a role in this study.[27]

Obesity

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.[28] The relationship between these conditions is not well understood. There are different factors that could contribute to this relationship, particularly diet and sunlight exposure.[28] Alternatively, vitamin D is fat-soluble therefore excess amounts can be stored in fat tissue and used during winter, when sun exposure is limited.[29]

Sun exposure

The use of sunscreen with a sun protection factor of 8 can theoretically inhibit more than 95% of vitamin D production in the skin.[24] In practice, however, sunscreen is applied so as to have a negligible effect on vitamin D status.[30] The vitamin D status of those in Australia and New Zealand is unlikely to have been affected by campaigns advocating sunscreen.[31] Instead, wearing clothing is more effective at reducing the amount of skin exposed to UVB and reducing natural vitamin D synthesis. Clothing which covers a large portion of the skin, when worn on a consistent and regular basis, such as the burqa, is correlated with lower vitamin D levels and an increased prevalence of hypovitaminosis D.[32]

Regions far from the equator have a high seasonal variation of the amount and intensity of sunlight. In the UK the prevalence of low vitamin D status in children and adolescents is found to be higher in winter than in summer.[33] Lifestyle factors such as indoor versus outdoor work and time spent in outdoor recreation play an important role.

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.[34]

Darker skin color

The reduced pigmentation of light-skinned individuals may result in higher vitamin D levels and that, because melanin acts like a sun-block, dark-skinned individuals, in particular, may require extra vitamin D to avoid deficiency at higher latitudes.[3] The natural selection hypothesis suggests that lighter skin color evolved to optimise vitamin D production in extreme northern and southern latitudes.[35]

Malabsorption

Rates of vitamin D deficiency are higher among people with untreated celiac disease,[36][37] inflammatory bowel disease, exocrine pancreatic insufficiency from cystic fibrosis, and short bowel syndrome,[37] which can all produce problems of malabsorption.

Critical illness

Vitamin D deficiency is associated with increased mortality in critical illness.[38] 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.[38] Additionally, vitamin D levels decline during stays in intensive care.[39] Vitamin D3 (cholecalciferol) or calcitriol given orally may reduce the mortality rate without significant adverse effects.[39]

Diagnosis

The serum concentration of 25(OH)D is typically used to determine vitamin D status. Most vitamin D is converted to 25(OH)D in the serum, giving an accurate picture of vitamin D status.[40]

The level of serum 1,25(OH)D is not usually used to determine vitamin D status because it often is regulated by other hormones in the body such as parathyroid hormone.[40] The levels of 1,25(OH)D can remain normal even when a person may be vitamin D deficient.[40]

Serum level of 25(OH)D is the laboratory test ordered to indicate whether or not a person has vitamin D deficiency or insufficiency.[40]

It is also considered reasonable to treat at-risk persons with vitamin D supplementation without checking the level of 25(OH)D in the serum, as vitamin D toxicity has only been rarely reported to occur.[40]

Levels of 25(OH)D that are consistently above 200 ng/mL (500 nmol/L) are thought to be potentially toxic, although data from humans are sparse. Vitamin D toxicity usually results from taking supplements in excess.[41] Hypercalcemia is often the cause of symptoms,[41] and levels of 25(OH)D above 150 ng/mL (375 nmol/L) are usually found, although in some cases 25(OH)D levels may appear to be normal. Periodic measurement of serum calcium in individuals receiving large doses of vitamin D is recommended.[2]

Screening

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.[42]

Treatment

In the United States and Canada as of 2016, the amount of vitamin D recommended is 400 IU per day for children, 600 IU per day for adults, and 800 IU per day for people over age 70.[43][44] 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.[45] Hypovitaminosis D is common in postmenopausal women, regardless of whether they are healthy or have other medical conditions.[46]

Treating vitamin D deficiency depends on the severity of the deficit.[47] 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.[47]

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).[48]

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,[49] other evidence indicates that D2 and D3 are equal for maintaining 25(OH)D status.[47]

Initial phase

Daily or weekly 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-months old infants, and 5,000 IU for patients over 1 year of age.[48]

For adults, other dosages have been called for. A review of 2008/2009 recommended dosages of 1,000 IU cholecalciferol per 10 ng/ml required serum increase, to be given daily over two to three months.[50] 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 nml/l) and, within certain body weight limits, to body weight.[51]

Single-dose therapy

Alternatively, a single-dose therapy is used for instance if there are concerns regarding the patient's compliance. The single-dose therapy can be given as an injection, but is normally given in form of an oral medication.[48]

Maintenance phase

Once the desired serum levels has been achieved, be it by a high daily or weekly 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.[48]

Special cases

To maintain blood levels of calcium, therapeutic vitamin D doses are sometimes administered (up to 100,000 IU or 2.5 mg daily) to patients who have had their parathyroid glands removed (most commonly kidney dialysis patients who have had tertiary hyperparathyroidism, but also to patients with primary hyperparathyroidism) or with hypoparathyroidism.[52] Patients with chronic liver disease or intestinal malabsorption disorders may also require larger doses of vitamin D (up to 40,000 IU or 1 mg (1000 micrograms) daily).

Epidemiology

The estimated percentage of the population with a vitamin D deficiency varies based on the threshold used to define a deficiency.

Percentage of

U.S. population

 Definition of

insufficiency

 Study Reference
69.5% 25(OH)D less than 30 ng/mL Chowdury et al. 2014 [53]
77% 25(OH)D less than 30 ng/mL Ginde et al. 2009 [54]
36% 25(OH)D less than 20 ng/mL Ginde et al. 2009 [54]
6% 25(OH)D less than 10 ng/mL Ginde et al. 2009 [54]

Recommendations for 25(OH)D serum levels vary across authorities, and probably vary based on factors like age; calculations for the epidemiology of vitamin D deficiency depend on the recommended level used.[55]

A 2011 Institute of Medicine (IOM) report set the sufficiency level at 20 ng/ml (50 nmol/l), while in the same year The Endocrine Society defined sufficient serum levels at 30 ng/ml and others have set the level as high as 60 ng/ml.[56] As of 2011 most reference labs used the 30 ng/ml standard.[56][57]:435[58]

Applying the IOM standard to NHANES data on serum levels, for the period from 1988 to 1994 22% of the US population was deficient, and 36% were deficient for the period between 2001 and 2004; applying the Endocrine Society standard, 55% of the US population was deficient between 1988 and 1994, and 77% were deficient for the period between 2001 and 2004.[56]

In 2011 the Centers for Disease Control and Prevention applied the IOM standard to NHANES data on serum levels collected between 2001 and 2006, and determined that 32% of Americans were deficient during that period (8% at risk of deficiency, and 24% at risk of inadequacy).[56][59]

History

The role of diet in the development of rickets was determined by Edward Mellanby between 1918 and 1920.[60] In 1921, Elmer McCollum identified an antirachitic substance found in certain fats that could prevent rickets. Because the newly discovered substance was the fourth vitamin identified, it was called vitamin D.[60] The 1928 Nobel Prize in Chemistry was awarded to Adolf Windaus, who discovered the steroid 7-dehydrocholesterol, the precursor of vitamin D.

Prior to the fortification of milk products with vitamin D, rickets was a major public health problem. 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.[24]

Research

A great deal of research has been conducted to understand whether low levels of vitamin D may cause or be a result of other conditions.

Some evidence suggests hypovitaminosis D may be associated with a worse outcome for some cancers, but evidence is insufficient to recommend that vitamin D be prescribed for people with cancer.[61] Taking vitamin D supplements has no significant effect on cancer risk.[62] Vitamin D3, however, appears to decrease the risk of death from cancer but concerns with the quality of the data exist.[63]

Vitamin D deficiency is thought to play a role in the pathogenesis of non-alcoholic fatty liver disease.[64][65]

Some studies have indicated that vitamin D deficiency may play a role in immunity. Those with vitamin D deficiency may have trouble fighting off certain types of infections. It has also been thought to correlated with cardiovascular disease, type 1 diabetes, type 2 diabetes, and some cancers.[66]

See also

References

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  2. 1 2 3 Vitamin D at Merck Manual of Diagnosis and Therapy Professional Edition
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