Vitamin D: Dosage, Sources, Deficiency, Toxicity

Vitamin D is an essential vitamin your skin makes when exposed to sunlight. You can also get small amounts of it from food. People who can’t regularly get enough of this vitamin often turn to supplementation to avoid deficiency, especially over the winter months. In this post, we cover the best vitamin D dietary sources, supplement formulations, and safe dosage.

What is Vitamin D?

Definition

Vitamin D, also known as the “sunshine vitamin”, is an essential fat-soluble vitamin.

The body naturally makes vitamin D when exposed to sunlight. Getting regular, moderate sun exposure is a safe way to maintain normal vitamin D levels during the summer months.

Many factors can impact the amount of vitamin D your skin makes–including the time of day, season, sunscreen use, latitude, and your skin pigmentation.

Vitamin D is also found in certain foods, such as fatty fish like salmon and sardines. Milk and cereal are often fortified with vitamin D. Additionally, many vitamin D supplements are available on the market [1].

Vitamin D is an essential fat-soluble vitamin produced in the skin when exposed to sunlight. Small amounts are also found in foods like fatty fish and fortified cereal.

Snapshot

Essential fat-soluble vitamin
The skin makes it when exposed to sunlight
The best food sources include fatty fish and beef liver
Bile salts enable its absorption in the gut
Most supplements contain vitamin D3 (better than D2)
Deficiency is relatively common in the US (especially during the winter)
Adults under 70 years old require 600 IU/day
Adults over 70 years old require 800 IU/day

Role in the Body

Although classified as a vitamin, some scientists consider vitamin D a prohormone because it is involved in many metabolic processes in the body [2, 3].

Vitamin D is extremely important for bone health since it plays a role in calcium and phosphorus balance. It also helps balance nerve and muscle activity. Additionally, it supports heart health, immune function, and healthy sugar balance [4, 5, 6].

Scientists are also exploring its potential role in the prevention of cancer and chronic conditions such as autoimmune disorders, allergies, and infections [7].

Taken at the recommended doses, vitamin D supplements are considered safe. However, taking too much can be harmful. Vitamin D supplements may also interact with prescription medications. Remember to talk to your doctor before supplementing!

Vitamin D helps maintain strong bones, muscle and metabolic health and immune balance in the body. It’s considered safe taken at the recommended dosage.

Vitamin D Forms, Sources & Absorption

Active vs Inactive Forms

There are two forms of vitamin D: vitamin D2 and vitamin D3:

Vitamin D2 (ergocalciferol) is mostly human-made, added to foods, and produced in plants, fungi, and yeasts.
Vitamin D3 (cholecalciferol) is synthesized in the skin and found in animal-based foods [8].

Both forms of vitamin D are biologically inactive and must be activated in the body.

In the liver, vitamin D is converted to calcidiol (25-hydroxyvitamin D [25(OH)D]), the major circulating form of vitamin D, and the indicator of vitamin D status [9].

In the kidneys, calcidiol is converted to calcitriol (1,25-dihydroxy vitamin D [1,25(OH)2D]), the active form of vitamin D [8].

In order to regulate bodily functions, calcitriol binds to vitamin D receptor (VDR) in the cytoplasm of target cells, and goes to the nucleus and binds with the retinoic acid X receptor (RXR).

This complex regulates the expression of vitamin D regulated genes [10].

Vitamin D2 is mostly human-made and used to fortify foods. Vitamin D3 is produced in the skin and naturally found in animal-based foods.

Sources

Vitamin D levels can be increased and maintained by:

Regularly getting natural sunlight
Eating foods high in vitamin D (fortified or natural)
Supplementing

People who don’t get enough vitamin D through sunlight or dietary sources might need vitamin D supplements.

Maintain normal vitamin D levels by getting regular sun exposure and eating foods high in vitamin D. If you can’t achieve this, you might need to consider supplementing.

Sun

Most of the vitamin D3 in humans is derived from synthesis in the skin.

7-dehydrocholesterol, a precursor for vitamin D3, after irradiation with UVB light is converted to vitamin D3 [11, 12].

Factors affecting UV radiation exposure and vitamin D synthesis include the season of the year, latitude, clouds, smog, aging, skin color, use of sunscreen, clothing, and amount of skin exposed [12, 13].

We get most of our vitamin D from production in the skin during sun exposure.

Food

Very few foods in nature contain vitamin D.

Major food sources are fatty fish (salmon, tuna, mackerel, sardines), cod liver oil, beef liver, egg yolks, cheese, and mushrooms (shiitake, portabella) [12].

Some foods are fortified with vitamin D. These include milk, soy milk, yogurt, margarine, orange juice, and cereals [12].

Both the United States and Canada mandate the fortification of infant formula with vitamin D: 40 – 100 IU/100 kcal in the United States and 40 – 80 IU/100 kcal in Canada [14].

Foods don’t contain much vitamin D. The best dietary sources are cod liver oil, fatty fish like salmon and sardines, beef liver, and fortified foods.

Supplements

In supplements, vitamin D is available in two forms, D2 (ergocalciferol) and D3 (cholecalciferol) [12].

Vitamin D3 is approximately 87% more effective in raising and maintaining the vitamin D levels in the body than vitamin D2.

Vitamin D3 should be used for supplementation and fortification [15, 16, 17].

Most multivitamins contain 400 IU of vitamin D, and single-ingredient vitamin D supplements are available for additional supplementation.

Intake of ordinary doses of vitamin D3 supplements seems to be associated with decreases in the overall risk of dying [18].

Studies suggest that vitamin D3 is more effective at increasing vitamin D blood levels than the D2 form. Most supplements contain vitamin D3.

Absorption

Vitamin D taken orally is absorbed in the gut with the aid of bile salts.

It is transported to the blood by chylomicrons and taken up by the liver or storage tissues (fat tissue and muscles) [19].

There are many factors that affect vitamin D absorption resulting in large differences in the bioavailability of vitamin D supplements in some populations [20].

In patients with celiac disease, cystic fibrosis, biliary obstruction, and chronic pancreatitis, the absorption of vitamin D is decreased [21, 19, 22].

Also, individuals taking medications that bind bile acid (such as cholestyramine) will also have impaired vitamin D absorption [23].

Obesity is also associated with decreased vitamin D levels [24].

Vitamin D is most efficiently absorbed when consumed with food containing fat.

Taking vitamin D with the largest meal improves absorption, and results in a 50% increase in blood levels of vitamin D [25].

Bile salts help absorb vitamin D in the gut. Gut disorders, blocked bile flow, and bile-binding medications reduce vitamin D absorption.

Vitamin D Dosage

Infants and children aged 0-1 yr should have a daily intake of at least 400 IU of vitamin D [26, 13].

Children 1 yrs and older and adolescents should have a daily intake of 600 IU of vitamin D.

Adults aged 19 – 70 and over 70 yr require at least 600 and 800 IU/d, respectively, of vitamin D to maximize bone health and muscle function, and prevent falls and fractures.

Pregnant and lactating women require at least 600 IU/d of vitamin D.

Some health professionals recommend 2,000 IU or more a day, though no hard evidence supports this practice.

Some sources suggest that obese and children and adults on anticonvulsant medications, glucocorticoids, antifungals, and medications for AIDS should be given at least two to three times more vitamin D for their age group to satisfy their body’s vitamin D requirement.

The recommended vitamin D dose for children up to 1 year old is 400 IU, for ages 1-70 years 600 IU, and for people over 70 years old 800 IU.

Vitamin D Deficiency

Prevalence & Diagnosis

Vitamin D deficiency is the most common nutritional deficiency worldwide in both children and adults.

It had been estimated that 20% to 80% of people in the United States, Canada, Mexico, Europe, Middle East, and Asia have a vitamin D deficiency [27, 28, 29].

The blood level of 25(OH)D is the best method to determine vitamin D status [7].

Vitamin D deficiency is defined as a 25(OH)D level of 20 ng/mL or less, and vitamin D insufficiency as 21 – 29 ng/mL [30, 31, 13].

Studies suggest that blood 25(OH)D levels between 30 ng/mL and 60 ng/mL are associated with lower risks of adverse health outcomes, including cardiovascular disease, cancers and autoimmune diseases [7].

Vitamin D deficiency is fairly common in the United States, according to some estimates. It is defined as 25(OH)D blood levels of 20 ng/mL or below.

Causes

Vitamin D deficiency can come from:

Inadequate sun exposure
Geographical locations (latitude and altitudes), atmospheric conditions (air pollution, the presence of clouds), and seasonal changes influence the intensity of UVB radiation and thus vitamin D production in the skin [32].
People with dark skin, African Americans and Hispanic Americans. Higher skin melanin content reduces the skin’s ability to make vitamin D [33, 34].
Wearing protective clothing and hats and applying sunscreen reduce skin exposure to sunlight [12, 13, 35].
Women in the Middle East and North Africa who wear a headscarf or cover all skin. 96% had blood vitamin D levels less than 20 ng/mL, and 60% had vitamin D levels below 12 ng/mL [36, 37].
Malabsorption of vitamin D
Patients with gut disease including Crohn’s disease, ulcerous colitis, celiac disease, cystic fibrosis [21, 19, 22].
Cholestatic and non-cholestatic liver disease [19].
Low vitamin D intake
Low vitamin D diet without fortified food or supplements (38).
A strict vegan diet [39].
Chronic kidney disease leads to reduced synthesis of 1,25-dihydroxy-vitamin D and an increased loss of 25-hydroxyvitamin D in urine [40].
Obesity [41].
The elderly have a reduced capacity to synthesize vitamin D in the skin and are more likely to stay indoors or use sunscreen [13, 42].
Pregnant women [43].
Infants who are exclusively breastfed and do not receive vitamin D supplementation [44].
Medication: glucocorticoids, antiepileptic drugs, rifampin, antiretroviral therapy [45].

Many factors can contribute to vitamin D deficiency. Some of them include inadequate sun exposure, gut disorders, liver disease, kidney disease, strict vegan diets, obesity, and certain medications.

Symptoms

People who are vitamin D deficient may not have any symptoms at all or symptoms may be vague.

Vitamin D deficiency manifests as fatigue, symmetric low back pain, throbbing bone pain, muscle weakness and aches, sweating, digestive problems, obesity, mood swings, and impaired immunity.

These symptoms are often misdiagnosed as fibromyalgia, chronic fatigue, age-related weakness, or even depression [46, 47, 48].

Severe vitamin D deficiency leads to rickets in children and osteomalacia in adults [44, 49].

Vitamin D deficiency may cause fatigue, back and bone pain, mood problems, and muscle weakness. Not all people with deficiency experience symptoms.

Vitamin D Toxicity & Contraindications

Toxicity

Vitamin D toxicity is an extremely rare but potentially serious condition.

It occurs from high intakes of supplements (>50,000 IU/d) for a long period of time.

Toxicity does not occur from dietary intake or prolonged sun exposure [50].

Vitamin D is a fat-soluble vitamin, and it is stored in fat tissues. Therefore, toxic effects may last for months after stopping supplementation [51].

High vitamin D levels lead to tissue calcification, and kidney and cardiovascular damage [50, 51, 52].

Symptoms and signs of vitamin D intoxication are headaches, metallic taste, poor appetite, nausea, vomiting, dehydration, constipation, pancreatitis, and kidney stones [50, 51, 53, 54].

Though vitamin D toxicity is rare, it can happen in people who take high vitamin D doses–well above the recommended intake. Toxicity can last for months and cause serious damage to the body.

Contraindications

Vitamin D supplementation must be considered carefully in patients with granulomatous diseases such as sarcoidosis and Crohn’s disease (active phase), metastatic bone disease, and Williams syndrome (infantile hypercalcemia) [55, 56, 57, 58].

Takeaway

Vitamin D is an essential fat-soluble vitamin important for bone and metabolic health and immunity. The skin makes vitamin D when exposed to natural sunlight. Small amounts of it are also naturally found in some foods like salmon, sardines, and beef liver. Milk and orange juice are often fortified with vitamin D. The recommended dose for children up to 12 months is 400 IU, for ages 1-70 years 600 IU, and for people over 70 years old 800 IU. Estimates suggest that vitamin D deficiency is fairly common in the US. People who can’t get enough vitamin D from sun exposure and diet may need to consider supplementing. Gut and liver disorders, obesity, and certain medications can lower vitamin D absorption and blood levels. Supplementing is generally safe, though intake above the recommended doses can be harmful.