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Vitamin D 101

Medically reviewed by 
Vitamin D 101

Many people are familiar with the “sunshine” vitamin, vitamin D, and often associate it with bone health. While that is true, vitamin D is much more than just a bone vitamin; vitamin D plays a role in regulating at least 1,000 genes. This article will discuss what vitamin D is, including its role in the body and the causes and symptoms of vitamin D deficiency. Additionally, this article will discuss how to test for vitamin D and good sources of vitamin D, including foods and supplements.


What is Vitamin D?

Vitamin D is a fat-soluble vitamin that we can obtain through sun exposure, food, and supplementation, although sun exposure is the largest source. When our bodies receive vitamin D, it has to go through two steps in order to be activated. The first step occurs in the liver, converting vitamin D into calcidiol, or 25-hydroxyvitamin D [25(OH)D]. The second step mainly occurs in the kidneys and takes 25-hydroxyvitamin D [25(OH)D] and converts it into calcitriol, or 1,25-dihydroxy vitamin D [1,25(OH)2D]. Many tissues in the body have receptors for vitamin D.

What is Vitamin D’s Role in The Body

Vitamin D has a vast effect on the body, affecting bones, inflammation and immune function, muscular function, and blood sugar.


Vitamin D affects bones in a few different ways. First, vitamin D enhances the absorption of calcium, a necessary component of bones, in the gastrointestinal tract. It also affects calcium levels and another critical bone mineral, phosphorus, in the blood, affecting bone mineralization. Cells of bones that influence the formation and remodeling of bones, called osteoblasts and osteoclasts, are also influenced by vitamin D.

Inflammation and Immune Function

Many chronic inflammatory diseases have been linked to low vitamin D levels, including inflammatory bowel disease (IBD), chronic kidney disease, asthma, atherosclerosis-related cardiovascular disease, and non-alcoholic fatty liver disease (NAFLD). Vitamin D can also support the immune response to acute infections. It is thought that vitamin D regulates the creation of inflammatory compounds and prevents the creation of pro-inflammatory cells.

Muscular Function

Muscle cells have vitamin D receptors as well as the enzyme that converts vitamin D into its active form. Vitamin D affects the mitochondria within muscle cells. Mitochondria are the parts of cells that produce energy, called ATP. Because of this, it is thought that vitamin D heralds the ability to affect muscular generation. Vitamin D has also been shown to improve muscular function.

Blood sugar

Vitamin D may play a role in blood sugar regulation. Blood sugar, or glucose, requires the hormone insulin in order to get into the cell, where it will then be converted into energy. For a number of different reasons, cells may become resistant to insulin, not allowing insulin, and thus, glucose, in. This process is referred to as insulin resistance.

What Causes Vitamin D Deficiency?

Vitamin D deficiencies occur when people have limited sunlight exposure, vitamin D absorption problems, and or diets devoid of the nutrient. Additionally, a deficiency can manifest if a person's kidneys cannot convert vitamin D into the active form.

Those at risk for a vitamin D deficiency include breastfed infants, as sun exposure is limited due to their sensitive skin and as breastmilk does not contain vitamin D. People with higher amounts of adipose tissue (fat tissue) and those with deeper skin tones are at a higher risk for vitamin D deficiency. Older adults tend to spend more time indoors, lessening the chance of sun exposure. Additionally, older adults' skin's ability to activate vitamin D decreases as we age. As vitamin D is a fat-soluble vitamin, it requires fat in order to be absorbed. Those with fat malabsorption issues, such as those with celiac disease, Crohn’s disease, ulcerative colitis, cystic fibrosis, and certain liver conditions, are at a greater risk for vitamin D deficiency. Those who have had gastric bypass surgery are also more likely to be vitamin D deficient, as the area of the intestine where vitamin D is absorbed may have been removed in the surgery. Those who wear clothing covering most of their bodies may have lower vitamin D levels. While sunscreen is important for reducing the risk of skin cancer, it’s use can also lead to lower vitamin D absorption and, thus, lower levels.

Vitamin D Deficiency Symptoms

Severe and or prolonged vitamin D deficiency can result in hypocalcemia or low blood calcium levels, as well as hyperparathyroidism or excess release of parathyroid hormones. Vitamin D deficiency symptoms include:

  • Fatigue
  • Depression
  • Muscle cramps and weakness
  • Heart rhythm irregularities
  • Seizures
  • Numbness and tingling around the mouth and in the extremities

In children, a disease called rickets is the result of a vitamin D deficiency. Rickets is characterized by soft bones, as the bones are not able to properly mineralize in the absence of vitamin D. This can also cause skeletal deformities and can be painful. If severe, it can lead to developmental delays, seizures, spasms, cardiomyopathy (heart struggles to pump blood), tooth deformities, and failure to thrive.

Osteomalacia is a condition characterized by existing bones becoming weak due to problems in mineralization during bone remodeling. This condition occurs in adults and adolescents and is due to a lack of vitamin D. Symptoms are similar to rickets and can include spasms, seizures, pain, and dental problems.

How to Test Vitamin D Levels

Vitamin D levels can be checked in the blood. Both calcidiol (25-hydroxycholecalciferol) and calcitriol (1,25-dihydroxycholecalciferol) can be checked in the blood, and both have their usefulness. Calcitriol, as offered by BioReference Laboratories as the 1,25-dihydroxy Vitamin D test, can be useful when assessing kidney function, as calcitriol will be converted into calcidiol in the kidneys, and thus levels reflect kidney function.

However, calcidiol is more commonly checked, as offered by Vibrant America as the 25-OH Vitamin D test. This test is more commonly ordered as it represents long-term vitamin D status since calcidiol is the storage form, and it has a longer half-life, meaning it stays in the body longer. Calcidiol can also be ordered as a fingerprick test, such as the vitamin D test offered by Ayumetrix. Levels of 25-OH vitamin D are described below.

Serum 25-OH Concentrations

Practitioners and patients alike should take care to pay attention to the units of measurement when interpreting 25-OH vitamin D lab results since a normal value in one unit can be abnormal in another, and thus misinterpretation can easily occur if not looking at reference values properly. Practitioners especially should know that there are two units of measurement when interpreting 25-OH vitamin D levels, especially when reading research papers.

It’s also important to note that most calcidiol, 25-hydroxycholecalciferol, tests are actually measuring both 25-hydroxycholecalciferol and vitamin D2, commonly called ergocalciferol. Vitamin D2 is the form of vitamin D found in some plants and fortified foods. It is also a form of supplemental vitamin D. Vitamin D2 will convert into calcidiol, which is why many tests measure both and include it as one value.

How to Make Sure You Are Getting Enough Vitamin D in Your Diet

The National Academies of Science, Engineering, and Medicine created the Food and Nutrition Board to create Dietary Reference Intakes (DRIs) for vitamins and minerals. One such DRI is the Recommended Dietary Allowance (RDA), which is defined as the average daily amount needed to reach nutritional adequacy in 97-98% of the healthy population. In the event there is insufficient evidence to form RDAs, Adequate Intake (AI) levels will be created. AIs are defined as levels assumed to reach nutritional adequacy.

The RDAs for vitamin D were created with bone health and calcium metabolism in mind and assuming people are receiving minimal sun exposure. RDAs for vitamin D are given in two different units: micrograms (mcg) and international units (IUs). For every 1 mcg of vitamin D, there are 40 IUs.

RDAs for Vitamin D

An AI was created for infants from birth to one year; for both males and females the AI is 10 mcg (400 IU).

Vitamin D can be found in foods and supplements.

Certain foods, including milk, breakfast cereals, and margarines have been fortified with vitamin D since the 1930s.

Food Sources of Vitamin D

Supplements containing vitamin D include multivitamins and stand-alone supplements. Vitamin D is found in both the D2 and D3 form in supplements, and both are well absorbed in the gut. However, D3 is the superior form, as it has been shown to increase serum levels more as well as keep serum levels higher longer. Dosages will range depending on age and vitamin D status. For example, for vitamin D deficiencies in adults, the Endocrine Society recommends a dose of 6,000IUs daily or 50,000 IUs once per week for at least six weeks. Once levels are out of the deficiency range, a maintenance dose of 1,500-2,000 IUs per day can begin.

Vitamin D toxicity can occur from supplementation, although it is rare; doses usually have to be above 10,000 IU/day for an extended period of time. Vitamin D toxicity can lead to elevated calcium levels (hypercalcemia), which can cause constipation, nausea, dehydration, and more.

Due to the lack of vitamin D found in breastmilk, the American Academy of Pediatrics recommends that breastfed or partially breastfed infants receive a 10 mcg (400 IU) vitamin D daily supplement. They also recommend at minimum 1,000 mL of vitamin D-fortified whole milk or formula for non-breastfed infants; if this requirement is not met, they then recommend a 10 mcg (400IU) daily supplement as well.



Vitamin D is an important fat-soluble vitamin that affects a wide array of processes in the body. Vitamin D status can be measured, and great care should be taken to ensure the level is properly interpreted, as reference ranges can change depending on age and units of measurement. Sun exposure, food, and supplement intake should be ascertained when evaluating vitamin D levels.

The information provided is not intended to be a substitute for professional medical advice. Always consult with your doctor or other qualified healthcare provider before taking any dietary supplement or making any changes to your diet or exercise routine.
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