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Reference Guide
Vitamin D3
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Vitamin D3

Vitamin D, recognized for its essential role in human health, encompasses various forms crucial for physiological functions such as bone health, immune system regulation, and cellular growth. 

Vitamin D3, or cholecalciferol, is a key form of vitamin D that is not only vital for maintaining bone health but also plays significant roles in immune function, mood regulation, and disease prevention.

Vitamin D3, produced from skin exposure to sunlight and from animal dietary sources, differs from its counterpart Vitamin D2 in both its source and potency. 

This informational article provides an in-depth exploration of vitamin D3, covering its definition and function, as well as its clinical uses and indications. Additionally, it discusses the mechanism of action in the body and examines the available testing options for assessing vitamin D.  

Understanding Vitamin D3

What is Vitamin D3?

Vitamin D3 is one of the two main forms of Vitamin D, the other being Vitamin D2 (ergocalciferol). Unlike Vitamin D2, which primarily comes from plant sources and fortified foods, Vitamin D3 is synthesized in the human skin in response to sunlight exposure and is also found in certain animal-based foods.

1,25-dihydroxyvitamin D3 is the active form of vitamin D3 in the human body.  It plays a vital role in regulating calcium and phosphate levels which are essential for bone health, muscle function, and nerve transmission.  It also is important in immune function.  

1,25-dihydroxyvitamin D3 is synthesized in the kidneys through a series of enzymatic reactions from calcidiol, or 25(OH) vitamin D, which is considered a prohormone.  

Together with 1,25-dihydroxyvitamin D2, 1,25-dihydroxyvitamin D3 makes up the body’s available pool of active vitamin D, calcitriol.  [10., 17.] 

While both forms are effective at raising serum 25-hydroxyvitamin D levels, some studies suggest that vitamin D3 may be more potent and bioavailable than vitamin D2. However, the clinical significance of this difference remains a topic of ongoing research and debate.  [3., 4.]

Natural Sources of Vitamin D3

  • Sunlight: the body produces Vitamin D3 from cholesterol precursors when the skin is exposed to UVB rays from the sun.
  • Dietary Sources: foods such as fatty fish (e.g. salmon, mackerel, and tuna), fish liver oils, egg yolks, and fortified products like milk and cereal provide Vitamin D3.

Functions and Health Benefits of Vitamin D3

What is Vitamin D3 Good For?

The functions of active vitamin D2 are the same as the active form of vitamin D.  These include:  [1., 2., 8., 12., 17., 19.]

  • Bone Health: some text
    • Calcium Absorption: It's crucial for calcium absorption in the gut, which is necessary for maintaining healthy bones and teeth. This is particularly important in preventing conditions like rickets in children and osteoporosis in adults.  Adequate calcium in the blood is also essential for muscle and nerve function.
    • Phosphate Absorption: vitamin D also regulates calcium absorption in the GI tract
    • Balances bone resorption and remodeling for healthy growth and bone preservation in adulthood
  • Immune System Function: Vitamin D modulates the immune system.  Specifically, vitamin D promotes a healthy immune balance, prioritizing TH1 cellular immunity and the generation of CD4+CD25+ regulatory T cells (Tregs), essential in immune regulation and mitigating autoimmune conditions.  It also helps enhance the body’s natural defense against infections.   
  • Mood Regulation and Brain Health: There's growing evidence linking Vitamin D levels to mood regulation and a reduced risk of depression. It's also thought to play a key role in brain health and cognitive function.
  • Muscle and Nerve Function: Adequate Vitamin D levels are important for muscle strength, coordination, and proper nerve signaling.  Adequate levels of Vitamin D3 are important for muscle strength and coordination, potentially reducing the risk of falls and muscle weakness, especially in older adults.
  • Cardiovascular Health: Emerging research suggests Vitamin D might play a role in heart health, although more research is needed to confirm this association and understand optimal vitamin D levels.  [6.]

Recommended Vitamin D3 Intake and Supplementation

RDA of Vitamin D

The recommended dietary allowance (RDA) of Vitamin D varies depending on age, gender, and other factors. Generally, these guidelines are:  [8., 14.]

Infants (0-12 months): 400 IU (10 mcg) per day.

Children and Teenagers (1-18 years): 600 IU (15 mcg) per day.

Adults (19-70 years): 600 IU (15 mcg) per day.

Older Adults (over 70 years): 800 IU (20 mcg) per day.

Pregnant and Breastfeeding Women: 600 IU (15 mcg) per day.

Indications for Vitamin D3 Supplementation

Vitamin D3 is used to treat or prevent deficiencies, especially in populations with limited sun exposure or dietary restrictions.

Deficiency Symptoms 

Vitamin D3 supplements are prescribed to elevate and maintain adequate Vitamin D levels in the body. Vitamin D deficiency can lead to bone pain, muscle weakness, and increased risk of bone fractures. In severe cases, it can cause rickets in children and osteomalacia or osteoporosis in adults.  [2., 8.]

Association with Chronic Conditions 

Deficient levels of Vitamin D have been associated with various chronic conditions such as autoimmune diseases, osteoporosis, neurodegenerative and mood conditions, rickets in children and may be associated with cardiovascular problems.  [1., 2., 6., 8.]

Cardiovascular Health

Adequate Vitamin D levels may be associated with a reduced risk of heart disease, possibly by influencing blood pressure and vascular health.  [6.]

Cancer Prevention 

Some studies suggest a link between adequate Vitamin D levels and a reduced risk of development of certain cancers, particularly of advanced or fatal cancers.  This benefit may be more apparent in non obese people.  [5.]

Prevention in At-risk Groups 

People who are at risk of Vitamin D deficiency, such as those living in areas with limited sunlight, those following strict vegan diets, and the elderly, may benefit from Vitamin D3 supplementation.

Other Benefits of Vitamin D3 Supplementation

Supplementing with Vitamin D3 can benefit health and wellness in the following contexts:

  • Enhancing Bone Health: to prevent or treat bone-related diseases.
  • Boosting Immune Function: especially in individuals with lower exposure to sunlight or during winter months.
  • Mood Regulation: supplementation may help improve mood, especially in cases of Seasonal Affective Disorder (SAD).  [11.]

Factors to Consider in Supplementation

  • Form of vitamin D in supplements: vitamin D3 is often recommended due to its higher potency compared to Vitamin D2.  It is important to discuss the ideal form of vitamin D with your healthcare provider.
  • Source and quality of supplements: choosing high-quality supplements and consulting with healthcare providers for personalized advice.
  • Monitoring and adjustment: regular monitoring of Vitamin D levels via appropriate lab testing is crucial, especially for those on high-dose supplements, to avoid the risk of toxicity.

Personalized Vitamin D3 Supplementation

How Much Vitamin D3 Should I Take Daily? 

The appropriate dosage can vary. Individuals at risk of deficiency or those with specific health conditions might need higher doses.

While Vitamin D3 is beneficial, the amount and form of supplementation may vary based on individual needs. Factors such as age, lifestyle, geographic location, and existing health conditions should be considered when determining the optimal intake of Vitamin D3.

How Much Vitamin D3 Per Day Should a Woman Take?

Generally, adult women require 600-800 IU daily, but the amount can increase in cases like pregnancy or certain health conditions.

Testing Options for Vitamin D3 Levels

Laboratory Testing Methods

Vitamin D measurements are typically done via serum, although they may also be done via plasma or blood spot analysis.  Fasting or other special preparation is not typically required for a vitamin D test, but patients should consult their healthcare provider to confirm.  

Sample Collection and Preparation

Sample collection and preparation for testing vitamin D levels typically involve obtaining blood samples through venipuncture. 

Alternatively, blood spot testing for vitamin D involves collecting a small sample of blood from a finger prick onto filter paper. This method offers convenience for both patients and healthcare providers and can provide accurate measurements of vitamin D levels, making it useful for screening and monitoring vitamin D status.  A blood spot sample can be taken from the comfort of a patient’s home.  

Interpretation of Vitamin D Blood Tests

What is Measured in a Vitamin D Blood Test?

Vitamin D blood testing measures the concentration of specific forms of vitamin D in the blood. 

This typically includes the measurement of 25-hydroxyvitamin D (25(OH)D), which is the major circulating form of vitamin D and is considered the best indicator of overall vitamin D status as it reflects dietary intake and skin synthesis.  [8.]

Serum analysis of 25(OH) vitamin D comprises 25(OH) vitamin D2 and 25(OH) vitamin D3.  [8.]

Some labs may also measure the active form of vitamin D [1,25-dihydroxyvitamin D (1,25(OH)2D) or calcitriol] although this is less commonly done as it has a shorter half-life and may not reflect long-term vitamin D status as accurately as 25(OH)D, calcidiol.  

When calcitriol  levels are run, this often includes 1,25-dihydroxyvitamin D2 and 1,25-dihydroxyvitamin D3 levels.  [18.]  

Some labs offer the option to test for levels of vitamin D2 and vitamin D3 as separate fractions, which may be clinically useful to monitor treatment progress.  [10.]

Reference Ranges for Vitamin D in the Blood

Typical reference ranges for the total 25-OH vitamin D, measured in ng/mL, is 25-80 ng/mL.  [15.]

Results may also be reported as nmol/L, which has a reference range of 75-125 nmol/L.  [9.]

Many experts define vitamin D deficiency according to the following levels, although it is important to note that there is some discrepancy regarding vitamin D insufficiency:  [9.]

  • Severe deficiency: a serum 25-hydroxyvitamin D (25(OH)D) level below 12 ng/mL (30 nmol/L)
  • Deficiency:  a serum 25-hydroxyvitamin D (25(OH)D) level between 12-20 ng/mL (30-50 nmol/L)
  • Vitamin D sufficiency:  a serum 25-hydroxyvitamin D (25(OH)D) level above 30 ng/mL (75 nmol/L)
  • Vitamin D toxicity: serum 25(OH)D levels greater than 150 ng/mL (374 nmol/L)  [21.]

Why Do Doctors Prescribe Vitamin D2 Instead of D3?

There are several reasons why Vitamin D2 may be prescribed over D3 in certain scenarios:

  • Availability and Cost: Vitamin D2 is often more readily available in higher doses and can be more cost-effective or more fully covered by insurance than D3 supplements.
  • Dietary Preferences: For individuals following a strict vegan diet, Vitamin D2, being plant-derived, is often the preferred choice.
  • Specific Clinical Protocols: Certain treatment protocols or medical guidelines may specify the use of Vitamin D2 based on the nature of the condition being treated.
  • Individual Response: The effectiveness of Vitamin D2 can vary from person to person, and doctors often consider individual factors when prescribing it.

Related Biomarkers and Their Testing

To gain a comprehensive understanding of Vitamin D3 status and its implications for health, it's important to consider related biomarkers. This section explores additional tests that are commonly performed alongside Vitamin D3 testing.

Key Biomarkers Related to Vitamin D3

Several biomarkers work in conjunction with Vitamin D3 to regulate various bodily functions.  It may be valuable to test these, which are also easily assessed in blood tests:

  • Calcium: Since Vitamin D3 is crucial for calcium absorption, testing for blood calcium levels can offer insights into Vitamin D3's effectiveness in the body.
  • Parathyroid Hormone (PTH): PTH helps regulate calcium and phosphorus levels in the body, and its balance is influenced by Vitamin D3. Testing for PTH can provide valuable information in cases of bone metabolism disorders or imbalances in calcium levels.
  • Phosphate: Similar to calcium, phosphate levels are influenced by Vitamin D3, and their balance is vital for bone health.
  • Vitamin D2 Levels: Testing for Vitamin D2 alongside D3 may provide a complete picture of the body’s vitamin D status.
  • Genetic factors influencing vitamin D function and status: genetic factors such as GC, CYP2R1, VDR, and CYP24A1 SNPs may carry clinical relevance for some people.  [9.]  This may be done as a blood test or via cheek swab.

Diet and Lifestyle Factors to Support Vitamin D Function in the Body

Dietary Sources of Vitamin D  [14.]

Vitamin D3 is synthesized in the skin in humans, and it can also be obtained through the consumption of animal-based foods in the diet.

  • Fatty fish such as salmon, mackerel, and tuna are rich dietary sources of vitamin D.
  • Egg yolks and fortified foods such as fortified milk, orange juice, and cereals also contribute to dietary intake of vitamin D.
  • Some mushrooms, particularly those exposed to ultraviolet light, contain small amounts of vitamin D.  Mushrooms contain a compound called ergosterol that is converted to vitamin D2, ergocalciferol, in the presence of UV light.  [7.]

Sun Exposure and Vitamin D Synthesis

Exposure of the skin to sunlight triggers vitamin D synthesis: Vitamin D3 (cholecalciferol) is produced in human skin from 7-dehydrocholesterol upon exposure to sunlight.  

Sun exposure is most effective when the sun is high in the sky, typically between 10 a.m. and 3 p.m.  Other factors such as latitude, season, cloud cover, sunscreen use, and skin pigmentation influence the amount of vitamin D synthesized through sun exposure.

Lifestyle Habits for Optimal Bone Health

  • Regular weight-bearing and muscle-strengthening exercises, such as walking, jogging, weightlifting, and resistance training, promote bone health by stimulating bone formation and density.
  • Adequate intake of calcium and vitamin D through diet or supplements supports bone health by providing essential nutrients for bone formation and maintenance.
  • Avoiding smoking and excessive alcohol consumption helps maintain optimal bone health, as smoking and alcohol can negatively impact bone density and increase the risk of fractures.

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Vitamin D3
Vitamin D3, also known as cholecalciferol, is a special nutrient that your body makes when your skin gets sunlight, especially from the sun's ultraviolet B (UVB) rays. You can also find it in some foods like fatty fish and fortified dairy products, but it's hard to get enough just from eating. Vitamin D3 is super important for your overall health. It helps make your bones and teeth strong by helping your body absorb calcium and phosphorus from the food you eat. It also boosts your immune system, helping your body fight off germs and lower inflammation. What's really cool is that Vitamin D3 is not just a vitamin, it's also a hormone, which means it affects many different functions and helps keep your body running smoothly.
If Your Levels Are High
Having higher than normal levels of Vitamin D3 might mean that your body is taking in more of this nutrient than it usually needs. This can happen for various reasons, such as spending more time in the sun, eating foods rich in Vitamin D3, or taking supplements. Some medications, like certain anticonvulsants or steroids, can also increase Vitamin D3 absorption. While Vitamin D3 is crucial for many body functions, too much of it can mess with the balance of other nutrients like calcium and phosphorus. It's also worth noting that some health conditions could make your body produce or absorb more Vitamin D3 than it typically does.
Symptoms of High Levels
Symptoms of high levels of Vitamin D3 could include nausea, vomiting, weakness, and frequent urination. In some cases, it might also lead to bone pain and kidney problems.
If Your Levels are Low
Low levels of Vitamin D3 might mean that your body isn't getting enough sunlight, which is the main way we get this important nutrient. It could also mean that you're not eating enough foods rich in Vitamin D3, like fatty fish or fortified dairy products. Factors like living far from the equator, having darker skin, or using sunscreen often can also lead to lower Vitamin D3 levels. Some medications, like certain anti-seizure drugs, can mess with how your body processes Vitamin D3. And if you have a condition that makes it hard for your body to absorb fat, like Crohn's disease or celiac disease, that could also cause low levels of Vitamin D3, since it's a fat-soluble vitamin.
Symptoms of Low Levels
Symptoms of low levels of Vitamin D3 may include fatigue, bone pain, muscle weakness, mood changes, and frequent illness or infections.
See References

[1.] Akpınar Ş, Karadağ MG. Is Vitamin D Important in Anxiety or Depression? What Is the Truth? Curr Nutr Rep. 2022 Dec;11(4):675-681. doi: 10.1007/s13668-022-00441-0. Epub 2022 Sep 13. PMID: 36097104; PMCID: PMC9468237.

[2.] Bhattarai HK, Shrestha S, Rokka K, Shakya R. Vitamin D, Calcium, Parathyroid Hormone, and Sex Steroids in Bone Health and Effects of Aging. J Osteoporos. 2020 Jun 17;2020:9324505. doi: 10.1155/2020/9324505. PMID: 32612801; PMCID: PMC7317615.

[3.] Biancuzzo RM, Clarke N, Reitz RE, Travison TG, Holick MF. Serum concentrations of 1,25-dihydroxyvitamin D2 and 1,25-dihydroxyvitamin D3 in response to vitamin D2 and vitamin D3 supplementation. J Clin Endocrinol Metab. 2013 Mar;98(3):973-9. doi: 10.1210/jc.2012-2114. Epub 2013 Feb 5. PMID: 23386645; PMCID: PMC3590486.

[4.] Biondi P, Pepe J, Biamonte F, Occhiuto M, Parisi M, Demofonti C, Baffa V, Minisola S, Cipriani C. Oral calcidiol is a good form of vitamin D supplementation. Clin Cases Miner Bone Metab. 2017 May-Aug;14(2):207-208. doi: 10.11138/ccmbm/2017.14.1.207. Epub 2017 Oct 25. PMID: 29263735; PMCID: PMC5726211.

[5.] Chandler PD, Chen WY, Ajala ON, et al. Effect of Vitamin D3 Supplements on Development of Advanced Cancer. JAMA Network Open. 2020;3(11):e2025850. doi:

[6.] Cosentino N, Campodonico J, Milazzo V, De Metrio M, Brambilla M, Camera M, Marenzi G. Vitamin D and Cardiovascular Disease: Current Evidence and Future Perspectives. Nutrients. 2021; 13(10):3603. 

[7.] Haytowitz DB. Vitamin D in Mushrooms.; 2009. 

[8.] Institute of Medicine (US) Committee to Review Dietary Reference Intakes for Vitamin D and Calcium; Ross AC, Taylor CL, Yaktine AL, et al., editors. Dietary Reference Intakes for Calcium and Vitamin D. Washington (DC): National Academies Press (US); 2011. 3, Overview of Vitamin D. Available from: 

[9.] Kahwati LC, Weber RP, Pan H, et al. Vitamin D, Calcium, or Combined Supplementation for the Primary Prevention of Fractures in Community-Dwelling Adults: An Evidence Review for the U.S. Preventive Services Task Force [Internet]. Rockville (MD): Agency for Healthcare Research and Quality (US); 2018 Apr. (Evidence Synthesis, No. 160.) Appendix A Table 1, Serum Vitamin D Level Reference Ranges. Available from: 

[10.] Kennel KA, Drake MT, Hurley DL. Vitamin D deficiency in adults: when to test and how to treat. Mayo Clin Proc. 2010 Aug;85(8):752-7; quiz 757-8. doi: 10.4065/mcp.2010.0138. PMID: 20675513; PMCID: PMC2912737.

[11.] Lansdowne AT, Provost SC. Vitamin D3 enhances mood in healthy subjects during winter. Psychopharmacology (Berl). 1998 Feb;135(4):319-23. doi: 10.1007/s002130050517. PMID: 9539254.

[12.] Latimer CS, Brewer LD, Searcy JL, et al. Vitamin D prevents cognitive decline and enhances hippocampal synaptic function in aging rats. Proceedings of the National Academy of Sciences. 2014;111(41):E4359-E4366. doi:

[13.] Lung BE, Mowery ML, Komatsu DEE. Calcitriol. [Updated 2023 Jul 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: 

[14.] National Institutes of Health. Vitamin D. Published September 18, 2023.

[15.] Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic & Laboratory Test Reference. 14th ed. St. Louis, Mo: Elsevier; 2019.

[16.] Pike JW, Shevde NK, Hollis BW, Cooke NE, Zella LA. Vitamin D—binding protein influences total circulating levels of 1,25-dihydroxyvitamin D-3 but does not directly modulate the bioactive levels of the hormone in vivo. Endocrinology. 2008;149(7):3656–67.

[17.] PubChem. Calcitriol. 

[18.] Quest Diagnostics: Test Directory. Accessed March 12, 2024. 

[19.] Sîrbe C, Rednic S, Grama A, Pop TL. An Update on the Effects of Vitamin D on the Immune System and Autoimmune Diseases. Int J Mol Sci. 2022 Aug 29;23(17):9784. doi: 10.3390/ijms23179784. PMID: 36077185; PMCID: PMC9456003.

[20.] Vieth R. Vitamin D supplementation: cholecalciferol, calcifediol, and calcitriol. Eur J Clin Nutr. 2020 Nov;74(11):1493-1497. doi: 10.1038/s41430-020-0697-1. Epub 2020 Jul 23. PMID: 32704098.

[21.] Vitamin D3 25-Hydroxyvitamin D: Reference Range, Interpretation, Collection and Panels. eMedicine. Published online April 27, 2021. Accessed March 12, 2024.

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