Understanding Vitamin D - What is it? Where to find it? How much do I need?

Written by Claudia Knight and Dietetic Intern, Hannah Swope

Vitamin D is a vital hormone that helps keep our bones and immune system strong (1). We get vitamin D through foods including salmon, sardines, eggs, and fortified foods like milk and cereals (2). With exposure to sunlight, we also make vitamin D in our skin.

Sunlight activation of vitamin D synthesis is a multi-step process that primarily involves the skin, liver, and kidneys. Our skin contains a molecule that reacts with UV light from the sun. The UV light forms cholecalciferol, or vitamin D3 in our skin. This is the same form of vitamin D that you would get when eating fish and meat sources. Cholecalciferol travels through the liver to become calcidiol, or 25(OH)D, and is activated in the kidneys to calcitriol, (1,25(OH)2D), which is the usable form of vitamin D (1).

Why is vitamin D so important?

Table 1. Functions of Vitamin D in our bodies (1).

Bone	Brain	Cardiovascular	Immunity
Formation Strength Healing Nutrient absorption	Reduces oxidative stress Neuroprotection	Reduces blood pressure Heart health Arterial flexibility	Anti-inflammatory Protective of respiratory tract

Vitamin D is vital for normal functioning and growth. It helps with the absorption of calcium, which is essential for bone density/strength and cardiovascular function. Optimal calcium and vitamin D levels in the body are associated with reduced hip fractures and reduced cardiovascular disease risk (1). 

How Many People are Actually Deficient in Vitamin D?

About 50% of the global population is vitamin D deficient (3). Changes in society over time have led to habits that reduce the skin's exposure to sunlight, which has led to a reduction in vitamin D intake. Some of these changes include: 

• Spending time indoors

• Wearing long clothing and hats

• Sunscreen 

• Increased air pollution

  
  

Who is at a Higher Risk of Vitamin D Deficiency?

Populations at a higher risk for vitamin D deficiency include (2,4):

• Infants that are breastfed. 

  • Breast milk does not contain adequate vitamin D levels, so infants are often given a vitamin D supplement (2). 

• People with a darker complexion. 

  • More melanin in the skin causes darker complexions. Melanin blocks UV light from traveling through the skin, reducing vitamin D production (5). 

• Older individuals

  • As we age, our skin does not produce as many pre-vitamin D molecules and our kidneys become less effective. This effect is seen most strongly in people between 62 and 80 years old (5).

• People with obesity

  • Vitamin D is a fat-soluble vitamin that can be absorbed into fat cells. If you have a high concentration of fat cells, extra vitamin D is removed from the bloodstream, resulting in low blood levels (4).

• Vegan Diet 

  • Vitamin D-rich food sources come from animal products (meat and dairy products). If you are not consuming these foods, your risk for deficiency is increased.  

In children, vitamin D deficiency can lead to Rickets. Rickets is a disorder that alters bone development, leading to height restriction, altered rib cage development, and associated symptoms such as fractures and pneumonia (2). Osteomalacia and osteoporosis are the most common complications in adults, which can lead to falls and fractures (6).

Vitamin D Deficiency Diagnostic Criteria and Symptoms

Vitamin D testing is performed through a blood test. There is a lack of conscensus on exact criteria for defining deficiency that is used across the board.

The National Institute of Health (NIH) states:

"Serum concentrations of 25(OH)D that are associated with vitamin D deficiency have not been definitively identified. The Food and Nutrition Board at the National Academies of Sciences, Engineering, and Medicine states that levels of 50 nmol/L (20 ng/mL) or more are sufficient for most people, and that the risk of deficiency increases at serum concentrations of less than 30 nmol/L (12 ng/mL)." (11)

According to the article, Recognition and Management of Vitamin D Deficiency published by the American Family Physician, deficiency is defined as a result of less than 20 ng/mL and insufficient vitamin D status is defined as less than 30 ng/mL. Listed below are the most common symptoms of vitamin D deficiency which a person may or may not experience (8): 

• Bone pain and throbbing, especially in the pelvis and legs

• Poor balance

• Muscle aches and pains

• Muscle weakness

• Low back pain (more common in older women)

• Correlated with sleep conditions (9)

How much do you need?

The National Institutes of Health have established guidelines for the minimum amount of vitamin D necessary for most people to avoid deficiency. These are called the Recommended Daily Allowances (RDA).

Table #. Vitamin D RDAs for Americans (10)

Recommended Daily Allowance of Vitamin D through Life
Infancy	Children and Adults Aged 1-70	Adults Over 70
400 IU	600 IU	800 IU

New research has found that taking more than the RDA for vitamin D can provide health benefits. The Egyptian Academy for Bone and Muscle Health recommends that adults consume a minimum of 800 IU vitamin D per day (4,8)

Putting Vitamin D in Perspective - How much vitamin D is in food?

Table #. List of good food sources of vitamin D (2,5,8):

Good Food Sources of Vitamin D
Food	Serving Size	Amount of Vitamin D (IU)
Cooked salmon	3 ounces	570
Cooked swordfish	3 ounces	566
Cod liver oil	1 teaspoon	450
Canned sardines	1 cup	288
Canned tuna	3 ounce	68
Cooked egg	1 large	44
Cooked beef liver	3 ounces	40
Swiss cheese	1 cup	25
Fortified milk	1 cup	100
Fortified cereal	1 serving	100
Fruit juices	1 cup	100

Supplementing Vitamin D - Is this Necessary?

According to the article Vitamin D: Evidence-Based Health Benefits and Recommendations for Population Guidelines, optimal 25(OH)D levels are between 30-50 ng/mL, which ideally can be obtained from the diet alone (7). For various reasons (listed above) if one is not able to consume enough through diet alone, supplementation of 2,000 IU per day for healthy people can be helpful to achieve these values. However, for someone who is deficient, kickstarting optimal levels with higher doses may be necessary. Clinical doses of 50,000 IU per week for 6-8 weeks can be prescribed by a doctor for patients that are vitamin D deficient (6).

Vitamin D is a fat-soluble vitamin. To absorb vitamin D most efficiently, we need to consume supplements with fat (7). It is important to be cautious when taking supplements. Since vitamin D is a fat-soluble vitamin, so it can accumulate in our bodies. This means that a toxic and detrimental threshold is possible. Research has found that blood levels greater than 150 ng/mL 25(OH)D are indicative of toxicity (5,11). Symptoms were apparent within just a few weeks of supplementation at levels between 50,000 - 300,000 IU vitamin D per day (2). Symptoms of toxicity were similar to the symptoms of deficiency listed above. 

If you are concerned that your vitamin D status is deficient, ask your primary care provider to order a blood test to check these levels. It is easy to get adequate vitamin D through supplementation. The upper limit is 4,000 IU. Supplements are usually sold in 400-5,000 IU doses.

Conclusion

Vitamin D is necessary for many bodily functions, especially strong and healthy bones at all ages. We can get vitamin D through many food sources, primarily meat and dairy products. Vitamin D is also added to various foods through fortification to decrease risk of deficiency in the general population (7). Examples of fortified foods include milk, yogurt, cereal, juice.

However, most people do not consume enough vitamin D-rich foods or have exposure to direct sunlight to maintain adequate levels of vitamin D year-round. Daily supplementation of 2,000 IU of vitamin D with a meal will help to maintain adequate vitamin D levels for most healthy adults (7). If there is concern for vitamin D deficiency, whether you are feeling symptoms or not, your doctor can order a blood test for 25(OH)D levels. Adequate levels are 30-50 ng/mL. If you are deficient, you can work with your doctor and registered dietitian to boost and maintain your levels of vitamin D. You may experience better sleep, balance, and reduced aches and pains!

References

1. Rebelos, E., Tentolouris, N. & Jude, E. The Role of Vitamin D in Health and Disease: A Narrative Review on the Mechanisms Linking Vitamin D with Disease and the Effects of Supplementation. Drugs 83, 665–685 (2023). https://doi.org/10.1007/s40265-023-01875-8

2. Roth, D.E., Abrams, S.A., Aloia, J., Bergeron, G., Bourassa, M.W., Brown, K.H., Calvo, M.S., Cashman, K.D., Combs, G., De-Regil, L.M., Jefferds, M.E., Jones, K.S., Kapner, H., Martineau, A.R., Neufeld, L.M., Schleicher, R.L., Thacher, T.D. and Whiting, S.J. (2018), Global prevalence and disease burden of vitamin D deficiency: a roadmap for action in low- and middle-income countries. Ann. N.Y. Acad. Sci., 1430: 44-79. https://doi-org.ezproxy2.library.drexel.edu/10.1111/nyas.13968

3. Kaur J, Khare S, Sizar O, et al. Vitamin D Deficiency. [Updated 2025 Feb 15]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532266/

4. El Miedany, Y., Toth, M., Mohamed El Gaafary, M. et al. Vitamin D management update: evidence-based guidelines for vitamin D optimization by the Egyptian Academy for bone and muscle health. Egypt Rheumatol Rehabil 52, 34 (2025). https://doi.org/10.1186/s43166-025-00330-8

5. MichaelF. Holick, LoisY. Matsuoka, Jacobo Wortsman https://doi.org/10.1016/S0140-6736(89)91124-0, https://doi.org/10.1093/ajcn/87.4.1080S

6. Chang, Szu-Wen et al. Vitamin D and health - The missing vitamin in humans. Pediatrics & Neonatology, Volume 60, Issue 3, 237 - 244. https://www.pediatr-neonatol.com/article/S1875-9572(18)30651-X/fulltext

7. Grant WB, Wimalawansa SJ, Pludowski P, Cheng RZ. Vitamin D: Evidence-Based Health Benefits and Recommendations for Population Guidelines. Nutrients. 2025; 17(2):277. https://doi.org/10.3390/nu17020277

8. Bordelon P, Ghetu MV, Langan RC. Recognition and management of vitamin D deficiency. Am Fam Physician. 2009 Oct 15;80(8):841-6. Erratum in: Am Fam Physician. 2009 Dec 15;80(12):1357. PMID: 19835345.https://pubmed.ncbi.nlm.nih.gov/19835345/

9. Schiza S, Bouloukaki I, Kaditis A, Lombardi C, Bonsignore MR. Vitamin D deficiency: A forgotten aspect in sleep disorders? A critical update. Sleep Med. 2024 Sep;121:77-84. doi: 10.1016/j.sleep.2024.06.023. Epub 2024 Jun 24. PMID: 38941960. https://pubmed.ncbi.nlm.nih.gov/38941960/

10. NIH: https://www.ncbi.nlm.nih.gov/books/NBK56068/table/summarytables.t2/?report=objectonly

11. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/