Vitamin D

by | Mar 27, 2020


Vitamin D is necessary for calcium regulation and immunity, among other functions. There are actually five different forms of vitamin D; we humans use the second and third forms (D2 and D3). Neither of these forms are active on their own, so you convert them into the active form (1,25-vitamin D) in your liver and kidneys. This active form then works on cells throughout your body.  


Vitamin D activation

Inactive vitamin D2 and vitamin D3 are both converted into 25-vitamin D in the liver, then into the active form 1,25-vitamin D in the kidneys


Vitamin D tells your small intestine to absorb more calcium from your food and your kidneys to excrete less calcium in the urine. Without enough vitamin D, your body cannot regulate calcium levels, so you may become calcium deficient even if you get enough calcium in your diet.1 

Vitamin D also helps regulate your immune system. Your white blood cells (leukocytes) use it to fight germs better and it helps prevent your immune cells from attacking your own body (called “autoimmunity”).2,3


The only way to diagnose a vitamin D deficiency is with a blood test. This is included in my nutrition checkups.

Vitamin D deficiency is very common; one estimate says 40% of American adults are deficient.4 This number may not be completely accurate, because the study population wasn’t a true sample of the whole population, but it does show there are a lot of people who aren’t getting enough vitamin D.

People who are deficient in vitamin D need a lot of calcium (additional 1,000 mg/day) to reduce their risk of osteoporosis, but people who aren’t deficient don’t need as much calcium (additional 400 mg/day) to reduce their risk, which fits with vitamin D’s role in regulating calcium.5 Some studies say that vitamin D doesn’t help reduce the risk of osteoporosis or fracture, but these studies don’t all see if the patients were deficient in the first place, and they only used 1,000 mg of calcium instead of comparing it to 400 mg of calcium.6,7

Because of its role in regulating the immune system, people with low levels of vitamin D are more likely to get sick as well as develop autoimmune diseases.8,9,10,11

There is also some evidence that links low levels of vitamin D and depression, although this is still being researched.12 Nevertheless, low levels of vitamin D are serious and should be treated.

Daily Need

Since vitamin D can easily enter your cells, blood tests are a reliable measurement of your status. The healthy range for your blood level of vitamin D is between 50 and 120 nmol/L (20 and 48 ng/mL).13,14 If your level is too low, you need more vitamin D. If it’s too high, you need less.

The Recommended Daily Allowance (RDA) for vitamin D is currently 600 International Units (IUs) a day. Other sources recommend 2,000 IUs a day, but I am unable to find any evidence supporting this specific number. Regardless, the goal is for your blood level to be in the healthy range, so it is important to get your blood level tested regularly and adjust your intake as needed. 


Vitamin D is not present in many foods. Oily fish have some vitamin D and egg yolks have a small amount. Cod liver oil has quite a bit, as can mushrooms, but only if they’ve been exposed to UV light. Many foods are fortified with vitamin D, such as some milk and cereals. This may or may not be enough to meet your needs.

If you do not have enough vitamin D, it’s easiest (and cheapest) to supplement it. To get 2,000 IUs of vitamin D every day, you could swallow four teaspoons of cod liver oil, eat two cups of raw mushrooms that have been exposed to UV radiation, or take one 21¢ pill. Personally, I prefer the one pill a day, so that’s what I recommend to patients. But if you’d rather use cod liver oil or mushrooms, that works too.

  • Vitamin D3 supplement – 2,000 IUs
  • Mushrooms, raw, 1 cup (UV) – 1,004 IUs
  • Mushrooms, raw, 1 cup (no UV) – 7 IUs
  • Cod liver oil, 1 tsp – 450 IUs
  • Atlantic salmon, farmed, raw, 3oz – 375 IUs
  • Sardines, canned, drained, 3oz – 157 IUs
  • Whole milk, fortified, 1 cup – 124 IUs
  • Egg yolk – 36 IUs

Optimal intake: 600 IUs a day or more if needed to normalize blood levels. Information from the USDA and NCCDB nutritional databases.

Vitamin D3 (cholecalciferol) can be made in your skin when exposed to the ultraviolet (UV) light in sunlight. However, this often isn’t enough to meet your needs and can cause skin cancer.15 High levels of UV radiation definitely cause cancer, but no safe limit for UV light has been established. That means I can’t say “as long as you get less than this much sunlight, you won’t get cancer.” We don’t know how much is safe. And the vitamin D3 that is made in the skin is the exact same molecule that you get from vitamin D3 supplements. Therefore, I do not recommend any sun exposure for vitamin D deficiency and instead recommend that people who are deficient use supplements to get their vitamin D.

Vitamin D3 (cholecalciferol) chemical structure

Vitamin D3 (cholecalciferol) produced in the skin by sunlight

Vitamin D3 (cholecalciferol) from supplement chemical structure

Vitamin D3 (cholecalciferol) from a vitamin D3 supplement


Vitamin D supplements are available over the counter, but it’s dangerous to get too much vitamin D. A deficiency requires blood tests to diagnose and monitor and treatment should only be done under medical supervision. This is included in the nutrition checkups I offer.


1. Steingrimsdottir L, Gunnarsson O, Indridason OS, Franzson L, Sigurdsson G. Relationship Between Serum Parathyroid Hormone Levels, Vitamin D Sufficiency, and Calcium Intake. JAMA. 2005;294(18):2336–2341. doi:10.1001/jama.294.18.2336

2. Edfeldt K, Liu PT, Chun R, et al. T-cell cytokines differentially control human monocyte antimicrobial responses by regulating vitamin D metabolism. Proc Natl Acad Sci U S A. 2010;107(52):22593–22598. doi:10.1073/pnas.1011624108

3. Saul L, Mair I, Ivens A, Brown P, Samuel K, Campbell JDM, Soong DY, Kamenjarin N and Mellanby RJ (2019) 1,25-Dihydroxyvitamin D3 Restrains CD4+ T Cell Priming Ability of CD11c+ Dendritic Cells by Upregulating Expression of CD31. Front. Immunol. 10:600. doi: 10.3389/fimmu.2019.00600

4. Parva NR, Tadepalli S, Singh P, et al. Prevalence of Vitamin D Deficiency and Associated Risk Factors in the US Population (2011-2012). Cureus. 2018;10(6):e2741. Published 2018 Jun 5. doi:10.7759/cureus.2741

5. Bischoff-Ferrari HA, Kiel DP, Dawson-Hughes B, et al. Dietary calcium and serum 25-hydroxyvitamin D status in relation to BMD among U.S. adults. J Bone Miner Res. 2009;24(5):935–942. doi:10.1359/jbmr.081242

6. Zhu K, Bruce D, Austin N, Devine A, Ebeling PR, Prince RL. Randomized Controlled Trial of the Effects of Calcium With or Without Vitamin D on Bone Structure and Bone-Related Chemistry in Elderly Women With Vitamin D Insufficiency. Journal of Bone and Mineral Research. 2008;23(8):1343-1348. doi:10.1359/jbmr.080327.

7. Avenell A, Mak JC, O’Connell D. Vitamin D and vitamin D analogues for preventing fractures in post-menopausal women and older men. Cochrane Database Syst Rev. 2014;2014(4):CD000227. Published 2014 Apr 14. doi:10.1002/14651858.CD000227.pub4

8. Ginde AA, Mansbach JM, Camargo CA Jr. Association between serum 25-hydroxyvitamin D level and upper respiratory tract infection in the Third National Health and Nutrition Examination Survey. Arch Intern Med. 2009;169(4):384–390. doi:10.1001/archinternmed.2008.560

9. Bodnar LM, Krohn MA, Simhan HN. Maternal vitamin D deficiency is associated with bacterial vaginosis in the first trimester of pregnancy. J Nutr. 2009;139(6):1157–1161. doi:10.3945/jn.108.103168

10. Littorin, B., Blom, P., Schölin, A. et al. Lower levels of plasma 25-hydroxyvitamin D among young adults at diagnosis of autoimmune type 1 diabetes compared with control subjects: results from the nationwide Diabetes Incidence Study in Sweden (DISS). Diabetologia 49, 2847–2852 (2006).

11. Merlino, L.A., Curtis, J., Mikuls, T.R., Cerhan, J.R., Criswell, L.A. and Saag, K.G. (2004), Vitamin D intake is inversely associated with rheumatoid arthritis: Results from the Iowa Women’s Health Study. Arthritis & Rheumatism, 50: 72-77. doi:10.1002/art.11434

12. Anglin RES, Samaan Z, Walter SD, Mcdonald SD. Vitamin D deficiency and depression in adults: systematic review and meta-analysis. British Journal of Psychiatry. 2013;202(2):100-107. doi:10.1192/bjp.bp.111.106666.

13. 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. 5, Dietary Reference Intakes for Adequacy: Calcium and Vitamin D. Available from:

14. Michael F. Holick, Neil C. Binkley, Heike A. Bischoff-Ferrari, Catherine M. Gordon, David A. Hanley, Robert P. Heaney, M. Hassan Murad, Connie M. Weaver, Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline, The Journal of Clinical Endocrinology & Metabolism, Volume 96, Issue 7, 1 July 2011, Pages 1911–1930,

15. Gruijl FD. Skin cancer and solar UV radiation. European Journal of Cancer. 1999;35(14):2003-2009. doi:10.1016/s0959-8049(99)00283-x.