Vitamin D supplementation has gained some popularity over the last few years and rightly so. It interacts with over 900 genes in the body. This is about 4% of all the genes in our genome! Having so many functions, you can imagine it acts in several different ways, sometimes like a hormone, sometimes like a vitamin, other times like a vasodilator.


What is Vitamin D?


Vitamin D is a vital substance that helps the body absorb calcium and helps maintain bones, muscles, nerves and the immune system. While there are many ways to get vitamin D (through your skin from sunlight, from vitamin D-rich foods such as some mushrooms and salmon and from supplements), if you are not taking a vitamin D supplement, it is very likely that you have vitamin D deficiency. Yes, you. 


Vitamin D is one of the four fat-soluble vitamins, meaning it is absorbed with fat and stored in fat cells throughout the body. Although it is called a vitamin, vitamin D it is not technically a vitamin because it is produced in the human body and must be synthesized by the body before it can provide a benefit. It is actually a hormone, providing messaging signals to many parts of the body. 


A vast swath of the population is vitamin D deficient. A review of the National Health and Nutrition Examination Survey (NHANES) of nearly 5,000 people over 20 years old who were hospitalized between 2011 and 2012 show that about 40 percent of patients had vitamin D levels less than 50 nmol/L (20ng/ml), below the recommended levels that authoritative sources such as the Endocrine Society and the Vitamin D Council recommend as normal levels. It’s obvious that a significant number of Americans have vitamin D deficiency. 


What are normal levels of vitamin D?


The best blood levels for 25-hydroxyvitamin D range from 30 to 100 ng/ml (75-250 nmol/l). The Institute of Medicine notes that almost all people are sufficient at levels of vitamin D greater than 50 nmol/L (greater than 20 ng/mL), while people with blood levels of vitamin D below 20–25 nmol (8-10 ng/ml) are considered to be at a higher risk for osteoporosis.  


Symptoms of vitamin D deficiency


When a person is vitamin D deficient, they are at risk for a myriad of health issues. One of such issues is compromised immune function. Research has now shown that most immune cells have a vitamin D receptor. Other risks include brittle or even misshapen bones and dental cavities. Even autoimmune diseases such as type 1 diabetes, rheumatoid arthritis, multiple sclerosis, and Crohn disease have been correlated with low vitamin D levels.




Some people who have a vitamin D deficiency may have no symptoms at all. But if someone has the following symptoms, it may be signs of low levels of vitamin D in the body. 


  • Low immune system 
  • Osteoporosis 
  • Muscle weakness or unexplained change in muscle strength
  • Mood changes such as anxiety or depression
  • Fatigue despite adequate sleep
  • Lower endurance during exercise
  • Chronic pain as a result of brittle bones or weak muscles


Who is at highest risk for vitamin D deficiency?


Honestly, the first world countries are so deficient in vitamin D, that just about everyone should be taking vitamin D supplements prophylactically. I don’t say this with many nutrients, but it is true for vitamin D. Having said that, there are certain populations and people with particular chronic diseases have an even higher risk of being vitamin D deficient. A 2018 study shows the following as the most at risk demographics:


Race: Studies indicate that non-Hispanic African-Americans had the highest percentage of vitamin D deficiency. Individuals with darker skin do not produce vitamin D from sunlight as well as those with lighter skin. Increased melanin scatters ultraviolet rays from sunlight, which results in the less efficient vitamin D conversion.


Age: As people age, the risk for vitamin D deficiency significantly increases. The percent of adults 65 and older suffering from vitamin D deficiency ranges from 20 to 100 percent in North America and Europe, according to one report. Older adults also do not convert sunlight to vitamin D as well as younger adults. The calcium absorbing function of vitamin D is important for older adults, so they are especially at risk for the harmful effects of being vitamin D deficient. 


Obesity: Individuals with a body mass index (BMI) of 30 or higher are at risk for vitamin D deficiency because body fat binds to vitamin D, preventing it from being absorbed into the bloodstream and used.


Chronic kidney disease: People with chronic kidney disease have abnormal vitamin D absorption, and usually require a vitamin D supplement to maintain bone and muscle strength. 


Digestive tract disorders: Certain medical problems, including Crohn’s disease, irritable bowel syndrome and celiac disease, can impact the intestine’s ability to absorb vitamin D from food.


How can I get vitamin D?


People can get vitamin D in three ways:



The most common source of vitamin D for children and adults is through sunlight. When skin is exposed to sunlight, it makes vitamin D from cholesterol. The sun’s ultraviolet B (UVB) rays strike cholesterol in skin cells, providing the energy for vitamin D synthesis to happen and for vitamin D to circulate in the body.




However, the damaging effects of sunlight on the skin and increased use of sunscreen and less time in direct sunlight have in turn limited the natural absorption of vitamin D from sunlight. Wearing a sunscreen with a sun protection factor (SPF) of 30 or more reduces vitamin D synthesis in the skin by more than 95 percent. Additionally, people with a darker skin tone need more exposure to the sun to synthesize vitamin D.


For my fellow nerds, here is the technical information about how we make vitamin D when exposed to sunlight. Just skip this section if you’re not interested!


The Vitamin D Pathway


  1. The process of the body manufacturing vitamin D3 starts with the cholesterol found naturally on the skin secreted by the sebaceous glands. An enzyme called 7-dehydrocholesterol reductase helps convert cholesterol into 7-dehydrocholesterol.
  2. 7-dehydrocholesterol absorbs UVB ultraviolet light from the sunlight and transforms into pre-vitamin D and then spontaneously turns into vitamin D3, also known as cholecalciferol. This occurs through spontaneous isomerization-where isomers (molecules composed of the exact same atoms but in different arrangements) transform into a different arrangement, a different isomer.
  3. Cholecalciferol (vitamin D3) binds to vitamin D binding/transport protein in the plasma and is transported to the liver.
  4. Cholecalciferol (vitamin D3) is converted to 25-hydroxy-vitamin D/calcidiol primarily in the liver (and some other places). This is the type that is usually tested for in blood lab test because it is found in higher amounts and lasts longer in the blood. 
  5. The kidneys and some other tissues convert the 25-hydroxy-vitamin D/calcidiol into the active metabolite 1,25-hydroxy-vitamin D/calcitriol. Both of these last two forms act as ligands for the vitamin D receptors (VDR) found throughout the body.


What all of that basically boils down to is this: vitamin D is manufactured by your body. See, that was easy!



There are very few foods in nature that contain vitamin D. The best sources are fatty fish such as salmon, mackerel, tuna. Other foods that contain vitamin D include egg yolks, mushrooms and beef liver.


In the United States, many foods are fortified with vitamin D such as cereals, orange juices and other products. 



There has been much confusion in regards to vitamin D dosing, partially due to a statistical error in the Institute of Medicine recommendations. The amount of vitamin D from food or supplements needed to get blood levels to the recommended 50 nmol/L or higher is much more significant than their original published report and what many authorities had thought. The adjusted recommended daily supplement intake for those not getting any direct sunlight are as follows: 


  • 1,000 IU for infants younger than a year who are on enriched formula
  • 1,500 IU for infants older than six months who are breastfed
  • 3,000 IU for children older than one year of age 
  • up to 8,000 IU for young adults and subsequent adult doses adapted to the body mass index until blood levels reach 100 nmol/L 


Supplement intake should be adjusted if you are getting regular direct sunlight. The best way to know if you’re taking the correct dose of vitamin D is to have your blood levels tested. Without that, you’re really just guessing that your body uses the supplement like the average person. Get it tested, know the facts. Easy peasy. 


There are two types of vitamin D supplements available on the market: D2 (ergocalciferol) and D3 (cholecalciferol). A majority of studies show that the D3 is the best type of supplement, as it is the same type of vitamin D the body makes on its own.


Can you have too much vitamin D?


For people who are taking vitamin D supplements, there is a risk of having too much vitamin D in the body, resulting in toxicity. Symptoms of too much vitamin D include:


  • No appetite
  • Vomiting/nausea
  • Weight loss
  • Constipation
  • Muscle weakness
  • Kidney stones


Vitamin D toxicity in turn causes the body to have increased levels of calcium, known as hypercalcemia. This can cause irregular heart rhythms, mental confusion and disorientation.


People who get excessive sun exposure are not at risk for vitamin D toxicity because the body naturally regulates the amount of vitamin D produced from sunlight.


A 2018 study indicates that vitamin D toxicity would likely be caused by doses of vitamin D in excess of 10,000 IU/day, long term. Many times, your doctor will recommend higher doses for short periods to try to get your vitamin D levels up quickly. If you’re taking vitamin D supplements, you should discuss the amount you’re taking with your doctor and monitor your vitamin D levels through blood work to ensure you are getting the correct dose. 


Vitamin D is a crucial substance that helps maintain many critical processes within the body. Vitamin D deficiency is fairly common and most people don’t get adequate amounts through sunlight and diet. Knowing the best ways to get enough vitamin D is key to maintaining normal levels and optimal health.


If you’re looking for a quality vitamin D product, this is the one my family and I use: Pure Encapsulations Liquid Vitamin D3.




  1. Kongsbak, M., Levring, T., Geisler, C., & von Essen, M. (2013). The Vitamin D Receptor and T Cell Function. Frontiers In Immunology, 4. doi:10.3389/fimmu.2013.00148 
  2. Office of Dietary Supplements – Vitamin D. (2019). Retrieved 6 September 2019, from 
  3. Parva, N., Tadepalli, S., Singh, P., Qian, A., Joshi, R., & Kandala, H. et al. (2018). Prevalence of Vitamin D Deficiency and Associated Risk Factors in the US Population (2011-2012). Cureus. doi:10.7759/cureus.2741
  4. Vitamin D Test Kit | DIY At-Home Vitamin D Testing. (2019). Vitamin D Council. Retrieved 6 September 2019, from 
  5. Hanley, D., & Davison, K. (2005). Vitamin D Insufficiency in North America. The Journal Of Nutrition, 135(2), 332-337. doi:10.1093/jn/135.2.332
  6. Kongsbak, M., Levring, T., Geisler, C., & von Essen, M. (2013). The Vitamin D Receptor and T Cell Function. Frontiers In Immunology, 4. doi:10.3389/fimmu.2013.00148 
  7. Podd, D. (2015). Hypovitaminosis D. Journal Of The American Academy Of Physician Assistants, 28(2), 20-26. doi:10.1097/01.jaa.0000459810.95512.14 
  8. Kennel, K., Drake, M., & Hurley, D. (2010). Vitamin D Deficiency in Adults: When to Test and How to Treat. Mayo Clinic Proceedings, 85(8), 752-758. doi:10.4065/mcp.2010.0138 
  9. Parva, N., Tadepalli, S., Singh, P., Qian, A., Joshi, R., & Kandala, H. et al. (2018). Prevalence of Vitamin D Deficiency and Associated Risk Factors in the US Population (2011-2012). Cureus. doi:10.7759/cureus.2741 
  10. Weishaar, T., Rajan, S., & Keller, B. (2016). Probability of Vitamin D Deficiency by Body Weight and Race/Ethnicity. The Journal Of The American Board Of Family Medicine, 29(2), 226-232. doi:10.3122/jabfm.2016.02.150251 
  11. Holick, M., Binkley, N., Bischoff-Ferrari, H., Gordon, C., Hanley, D., & Heaney, R. et al. (2011). Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. The Journal Of Clinical Endocrinology & Metabolism, 96(7), 1911-1930. doi:10.1210/jc.2011-0385 
  12. Vanlint, S. (2013). Vitamin D and Obesity. Nutrients, 5(3), 949-956. doi:10.3390/nu5030949
  13. Al-Badr, W., & Martin, K. (2008). Vitamin D and Kidney Disease. Clinical Journal Of The American Society Of Nephrology, 3(5), 1555-1560. doi:10.2215/cjn.01150308
  14. Vitamin D and gastrointestinal diseases: inflammatory bowel disease and colorectal cancer – Maitreyi Raman, Andrew N. Milestone, Julian R.F. Walters, Ailsa L. Hart, Subrata Ghosh, 2011. (2019). Therapeutic Advances In Gastroenterology. Retrieved from 
  15. Rathish Nair, A. (2012). Vitamin D: The “sunshine” vitamin. Journal Of Pharmacology & Pharmacotherapeutics, 3(2), 118. Retrieved from 
  16. Bikle, D. (2014). Vitamin D Metabolism, Mechanism of Action, and Clinical Applications. Chemistry & Biology, 21(3), 319-329. doi:10.1016/j.chembiol.2013.12.016
  17. Office of Dietary Supplements – Vitamin D. (2019). Retrieved 6 September 2019, from 
  18. Veugelers, P., & Ekwaru, J. (2014). A Statistical Error in the Estimation of the Recommended Dietary Allowance for Vitamin D. Nutrients, 6(10), 4472-4475. doi:10.3390/nu6104472 
  19. Stephensen, C., Zerofsky, M., Burnett, D., Lin, Y., Hammock, B., Hall, L., & McHugh, T. (2012). Ergocalciferol from Mushrooms or Supplements Consumed with a Standard Meal Increases 25-Hydroxyergocalciferol but Decreases 25-Hydroxycholecalciferol in the Serum of Healthy Adults. The Journal Of Nutrition, 142(7), 1246-1252. doi:10.3945/jn.112.159764 
  20. Marcinowska-Suchowierska, E., Kupisz-Urbańska, M., Łukaszkiewicz, J., Płudowski, P., & Jones, G. (2018). Vitamin D Toxicity–A Clinical Perspective. Frontiers In Endocrinology, 9. doi:10.3389/fendo.2018.00550