Disclaimer: This post is sponsored by NOW®, but all opinions are my own. Thank you for supporting the brands that make the Nutrition by RD blog possible.

Vitamin D is a hot topic. Several studies have linked it with many aspects of health including immune function, cardiovascular health and most notably bone health.* Additionally, vitamin D deficiency has been associated both physical and mental health problems.

What exactly is Vitamin D?

Vitamin D is a fat-soluble vitamin that promotes the absorption of calcium, which is an important mineral required for the maintenance of strong bones.* Vitamin D’s name may be a bit misleading since it is actually a hormone and not a vitamin. It is unique in that it can be made in the body after exposure to sunlight, and unlike most vitamins its primary source is sunlight, not food. It is found in two forms: Vitamin D2 (ergocalciferol), which mainly comes from plant sources and fortified foods, and Vitamin D3 (cholecalciferol), which is produced in the skin and found only in animal-sourced foods.(1)

Sources of Vitamin D:


Regular sun exposure is the most natural way to get sufficient vitamin D. Our bodies produce vitamin D after exposure to UV rays from the sun. This process begins when the UVB rays from the sun convert 7-dehydrocholesterol, a substance present in our skin, into pre-vitamin D3 (cholecalciferol). The pre-vitamin D3 then travels to the liver, where it transforms into 25-hydroxyvitamin D, also known as calcidiol. Next, the calcidiol is carried into the kidneys, where it’s converted into its active form, calcitriol.(2)

Image taken from https://www.health.harvard.edu/staying-healthy/vitamin-d-and-your-health-breaking-old-rules-raising-new-hopes


Food sources that naturally contain vitamin D are limited. Fortified food sources, including milk, orange juice and cereal, make up the majority of Vitamin D in Americans’ diets. Alternative sources of Vitamin D include fatty fish, such as salmon. Mushrooms that are exposed to ultraviolet light (noted on the product label) are also a good source of Vitamin D2.  


In the northeast, Vitamin D deficiency is very common because exposure to sunshine is limited. Approximately 42% of US adults are Vitamin D deficient.(1) Dosing is individualized and depends on current Vitamin D status and therefore before making any recommendations, it is important to assess lab work to verify if and how much supplementation is required.

How to check your Vitamin D Status:

Ask your doctor to administer a blood test. The best test to measure vitamin D count is through the 25-hydroxy vitamin D blood test. It is usually measured in nanograms per milliliter (ng/mL).  Check out the table below, which lists the health status associated with varying Vitamin D levels.(1)

What problems does Vitamin D deficiency lead to?

Vitamin D deficiency is linked to decreased bone density, which can cause osteoporosis, a medical condition in which the bones become brittle and fragile due to tissue loss. Currently, researchers are studying how vitamin D deficiency may be connected to several chronic conditions, including cancer (breast and bowel),(3,4) chronic pain and autoimmune diseases,(5) diabetes and cardiovascular disease.(6) Depression has also been linked to vitamin D deficiency.(7) In severe cases, vitamin D deficiency can lead to rickets, resulting in skeletal deformities seen in young children.  Rickets is currently rare in developed countries, but not uncommon in other areas of the world.(1)

Who is at risk for Vitamin D deficiency?

Since the primary source of Vitamin D is through exposure to sunshine, those that spend most of their time indoors, wear clothing that cover most of their skin, or live far from the equator tend to be more likely to be deficient. Individuals with naturally darker skin may also be at greater risk for deficiency since the pigment (melanin) in darker skin absorbs less UV radiation. Individuals with digestive issues impeding fat absorption, such as irritable bowel disease (IBD – such as Chrohn’s or Ulcerative Colitis) or Celiac disease may also be more likely to become Vitamin D deficient. Finally, those above 70 years old, or those with kidney or liver disease, are at an increased risk for vitamin D deficiency due to a decreased ability to create the active form of Vitamin D3 in the body.(1)

Recommendations for adequate Vitamin D:

1. Regular sun exposure is the most natural way to get enough vitamin D. Aim to get 15-20 minutes of sunlight a few times per week.(1)

2. Men and women (19 – 70 years): 600 IU each day. Men and women (71 years and older): 800 IU each day. To illustrate, a 3oz serving of salmon has about 445 IU of salmon, fortified milk has about 115 IU and fortified orange juice has about 135 IU.(1)

3. If you are Vitamin D deficient, a common rule of thumb is that for every 1 ngl/mL increase, you’ll need to get an additional 100-150 IU of vitamin D per day.9 It can take several weeks to see an improvement. Additionally, studies show that D3 is preferred over D2 since it will raise blood levels more effectively over time. If you do need a supplement, remember to take it with food. The best food to pair the supplement with is food containing fat, such as olive oil or avocado, since it is a fat-soluble vitamin that requires fat to help absorb it.

4. It’s important not to overdo supplementation as this can have negative consequences. The upper limit to which safety is assumed is at 4,000 IUs per day.(1) My favorite Vitamin D supplement is by NOW®. They sell their soft-gels in quantities of 1,000 IU, 2,000 IU and 5,000 IU.

You can learn more about Now Foods Vitamin D-3 supplement here: https://www.nowfoods.com/supplements/vitamin-d-3-1000-iu-softgels

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.


  1. https://ods.od.nih.gov/factsheets/Vitamin%20D-HealthProfessional/
  2. https://www.health.harvard.edu/staying-healthy/vitamin-d-and-your-health-breaking-old-rules-raising-new-hopes
  3. https://www.ncbi.nlm.nih.gov/pubmed/29912394
  4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5802611/
  5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6047889
  6. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6013996/
  7. http://www.jneuropsychiatry.org/peer-review/depression-and-vitamin-d-deficiency-causality-assessment-and-clinical-practice-implications-12051.html
  8. https://news.northwestern.edu/stories/2020/05/vitamin-d-appears-to-play-role-in-covid-19-mortality-rates/
  9. Armas L.A.G., Hanson C. (2017) Bone Health: Sound Suggestions for Stronger Bones. In: Temple N., Wilson T., Bray G. (eds) Nutrition Guide for Physicians and Related Healthcare Professionals. Nutrition and Health. Humana Press, Cham