You may have heard of Vitamin D as “the sunshine vitamin” or that it is important for your bones. It’s also common to see whole milk labeled as “Vitamin D milk”. We will explain this essential vitamin’s role in the body, how much you need, and good sources, including how it’s absorbed from the sun.

Getting Your Vitamin D

Function

Vitamin D is a fat-soluble vitamin essential for multiple functions in the body, including immune system support, aiding the calcium absorption, and promoting cell growth. Adequate Vitamin D is also shown to prevent against colorectal cancer and heart disease. There are different forms of Vitamin D. Precursors include Vitamin D2, D3, commonly found as dietary supplements. 25-hydroxyvitamin D is formed from Vitamin D2 and D3 in the liver and is used to determine Vitamin D status in the body. 1,25-dihydroxyvitamin D is formed in the kidneys and is active form for Vitamin D.

 

Recommended Dietary Allowance

Below is a table showing the recommended amount of Vitamin D for different age groups and gender. For comparison, 1 cup of whole milk has around 120 IU and the average dietary supplement has 10,000 IU and is taken once per week. Note that supplements are generally only recommended in setting of a deficiency.

Age Male Female Pregnancy Lactation
0-12 months 400 IU 400 IU
1-3 years 600 IU 600 IU
14-18 yrs 600 IU 600 IU 600 IU 600 IU
19-50 yrs 600 IU 600 IU 600 IU 600 IU
51-70 yrs 600 IU 600 IU
>71 yrs 800 IU 800 IU

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

 

Sources

Very few foods are naturally good sources of Vitamin D. Some of these are fatty fish, such as salmon, tuna, and swordfish. Dairy foods like cheese and milk also contain naturally-occurring Vitamin D. Mushrooms specifically grown in ultraviolet light are also a source of Vitamin D. Most dietary intake of Vitamin D comes from fortified foods, including orange juice, yogurt, and breakfast cereals.

The best source of Vitamin D actually comes from sunlight. Ultraviolet light from the sun converts cutaneous (present in the skin) 7-dehydrocholesterol to previtamin D3, thus converting to Vitamin D3. If not needed, the previtamin D3 is converted to tachysterol and lumisterol to be stored. If needed, previtamin D3 is converted to Vitamin D3, then 25-hydroxyvitamin D in the liver, then 1,25-dihydroxyvitamin D in the kidneys. The amount of time recommended for sun exposure is variable depending on factors such as skin pigmentation, elevation, and body surface exposure. When it comes to tanning beds, research concludes to not use tanning beds as a source of Vitamin D due to risk of developing skin cancer. In cases where UVB light is not available, it is recommended to use dietary supplements to prevent Vitamin D deficiency.

 

Deficiency

According to multiple studies, children are less likely at risk for developing Vitamin D deficiency because they generally spend more time outdoors compared to adults and elderly. However, deficiency in children is possible and is known as rickets, which is a bowing and frailty of the bones. In pregnant and lactating women, Vitamin D deficiency can cause osteomalacia, which a softening of the bones. Vitamin D deficiency in elderly can result in more bone fracture due to poor calcium storage and bone frailty. On the other hand, it is possible to have too much Vitamin D in the body, known as hypervitaminosis. Fat-soluble vitamins are at higher risk of hypervitaminosis because they are absorbed by fat substrates, such as bile, which is reabsorbed into the body. Excess of water-soluble vitamins (think B12, B6 and C to name a few) are excreted by the kidneys via urine.