Why Do I Need Vitamin D?

by Sarah Schlichter
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Why Do I Need Vitamin D?

We all know we need vitamin D for its role in bone health, but it has several other important functions. It also aids in immunity, calcium absorption and muscle and cardiovascular function, as well as brain development. There is also ongoing research on vitamin D’s role in muscle function, recovery time and athletic performance.

Since it has so many functions, a lack of vitamin D can be detrimental. Research links low vitamin-D levels to higher mortality rates and increased autoimmune diseases, such as Type 1 diabetes, multiple sclerosis and rheumatoid arthritis. No wonder vitamin D is a hot topic.


The general recommendation for vitamin D in adults is 600-800 IU per day according to the Food and Nutrition Board, which is responsible for the Dietary Reference Intakes (DRIs).


Nearly 50% of the world’s population has insufficient vitamin-D levels, with nearly 1 billion people diagnosed as deficient. There is some debate about what is considered a normal vitamin-D level. The Institute of Medicine defines deficiency as serum levels less than 12 ng/ml, while the Endocrine Society considers levels less than 20 ng/ml as deficient. The Vitamin D Council has the strictest measure, defining levels below 30 ng/ml as deficient. Generally, in healthy individuals, anything less than 20 ng/mL is considered inadequate for bone and overall health and increases the risk of bone fractures.


Good news — you can get all the vitamin D you need from sunshine! Of importance to note, however, vitamin D absorption from sunlight depends on a few factors, including the time of day, where you live, how much skin you expose, air pollution, your skin color and sunscreen use.

The middle of the day is prime time to absorb the most vitamin D from the sun, and research shows that just 1015 minutes a day is enough. A trick is determining the length of your shadow. If your shadow is longer than you are tall (this usually happens in the winter), you’re likely not able to make sufficient vitamin D. Those with lighter skin can synthesize more vitamin D than those with darker skin pigments, and the closer you live to the equator, the more efficient and easier it is for your body to produce vitamin D. Those who live at northern latitudes have less UVB light available from October to April, which also translates to lower absorption.   

While sunscreen is important for our skin health by blocking UV radiation, it hinders vitamin D absorption since UVB is the portion of sunlight that stimulates our skin to produce vitamin D. Wearing sunscreen with SPF 15 or greater can decrease vitamin D3 production by 99%.    


While we can make vitamin D from sunlight, it’s found in several foods too, including fatty fish such as salmon, sardines, tuna and mackerel, fortified dairy and cereal products, margarine and egg yolks. For those who don’t include a variety of fatty fish and fortified dairy in their diets, getting enough vitamin D through food can be virtually impossible.  

So get outside and enjoy a bit of summer sunshine! Sunlight is by far the best way to absorb vitamin D. It’s so good that a little goes a long way: spend just 10-30 minutes in the sun a few days a week without sunscreen and revel in a dose of vitamin D.


  • Cassondra Kennedy

    I’ve always wondered if enough Vitamin D is absorbed solely through the arms, face, and legs. For 10-15 minutes in the sun to be sufficient, does one have to be dressed down to bathing suit levels? Or will it be sufficient in normal clothing as well?

    • Jerome Barry

      Normal clothing outdoors during daylight is exposure enough. Take a little walk outside during the noon time and you’ll be doing it right.

    • DaBoss

      Depending where you live be cautious about going out in the full sun. We need Vit D but we don’t need melanomas. Dermatologists in this area recommend 15-20 mins in the morning.

    • cwolf


      Folks like Holick say 40% of your skin at solar noon at a UVB sufficient latitude and season. Zero UVB in Boston in winter (Holick put his grad students on his roof to prove this).

      See Rule of 9s.

  • Bet Rob

    You don’t “absorb” Vitamin D from the sun, as the writer keeps saying. You produce Vitamin D when the skin absorbs UVB rays.

    So the “experts” tell us we should never expose ourselves to the sun without coating every inch of bare skin in high-SPF sunscreen, because the sun is dangerous and we’ll all get skin cancer, or at least wrinkles. The “experts” also tell us we should eat as little cholesterol as possible, and suppress our natural cholesterol production with statins, because cholesterol is dangerous and gives us heart disease. But cholesterol and UVB rays are both necessary for Vitamin D production. Then the “experts” act puzzled as to why so many people are now deficient in Vitamin D…and recommend taking supplements, which aren’t as effective as simply eating well, letting the body make the cholesterol it NEEDS, and getting some sun regularly.

    Wonder who’s paying these “experts”?

    • Zoe

      Well said.

  • Lizy

    Every bit of information in this article is inaccurate. You do not get vitamin D from the sun. The sun converts the vitamin D that has gone from your food as D1 to your kidneys as D2 then to the skin to become D3. We are deficient because our diets are lacking in the type of food that is rich in Vitamin D (sardines, fatty organ meats, mackerel, salmon, etc). If you take a vitamin D supplement it has already been converted to the D3 form that your body is able to absorb. Herein is the problem with internet “experts” and all of the misinformation portrayed as fact.