Facts About Vitamin D
Perhaps one of the most fascinating facts about vitamin D is that it’s not really a vitamin. It’s actually a fat-soluble hormone that is produced in the body when skin is exposed to sun. We can also get some vitamin D from foods and dietary supplements. Vitamin D is found in fatty fish, fish oils, egg yolk, butter, liver, and fortified foods such as milk. But it’s very difficult to get enough vitamin D from diet alone.
In order for vitamin D to become activated in the body, it has to undergo two hydroxylations. First, it is converted to 25-hydroxyvitamin D [25(OH)D] in the liver. And then the physiologically active 1,25-dihydroxyvitamin D, also known as calcitriol, is formed primarily in the kidneys. The circulating form of vitamin D is 25(OH)D and that’s what is measured to determine vitamin D status. Adequate serum vitamin D status is defined as 50 nmol/L or higher.
Vitamin D is critical to the function of many body systems including musculoskeletal, gastrointestinal, endocrine, immune and cardiovascular. That’s why maintaining adequate vitamin D levels is critically important to overall human health.
According to the National Institutes of Health Office of Dietary Supplements, “There is considerable scientific discussion about the serum concentrations of 25(OH)D associated with deficiency (e.g. rickets), adequacy for bone health, and optimal overall health, and cut points have not been developed by a scientific consensus process. Based on its review of data of vitamin D needs, a committee of the Institute of Medicine concluded that “persons are at risk of vitamin D deficiency at serum 25(OH)D concentrations less than 30 nmol/L (<12 ng/mL).”
In the recent past, across the country, vitamin D inadequacy was so common it became a public health concern and still is, and supplementation has now become routine medical practice for anyone testing low on their 25(OH)D test.
As for an upper limit, The Institute of Medicine reported in 2010 that serum levels of 25(OH)D greater than 125 nml/L are associated with adverse effects.
Vitamin D3 (cholecalciferol) is the natural form of vitamin D that the body makes from sunlight. This is also the form of vitamin D that is found in most dietary supplements. The authors of a 2017 review in Evidence-Based Clinical Review concluded, “Despite known dietary sources of vitamin D and the role of sunlight in its production, much of the US population may have inadequate levels of serum 25-hydroxyvitamin D.” Why is this?
During warm summer months in most of the country, there’s plenty of sunlight to boost vitamin D levels in your body, if you can spend some time outside daily for skin exposure. However, unless you live in a warm southern state, this skin exposure will not occur during the fall, winter and spring and the body will not make adequate vitamin D3. In addition, the challenge lies in preventing sun overexposure while still trying to reap vitamin D’s health benefits. Concern about skin cancer has led many health authorities to warn against any outdoor sun exposure unless you’re wearing sunblock or sunscreen. This practice prevents the formation of vitamin D. Allowing 15-30 minutes or so of unprotected sun exposure to your arms, legs, abdomen and back is required in order to produce enough vitamin D, but it also has to be during the optimal time of day, generally between 10 a.m. to 3 p.m. This is simply impossible for most people who are working, and for children in school or daycare all day. Without weekly sun exposure to the majority of our body, we simply can’t produce enough vitamin D to store and sustain us year round. These changes in lifestyle and the fairly universal use of sunscreen has led to the common public health problem of vitamin D inadequacy.
Maintaining adequate vitamin D levels often requires a combination of diet, sunlight exposure and dietary supplements. The National Institutes of Health Office of Dietary Supplements has identified the following groups at increased risk of vitamin D inadequacy:
- Breastfed infants
- Older adults
- People with limited sun exposure
- People with dark skin
- People with fat malabsorption issues
- Obese individuals or those who have undergone gastric bypass surgery
Vitamin D is a critical nutrient serving many key functions in the human body. Levels should be tracked consistently, especially in all patient populations that may be at risk of deficiency.
|Vitamin D Recommendations||Vitamin D Council||Food and Nutrition Board|
|Infants||1,000 IU daily||400 to 1,000 IU daily|
|Children||1,000 IU daily per 25 lbs of body weight||600 to 1,000 IU daily|
|Adults||5,000 IU daily||1,500 to 2,000 IU daily|
Autier P, Gandini S, Mullie P. The Journal of Clinial Endorcrinology & Metabolism. 2012;97(8):2606-2613.
Pfotenhauer KM, Shubrook JH. Evidence-Based Clinical Review. 2017;117:301-305.
Sassi F, Tamone C, D’Amelio P. Nutrients. 2018;10(11):1656.
van Schoor N, Lips P. Endocrine & Metabolism Clinics of North America. 2017;46(4):845-870.