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 so important to overall human health.
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 get some, but very minimal, vitamin D from diet. Very few foods in nature contain vitamin D. Vitamin D is found in some fatty fish like wild caught salmon, (but not in farm raised salmon), fish liver oils, egg yolk, mushrooms, butter, liver, and fortified foods such as milk. But it’s really 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.
According to the Office of Dietary Supplements (ODS), levels of vitamin D are measured by the 25-hydroxy level in nanomoles/liter (nmol/L) or nanograms/milliliter (ng/mL). The results can indicate the following:
deficiency: less than 30 nmol/L (12 ng/mL)
potential deficiency: between 30 nmol/L (12 ng/mL) and 50 nmol/L (20 ng/mL)
normal levels: between 50 nmol/L (20 ng/mL) and 125 nmol/L (50 ng/mL)
Across the country, vitamin D inadequacy is 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.
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 with skin exposure at least 3 days a week . However, because the angle of the sun changes and drops lower during the fall, winter and spring, (generally November through March) even with skin exposure during this time, studies have shown we do not make adequate vitamin D, and in the northern half of the US, none at all.
If you live in a warm southern state below the 32 degrees latitude, which runs through Atlanta, Georgia and includes Florida, Alabama, Mississippi, Louisiana, Texas and southern New Mexico, Arizona and California, it is possible to make vitamin D almost all year. However, this is still only true, if you are outside getting sun exposure during the optimal time of day between 10 a.m. to 3 p.m. 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. This is simply impossible for most people who are working, and for children in school or daycare all day. Without adequate sun exposure to the majority of our body several times a week, we simply can’t produce enough vitamin D to store and sustain us.
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 in the skin.
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 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:
People with limited sun exposure
People with dark skin
People with fat malabsorption issues
Obese individuals need 2-3 times more vitamin D3 than normal weight adults
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 are at higher risk of deficiency.
Age Group Recommendations from Vitamin D Council
Infants: 1,000 IU daily (25 mcg)
Children: 1,000 IU daily (25 mcg) per 25 lbs of body weight
Adults: 5,000 IU daily (125 mcg)
Age Group Recommendations from Endocrine Society
Infants: 400 to 1,000 IU daily (10-25 mcg)
Children: 600 to 1,000 IU daily (15-25 mcg)
Adults: 1,500 to 2,000 IU daily (37-50 mcg)
- Autier P, Gandini S, Mullie P. The Journal of Clinial Endocrinology & 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.