Vitamin D, commonly known as the "sunshine vitamin," is actually not a vitamin but a prohormone. It is also known as calcitriol, ergocalciferol, calcidiol and cholecalciferol. Of those, calcidiol is the form doctors most commonly focus on when measuring vitamin D levels in the blood.

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Where Do We Get Vitamin D?

Vitamin D comes from three primary sources: sun exposure, diet, and supplements.

Sun Exposure and Vitamin D Absorption

During exposure to sunlight, a cholesterol in the skin absorbs UV B radiation which converts it into vitamin D3. Vitamin D3 is then metabolized in the liver and kidneys and converted into 25-hydroxyvitamin D, the primary circulating form in our blood serum.

Diet and Supplements and Vitamin D Absorption

In foods and dietary supplements, vitamin D has two primary forms, D2 (ergocalciferol) and D3 (cholecalciferol). Both forms are well absorbed in the small intestine and then stored in the liver and body's fat tissues for long-term use. (1), (2), (3)

Vitamin D2 and Vitamin D3: What's The Difference?

• Vitamin D2 originates from plants and is found in the human diet. It is also largely human-made and added to fortified foods.
• Vitamin D3 is synthesized in the skin of humans from 7-dehydrocholesterol and is also consumed in the diet via the intake of animal-based foods.
• Both vitamin D2 and D3 are found in dietary supplements or fortified foods.
• The D2 and D3 forms differ only in their side-chain structure. The differences do not affect metabolism, and both forms function as prohormones.

What Causes Low Vitamin D

Despite the presence of fortified foods, 40% of the adult population in North America continues to be deficient in Vitamin D.  

There are several root causes of low vitamin D levels, and certain groups of people are at a higher risk of developing Vitamin D deficiency.

Living in Northern Latitudes

If you live above 37 degrees latitude, you are at a higher risk of Vitamin D deficiency. The skin makes little vitamin D from the sun at these latitudes except in the summer months due to the inability of the necessary UVB rays to penetrate the higher latitudes at other times of the year.

Breastfed Infants

Unfortunately, human breast milk is a poor source of vitamin D on its own therefore strictly breastfed infants are at higher risk of not getting enough of this essential vitamin.

The good news is with appropriate vitamin D intake or supplementation, the lactating mother can fully transfer from her blood to her milk the vitamin D required to sustain optimal vitamin D nutrition in the nursing infant with no additional supplementation needed for the infant.

Older Adults

The skin's ability to synthesize Vitamin D declines with age. This is because older adults have lower skin concentrations of the vitamin D precursor, 7-DHC, compared to younger individuals.

Limited Sun Exposure

UV sunlight is essential for the body to make Vitamin D; it only takes 15-20 mins two to three days per week to get sufficient UV exposure. However, many people avoid sun exposure for many reasons. If this is the case, supplementing Vitamin D may be your best option.

Darker skin

Greater amounts of melanin in the skin produce darker skin color. While higher amounts of melanin protect the skin against sunburn, it unfortunately also decreases the skin's ability to synthesize vitamin D from direct sunlight.

Underlying Health Conditions

Vitamin D absorption is fat-soluble, which means that it is dependent upon the body's ability to absorb dietary fat to utilize and store it.

Conditions such as Celiac disease, Crohn's disease, Ulcerative colitis, Cystic Fibrosis, and some liver diseases, as well as post-gastric bypass surgery, have a decreased ability to absorb dietary fat, which is essential to maintaining optimal Vitamin D levels.

Medications

Certain prescription medications can lower Vitamin D levels in the body, mainly by decreasing absorption of Vitamin D from food and supplements. Common medications that deplete or reduce Vitamin D absorption include:

• Laxatives
• Steroids
• Statins
• Some seizure medications
• Thiazide diuretics
• The weight loss drug Orlistat

Plant-Based Diets

Plant-based diets avoid many of the foods naturally rich in Vitamin D, such as dairy and oily fish. Vegan supplements are available to help bring up levels if necessary.

Obesity

Increased adipose tissue keeps vitamin D isolated so that it is not easily released. Therefore people with a body mass index over 30 are at risk for lower vitamin D levels and often need to take larger doses of vitamin D supplements to reach and maintain normal D levels.

Vitamin D Deficiency Signs & Symptoms

Common signs and symptoms of low vitamin D levels:

• Fatigue
• Bone pain
• Muscle weakness, muscle aches, or muscle cramps
• Mood changes, like depression and anxiety

How to Test Your Vitamin D Levels

Low vitamin D levels have been associated with heart disease, cancer, and impaired immune function. Prolonged severe deficiencies can lead to muscle and bone weakness conditions like rickets, osteoporosis, or osteomalacia. So keeping track of your levels should be a top priority.

Determining your Vitamin D levels is simple and can be done at home via blood spot or at a lab via a blood draw.

Other Beneficial Root Cause Testing

Comprehensive Metabolic panels can be helpful to determine if there is an underlying liver or kidney function root cause of low Vitamin D levels.

Comprehensive Gut Test

Since conditions that affect digestion and absorption can lower vitamin D levels, it's essential to rule out underlying digestive disorders. A comprehensive stool test can show a patient's ability to break down and absorb dietary fats and rule out any inflammatory markers that can limit a patient's ability to absorb nutrients properly.

Celiac Testing

If you suspect a patient is sensitive to gluten or has celiac disease, a full Celiac + Gluten Sensitivity Panel can be run.

Treatment for Vitamin D Deficiency

Sun Exposure

If it is summer or you live in a lower latitude, 15-20 minutes of direct sunlight 2 or 3 times per week at a UV index of 3 or higher is usually sufficient for Vitamin D synthesis for most people.  

A quick search for "local UV Index" can help determine the amount of UVB available to you at any given time. In general, the sun's rays are strongest between 10 am and 3 pm, but the Shadow Rule is a quick guide to gauge the intensity of UV rays.

Shadow Rule

If you are taller than your shadow, you are being exposed to higher levels of UV rays.

If your shadow is taller than you, your UV exposure is likely to be lower.

Keep in mind that wearing sunscreen and standing behind windows block UV rays necessary for Vitamin D production.  

Foods High in Vitamin D

Vitamin D is found naturally in oily fish, like salmon, sardines, swordfish, and tuna. Cod liver oil is highest in Vitamin D content, with over 1300 IU per serving. Dairy also contains Vitamin D but much at lower levels.

Processed foods are now commonly fortified with Vitamin D to help prevent deficiency worldwide. Be sure you check labels on fortified foods because Vitamin D content can vary widely.

*Note most Functional Medicine practitioners will request avoiding dairy products and processed foods to maintain optimal gut health.

Vitamin D Supplements

Supplementation is the most efficient way to raise deficient levels of Vitamin D. It is also a safer option for people at high risk for skin cancers from sun exposure. Various supplements are available both by prescription and over the counter.  

Vitamin D2 and D3 are slightly different. Both forms raise your blood vitamin D levels and can treat and prevent rickets from vitamin D deficiency. However, studies have found that vitamin D3 effectively increases blood vitamin D levels for a more extended period than vitamin D2.

Vitamin D3 (cholecalciferol) is available over the counter and comes from animals.

Vitamin D2 (ergocalciferol) comes from plants but requires a prescription.  

Why is Vitamin D2 Prescribed? Vitamin D2 is indicated to treat calcium disorders and parathyroid disorders. It is also the preferred form for patients with chronic kidney disease.

Supplementation can be tricky for those who follow a plant-based or vegan diet due to dietary restrictions around dairy and fish. While most vitamin D3 is produced from animal sources, it is possible to find D3 from a vegan source, such as lichen.

Lastly, Vitamin K2 works synergistically with Vitamin D. Studies show that taking both Vitamin D with Vitamin K2 increases the body's ability to metabolize calcium effectively versus taking Vitamin D alone.

UV Lamp Exposure

Ultraviolet light sessions have been shown to elevate Vitamin D levels in people with malabsorption syndromes or those who live in Northern areas when natural sunlight cannot provide enough exposure for the body naturally. UV light therapy emits UVB rays similar to that of natural sunlight.

When to Retest Vitamin D Levels

Vitamin D is a fat-soluble vitamin, which means it is stored in the fat in the body, and it takes time for these levels to change. It is recommended to wait 3 to 6 months to recheck vitamin D levels after starting supplementation or UV light treatment. Keep in mind that getting too much Vitamin D is possible, so it is vital to recheck levels periodically and after treatment.

Summary

Vitamin D is essential to the body's ability to function. It is a prohormone that aids neurotransmitter function and supports a healthy immune system.

Despite advances in medicine and technology, low vitamin D continues to be prevalent worldwide.

Checking a blood vitamin D level is the best way to tell if one is deficient. There are many potential root causes for low vitamin D levels. Addressing a root cause combined with a diet rich in fatty fish and 2-3 times per week of sunshine can support healthy vitamin D levels.