Vitamin D3, Cholecalciferol, Vitamin D2, Ergocalciferol, Calcidiol, Calcitriol.

What are all of these things? They are all referring to the various forms of vitamin D in our body, or in the food we eat. Vitamin D3 is also known as cholecalciferol, this is the form made naturally by the body in response to sunlight.

Vitamin D2 is also known as Ergocalciferol and comes from plants. Vitamin D supplements come in the form of either Vitamin D3 or Vitamin D2. Calcitriol, or 1,25-Dihydroxyvitamin D if you prefer, is the active form of vitamin D in our body.

When sunlight contains ultraviolet B (UVB) radiation, and when it hits your skin cells, it turns 7-dehydrocholesterol into vitamin D3, meaning cholecalciferol. Vitamin D3 in turn binds to vitamin D-binding protein (VDBP) and is transported to the liver, where it gets converted to calcidiol, aka 25-hydroxyvitamin D.

Calcidiol is the storage form of vitamin D in the body. Calcidiol is later converted to the active form of vitamin D in the body, meaning calcitriol, or 1,25-Dihydroxyvitamin D. This conversion of calcidiol into calcitriol mainly occurs in the kidney. But it also takes place in different tissues and cells of the immune system, such as lymph nodes and alveolar macrophages. And not only in alveolar macrophages, but the alveoli themselves.

Vitamin D3

Vitamin D3 is only found naturally in a few different food sources, mainly fatty fish, like cod, swordfish, tuna, and salmon. Milk doesn’t naturally contain vitamin D3, but it has been fortified with it for almost 100 years now. But other dairy products made from milk like cheese and ice cream aren’t typically fortified with vitamin D and contain only small amounts.

For older adults to meet the RDA of 800 IU, they would have to drink about 4 cups of fortified milk per day. Vitamin D3 mainly comes from sunlight though, not food. But the skin’s production of vitamin D depends on a number of factors, only some of which you have control.

Depending on the season, where you live, where you travel, the time of day, the clothing you wear, the umbrella that you use, all of these factors determine how much UVB light hits your skin. The sun’s rays are most direct between 10 a.m. and 3 p.m.

However, the farther you live from the equator, the less UVB radiation you receive. People who live north of about 37° latitude can’t make any vitamin D from sunlight from November to March, even if they spend all day outside bare naked. Why is this the case?

During the winter months, the earth tilts away from the sun, ultimately leading to fewer sun rays hitting the earth. Having darker skin means less UVB absorption, which means less vitamin D3 production. Also, as we age, our body becomes less efficient at converting UVB light into vitamin D3.

This is why older people, as well as darker skin people, are more prone to have lower vitamin D levels. To make matters worse, many older people have reduced exposure to sunlight for different reasons, and may not be getting enough vitamin D in their diet.

Vitamin D signals the intestines to absorb calcium into the bloodstream.
This happens even if you have enough calcium in your diet. Besides strengthening bones, vitamin D helps reduce fractures in the elderly by preventing muscle deterioration, and reducing the chances of falling.

The official definition of a vitamin deficiency means that specific health problems arise as a direct result of not having enough of a specific nutrient. True vitamin D deficiency in children causes rickets, a bone disease where the legs become bowed. This is rare in the United States.

Vitamin D deficiency in adults can lead to osteomalacia and osteoporosis, leading to bone fractures. So what if you don’t have deficiency per se, but you have less-than-ideal levels of a specific vitamin? Well, this can increase your risk of various health issues, even though they are not solely responsible for these problems. This is what we call “Insufficiency.”

Most medical societies consider someone vitamin D insufficiency if their level is between 12 to 20 ng/mL (30 to 50 nmol/L), and Vitamin D deficiency is a level less than 12 ng/mL (30 nmol/L). Most experts consider normal levels being above 20 and less than 50 ng/ml. Most people in the United States have values around 20 ng/ml.

But having too high levels can cause other health issues. In one study, levels above 32 ng/ml resulted in people getting less quality sleep. The risk of vitamin D toxicity typically occurs at levels over 100 ng/mL in adults who are also ingesting substantial amounts of calcium. Vitamin D toxicity, whether it’s from vitamin D3 or vitamin D2, generally occurs after inappropriate use of vitamin D.

It’s especially important to avoid excess vitamin D in pregnancy because that can cause calcium levels to rise to the point of causing seizures in the mother and developmental problems in the baby.

Dr. Mike Hansen, MD
Internal Medicine | Pulmonary Disease | Critical Care Medicine

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