vitamin D: what you need to know
It seemed like no one really cared about vitamin D until a few years ago, and now it's on everyone's lips. If you're a human and you don't live at the equator, your levels are probably low. Doctors used to test this rarely, worrying only about the disease of severe deficiency (rickets), but now it's tested routinely. Low vitamin D has been implicated in everything from fatigue to psychiatric disorders to low bone density. Huh? What's the unifying thread here? How does this work? Why should you care?
What even IS vitamin D?
Vitamin D (calciferol) is a group of compounds that are crucial to some physiological processes in the body. The most common forms are D2 (ergocalciferol) and D3 (cholecalciferol). There are others, but this isn't important to understand, nor is the chemical differences among the different types. They're all modified by the liver and then the kidneys into the biologically active form, known as calcitrol.
The major known function is to promote calcium absorption in the gut, which helps to maintain necessary calcium levels in the blood, muscles, and bone. There are lots of tissues in the body with vitamin D receptors, though, and it is also involved in modulating the inflammatory process and in cell growth. This is probably why such a wide variety of findings are associated with low levels of vitamin D. Clinically, low vitamin D seems to be associated with low mood, low energy, pain, and decreased immunity.
Should I test?
The correct form to test is 25(OH)D. This test definitely shows how much vitamin D you have in the body, but it's less clear whether this translates to the effect it has on the body.
Some medical conditions are associated with low vitamin D (including autoimmune disease, digestive diseases causing malabsorption, and cancer), and deficiency is common in people over 65. This is a correlation, NOT a causation.
Because a severe deficiency causes disease and it is possible to ingest too much, testing for vitamin D is wise if you have any risk factors, including medical conditions, age over 65, darker skin, limited exposure to sunlight, and residence far from the equator.
Should I supplement?
The totality of scientific evidence doesn't definitely show benefit for vitamin D supplementation in most situations-- but it's widely recommended by health care practitioners anyway, and preventing severe deficiency (leading to rickets) is certainly prudent. There does not seem to be any association with mortality (Zhang et al., 2019), though that's probably not the only thing you care about! Raising vitamin D levels into an optimal range makes sense (we call this "face validity") even if we don't yet have high-quality studies showing a direct impact on particular health outcomes.
Because it's a fat-soluble vitamin, it's stored in the body (unlike water-soluble vitamins, which are generally excreted when there's excess). This means two things: first, you CAN take too much, and second, it's absorbed better when you have some dietary fat with it.
D2 is usually made from yeast and is generally vegan; D3 is usually made from lanolin, fish oil, or sometimes algae. Both work; some evidence suggests D3 raises levels more, so if you are not vegan, this might be a better choice. There's not clear, strong evidence of superiority, though, so either is an OK choice.
There's also no evidence that a liquid, "encapsulated", "micellar", or other form is superior to any other, though they are typically more expensive.
Look for a third-party quality seal on the bottle from NSF or USP: these mean the company follows good manufacturing practices and the product contains what it says it does.
If you take a multivitamin, check the label to see how much vitamin D you might be getting already.
A healthcare provider can prescribe a high-dose supplement, often dosed weekly, if you have very low levels. Otherwise, you can easily find supplements over the counter.
Should I focus on food?
The recommended RDA is 600 IU, which is equal to 15 mcg.
Some foods are fortified with vitamin D in the US, meaning it has been added. Cow's milk is fortified in the US and Canada, as are many plant-based beverages like soy, almond, or oatmilk (but read the label; not all are). Some breakfast cereals, orange juice, and other foods are fortified with vitamin D, too.
There aren't a lot of good natural dietary sources. Fatty fish, egg yolks, and beef liver have some. So do mushroom, especially those that have been treated with UV light (which seems to increase the amount present).
Should I rely on the sun?
UVB on the skin light promotes vitamin D synthesis in humans. However, exposure to UV light has risks as well, including skin cancer. UVB light penetration is limited by glass/windows, sunscreen, clothing, cloud cover, smog, and melanin content of skin. Vitamin D synthesis also appears to decrease with age. Some evidence suggests that daily exposure of face, arms, hands, and legs without clothing or sunscreen for 5-30 minutes is sufficient, but in reality, this may not be realistic and/or may not be truly adequate in many circumstances.
TL;DR
The upshot is, vitamin D is clearly important for health, but science hasn't identified all the mechanisms yet. Getting blood levels tested is a good idea, and getting to an optimal level by any combination of sun exposure, foods, and supplements that's appropriate for you can prevent disease caused by severe deficiency, may be helpful in overall health, and is unlikely to have any downsides.
