What actually is vitamin D? A plain-English guide

It isn't really a vitamin. Your body makes it from sunlight. There are two forms — D2 and D3 — and they aren't quite equivalent. Here's what vitamin D actually is, where it comes from, and what it does in your body.

Most people learn about vitamin D in a sentence: "It's for your bones, you get it from sunlight, and Australians don't have to worry because we have plenty of sun." Two of those three things turn out to be roughly true, and one of them isn't true at all. But before we get to who should worry, it helps to understand what vitamin D actually is — because almost everything interesting about it comes from a single odd fact: it isn't a vitamin.

It's not really a vitamin

By the textbook definition, a vitamin is a substance your body can't make and therefore has to get from food. Vitamin D doesn't fit that description. Your body makes vitamin D itself, given the right inputs — specifically, ultraviolet B (UVB) radiation hitting a cholesterol precursor in your skin. That makes vitamin D, technically, a secosteroid hormone: a molecule structurally related to cholesterol that your body produces, modifies, and uses as a signalling compound in dozens of different systems.

The "vitamin" label is a historical accident. When vitamin D was first discovered in the early 20th century, scientists assumed it was a vitamin because they could cure rickets by feeding cod liver oil to deficient kids. By the time the full story was understood, the name had stuck.

This matters more than it sounds. Because vitamin D acts like a hormone, almost every cell in your body has a vitamin D receptor (VDR). That's why its influence reaches well beyond bones: into immune function, muscle tissue, brain receptors, cardiovascular health, and cellular growth and repair.

The two forms — D2 and D3

D3 (cholecalciferol) is the form your skin produces when UVB radiation interacts with 7-dehydrocholesterol. It's also found in animal-source foods like oily fish, egg yolks, and liver. Most over-the-counter vitamin D supplements in Australia are D3.

D2 (ergocalciferol) is produced when UV light hits a similar precursor in plant and fungal cell membranes. The main natural source is mushrooms (particularly UV-exposed varieties). D2 is also used in some fortified foods and prescription products like Ostevit-D.

Recent evidence suggests D3 is somewhat more effective than D2 at raising and maintaining blood vitamin D levels, particularly when taken as a weekly or monthly dose. Both work; D3 generally works better.

How your body makes vitamin D from sunlight

• UVB radiation penetrates the upper layers of skin and interacts with 7-dehydrocholesterol, a sterol present in skin cells.
• That interaction produces previtamin D3, which spontaneously isomerises into vitamin D3 (cholecalciferol) over 24–48 hours.
• Vitamin D3 travels to the liver, where it's converted to 25-hydroxyvitamin D (25(OH)D) — the storage form that circulates in your blood. This is what's measured in vitamin D blood tests.
• When needed, the kidneys convert 25(OH)D into 1,25-dihydroxyvitamin D (calcitriol) — the biologically active hormone form.

Vitamin D blood tests measure 25(OH)D rather than 1,25(OH)2D because 25(OH)D has a half-life of about three weeks, making it a stable, reliable indicator of your overall vitamin D status.

The limits of skin synthesis

• UV Index. When the UV Index is below 3, the UVB component is too weak to drive meaningful vitamin D production.
• Skin tone. Darker skin tones need substantially more UVB exposure to produce the same amount of vitamin D.
• Age. A 70-year-old produces roughly a quarter of the vitamin D from the same UV exposure as a 20-year-old.
• Window glass blocks UVB. A sunny office contributes essentially nothing to vitamin D production.

What vitamin D does in your body

• Calcium and phosphorus absorption. Without enough vitamin D, calcium absorption drops sharply and the body starts drawing calcium out of bone.
• Immune function. Almost every immune cell carries a vitamin D receptor.
• Muscle function. Deficiency is associated with muscle weakness and (in older adults) higher falls risk.
• Mood and brain function. Adequate levels are associated with more stable mood, particularly through low-light months.
• Cell growth and differentiation. Vitamin D plays a role in how cells mature, divide, and die.

How much do you need?

The NHMRC publishes Nutrient Reference Values for vitamin D, based on the assumption of minimal sun exposure.

How to know if you have enough

You can't tell from how you feel. Mild and moderate deficiency typically produces no obvious symptoms — or symptoms easily mistaken for stress, age, or "just winter."

The only definitive way to know is a blood test that measures your 25(OH)D level. A baseline test, a follow-up three months after starting supplementation, and an annual maintenance test is reasonable for most adults.