Vitamin D3

What it actually is

Step outside on a clear day and put your forearms in the sun for a few minutes. Something is already happening in your skin.

Ultraviolet B light hits a cholesterol derivative sitting in the skin and converts it. The liver picks that up and turns it into 25-hydroxy vitamin D, the storage form, which is what a blood test measures. The kidneys then convert what is needed into the active hormone, which does the actual work.

That word hormone matters more than the word vitamin. A vitamin is something you must eat because your body cannot make it. Vitamin D fails that definition, because given sunlight your body makes it perfectly well. It behaves like a hormone: made in one place, acting in many others, on receptors found in most tissues in the body.

Which is why the deficiency picture is so broad. Bone, immune function, muscle, mood. Not because vitamin D is magic, but because the receptor is nearly everywhere.

Normal, optimal, and the year the goalposts moved

Here is where I have to be more careful than most pages on this topic, because the ground genuinely shifted and a lot of writing has not caught up.

The reference point almost everyone still works from came from the 2011 Endocrine Society clinical practice guideline: deficiency below 20 ng/mL, insufficiency from 20 to 30, sufficiency at 30 and above. That is the source of get above 30, and it is what most labs, most practitioners and most articles still quote.

In 2024 the Endocrine Society published a guideline communication stating that it no longer endorses its previously proposed definitions of sufficiency at or above 30, or insufficiency between 20 and 30. The reasoning given is that in generally healthy populations, available clinical trial evidence does not permit assigning specific thresholds that predict net benefit from supplementation.

Read that carefully, because it is easy to misread in both directions.

It does not say vitamin D does not matter. It does not say deficiency is not real. It says something narrower: the trials do not support drawing a confident line and declaring everyone above it fine and everyone below it in need of a supplement.

I will be honest with you, because I always will be. This is one of the more interesting corrections in nutritional medicine in recent years, and it is uncomfortable for an industry that has been quoting 30 as settled for a decade. Your number is real. What it means for you specifically is less settled than almost anyone selling you something will admit.

What has not changed

Genuinely low values still matter. Well below 20 is associated with real bone and musculoskeletal consequences, and that part of the picture was not withdrawn.

What changed is the confident middle. A result of 28 being labelled insufficient and triggering an automatic supplement recommendation has considerably less behind it than the last decade implied.

So the practical read: a low number is worth acting on with a provider. A middling number is a conversation rather than a diagnosis. And a high number in someone who has been supplementing heavily is worth taking seriously in the other direction.

D2 and D3 are not the same thing

There are two forms sold as vitamin D, and the difference matters more than the shared name suggests.

D3, cholecalciferol, is what your skin makes from sunlight and what is found in animal foods. D2, ergocalciferol, comes from fungal or plant sources and is the form used in a lot of high-dose prescription vitamin D.

They are both converted to a storage form, and the storage forms are chemically distinct: 25(OH)D3 and 25(OH)D2. Most lab reports give you a total that adds the two together, which is where this gets interesting.

A 2024 systematic review and meta-analysis in Advances in Nutrition compared them head to head across 20 comparative randomized trials. D3 was superior to D2 at raising total 25(OH)D, and in the daily-dose comparisons analysed by mass spectrometry the D2 group ended up 10.39 nmol/L lower, a difference of 40 percent.

So on raising your level, D3 simply works better.

Now the finding that should change what you buy. A 2024 dose-response meta-analysis in Steroids pooled 33 study arms of D2 supplementation and found it raised total vitamin D by 11.47 ng/mL, raised 25(OH)D2 as expected, and lowered 25(OH)D3 by 4.63 ng/mL.

Read that carefully, because it is precise and it is easy to overstate. Taking D2 does not lower your vitamin D. It raises your total while actively pushing down the D3 fraction, the form your body makes itself and the form the outcome research was largely built on.

That is the argument for D3 stated at its actual strength: better at raising the total, and it does not displace the form you already had.

One practical consequence worth knowing. If you take D2 and your total looks acceptable, that total is partly D2 propping up a falling D3. A report giving only a total cannot show you that, and asking for the fractions is what reveals it.

What moves it

Down: winter and northern latitude, where the sun sits too low for enough UVB to reach you for months at a time. Spending daylight hours indoors. Darker skin, which needs considerably more exposure to make the same amount. Sunscreen, used properly. Age, since skin makes less of it over time. Higher body fat, since vitamin D distributes into fat tissue. Conditions affecting fat absorption, because it is fat soluble. Some medications.

Up: sunlight, by far the most efficient route. Oily fish, egg yolks and fortified foods, modestly. Supplementation, reliably. Note that sun exposure alone will not push you into excess, because the skin self-limits production. Supplements can.

The honest hedge

I am not a doctor and I am not diagnosing anyone.

Vitamin D is the single most over-claimed nutrient in wellness writing, and I would rather tell you that than add to it. It has been proposed as helpful for a very long list of conditions, and the trial results for most of those have been considerably less impressive than the observational associations suggested. Low vitamin D turns out to accompany poor health often without being the thing that caused it.

What holds up: it matters for bone and muscle, deficiency is real and common, and correcting genuine deficiency is worth doing. What is shakier: precise optimal targets, and the idea that pushing a normal person higher produces benefits.

It is a variable worth knowing, and it is one almost nobody measures.

So. What to actually do.

Tonight, at no cost. Look at your last blood panel and see whether vitamin D is even on it. For most people it is not, which means the honest answer to what is your level is that you do not know.

This week. If you have never measured it and you live somewhere with real winters, spend most of your day indoors, or have darker skin, that is a reasonable case for testing. The 25-hydroxy vitamin D test is one of the cheaper ones on the menu and it does not require fasting.

If you are supplementing, take D3 rather than D2. That is the clearest practical instruction on this page. D3 raises total 25(OH)D more effectively across 20 comparative trials, and D2 lowers the 25(OH)D3 fraction while it lifts the total. Check the label on what you already own, since it will say cholecalciferol for D3 or ergocalciferol for D2.

If your D2 was prescribed, raise it with your prescriber rather than switching quietly. High-dose prescription vitamin D is commonly D2, and the dosing between the two is not interchangeable.

If you are already supplementing, measure. Particularly if you have been taking a high dose for years on the strength of general advice rather than a number. This is the situation where people are most often surprised.

Whatever you change, retest at about three months. The storage form moves slowly, so that is roughly how long it takes to settle at a new level.

None of this has to happen this week. But tonight you can check whether the number even exists on your last panel.

You are not a person who is bad at making vitamin D. You are an animal built for more sun than modern life gives you, running a hormone system on an input your ancestors got for free and you now have to plan for.

Your body is not broken. It is blocked. And sometimes the block is simply a winter that is longer than your skin was designed for.

Go find out what your number actually is.