The Question I Get Every January
Someone walks in, usually in early January, looking a bit fed up. They tell me they have been taking a vitamin D supplement since last summer. They got their bloods done. The level has barely moved. They want to know what went wrong.
- Someone walks in, usually in early January, looking a bit fed up.
- Worth knowing This post is about supplementing sensibly, not diagnosing anything.
It is a fair question. In thirty years behind this counter in Paignton, I have heard this story more times than I can count. The supplement was not fake. The person was not imagining it. Something in the chain broke down, and a short GP appointment rarely gets to the bottom of it.
That is what this post is for. The three most common reasons a vitamin D supplement stops working as well as it should, and what you can actually do about each one.
A Quick Note Before We Start
This post is about supplementing sensibly, not diagnosing anything. If your vitamin D level is genuinely and persistently low despite supplementing, please do talk to your GP. There are medical reasons that can affect absorption, and a healthcare professional needs to rule those out.
If you want to understand how much to take in the first place, I have covered that separately in The NHS Vitamin D Advice Is Probably Leaving You Short: What Adults in the UK Actually Need. This post picks up from there.
Reason One: You Are Taking It at the Wrong Time
Vitamin D is fat-soluble. That sounds straightforward, but it has a real practical implication. If you are taking your tablet on an empty stomach first thing in the morning with a glass of water, a meaningful portion of it may not be absorbed properly. Fat-soluble vitamins need dietary fat present to be taken up through the gut wall.
This is probably the single most common reason I see people plateau. They are consistent, they are taking a reasonable dose, but they are taking it at the worst possible time.
The fix is simple. Take your vitamin D with your biggest meal of the day. If that meal contains some fat, even a small amount, you are giving the vitamin somewhere to go.
The form matters too. There are two forms you will mostly encounter:
- D3 (cholecalciferol) - The form your skin makes when exposed to sunlight. Generally more effective at raising and maintaining blood levels.
- D2 (ergocalciferol) - Plant-derived, used in some supplements and some prescription versions. Less efficient at raising levels in most people.
If your supplement says D2 on the label, that may be part of the problem. Most quality UK supplements use D3, but it is worth checking. Prescription vitamin D in the UK is sometimes dispensed as D2, which is worth mentioning to your GP if your levels are not moving.
Reason Two: You Are Missing the Cofactors
This is the one that surprises people most, and the one that gets the least attention in a standard GP appointment.
Vitamin D does not work in isolation. It relies on other nutrients to be properly metabolised and used by the body. If those nutrients are low, supplementing with vitamin D alone is like trying to run an engine with no oil. You can pour in all the fuel you like.
The two main ones to know about are magnesium and vitamin K2.
- Magnesium - Required for multiple steps in the conversion and activation of vitamin D in the body. Low magnesium is very common in the UK, and many people are running short without knowing it.
- Vitamin K2 (specifically MK-7) - Works alongside vitamin D to support where calcium ends up in the body. K2 is genuinely difficult to get from diet alone unless you eat a lot of fermented foods, which most people in the UK do not.
I stock Viridian Vitamin K2 Vegan 60 Capsules for exactly this reason. It is the form I point people toward when they tell me they are already taking D3 and not getting anywhere. I have written about this pairing in more detail in Vitamin D3 + K2: Why You Should Always Take Them Together.
View in the shop
If you are supplementing vitamin D without adequate magnesium and K2, you are probably leaving a good chunk of the work undone.
Reason Three: The Dose Was Never High Enough
Standard UK advice has historically pointed people toward very low doses. Those doses do not account for people who start from a genuinely depleted position, people with higher body weight, or people whose absorption is naturally less efficient.
If you started at 400 IU daily in October and tested in April, a modest improvement is possible. But if you started from a low baseline, that dose may not be enough to move the needle meaningfully through a UK winter when sunlight is effectively absent as a source.
There is no single dose that is right for everyone. What I will say is that the dose you were told to take may have been designed for maintenance, not recovery from a low starting point. That distinction matters. If your GP has not discussed your starting level and your dose in the same conversation, that is worth raising next time.
D3 vs D2: A Quick Comparison
| Feature | D3 (Cholecalciferol) | D2 (Ergocalciferol) |
|---|---|---|
| Source | Animal-derived or lichen-based vegan versions | Plant-derived, UV-irradiated yeast or fungus |
| Effectiveness at raising blood levels | Generally more effective | Less efficient in most people |
| Vegan options available | Yes, lichen-derived D3 | Yes, less relevant given lower potency |
| Half-life in the body | Longer | Shorter |
FAQ
How long should it take for a vitamin D supplement to raise my blood level?
It varies depending on your starting level, your dose, and how well you are absorbing it. Most people see a meaningful change within eight to twelve weeks of consistent supplementation at an appropriate dose, taken with food. If you have been supplementing for longer with no movement at all, go back to your GP.
Can I just take more vitamin D to compensate for poor absorption?
Not simply, no. High doses taken without adequate cofactors and without monitoring can cause problems. Vitamin D toxicity is rare but real. Address the why first, which is usually cofactors or timing, rather than increasing the dose. Talk to your GP before significantly changing your intake.
Is a liquid or oil-based vitamin D supplement easier to absorb?
Liquid and oil-based formats tend to be absorbed more readily than dry tablets, because the fat-soluble vitamin is already in a lipid environment. A good-quality D3 softgel taken with a meal containing fat is usually sufficient. Timing and cofactors still matter more than format alone.
Can vitamin K2 interfere with any medications?
Vitamin K in any form can interact with anticoagulant medications such as warfarin. If you are on any blood-thinning medication, please speak to your GP or pharmacist before adding K2 to your routine. This is not something to navigate on your own.