Why vitamin D is hard to feel
Most deficiencies announce themselves fairly quickly. Iron drops and you feel breathless on the stairs. B12 falls and your hands start tingling. Vitamin D is different. It declines slowly, over weeks and months, and the symptoms it produces are easy to attribute to the season, the workload, the weather.
That's part of why the NHS doesn't wait for people to report symptoms before making a recommendation. The guidance is population-wide: everyone aged four and over should consider taking a daily supplement containing 10 micrograms of vitamin D during the autumn and winter. The reason is that between October and March there simply isn't enough sunlight at this latitude to produce meaningful amounts of vitamin D from skin, regardless of how much time you spend outside.
Most people don't know they're running low because vitamin D doesn't ring a sharp, obvious bell. It accumulates as a background drag.
The recognised pattern
If you're looking for a pattern to match rather than a diagnosis, these are the symptoms the NHS associates with low vitamin D. None of them is specific to vitamin D alone, which is part of why they're easy to miss. What's useful is the grouping: when two or three appear together, especially from autumn through spring, the case for acting on the food and supplement side is stronger.
Bone and muscle pain or tenderness. Vitamin D is needed for calcium absorption; a sustained shortage can leave bones less dense and more prone to aching. In adults, prolonged deficiency can lead to osteomalacia, a softening and tenderness of the bones.
Getting ill more often. Vitamin D plays a role in immune function; running low may leave you more susceptible to colds and infections.
Hair feeling thinner. Less commonly noted, but included in the pattern. If the other three are also present, it's worth taking seriously.
A few things to bear in mind: each of these symptoms has many possible causes. If this pattern fits but you've been taking the 10µg supplement consistently for four to six weeks and things haven't shifted, that's the point to speak to your GP, not because you necessarily need a blood test, but because a broader triage is more useful than continuing to guess.
What the body is doing
Vitamin D's main job is to regulate how much calcium and phosphate the body absorbs. Those two minerals are the structural material of bones and teeth, and they also keep muscles contracting correctly. Without enough vitamin D, the body can absorb calcium from food but can't deploy it efficiently, which is why the ache tends to land in bones and muscles rather than anywhere more obvious.
There's a second role that matters for how you feel day to day: vitamin D appears to influence the pathways involved in mood regulation. The relationship isn't fully mapped yet, but the seasonal overlap between low sunlight, low vitamin D, and low mood is observed consistently enough that the NHS lists persistent low mood as a recognised symptom of deficiency.
Vitamin D is also unusual in how the body makes it. Unlike most vitamins, which come entirely from food, the main source is skin exposed to sunlight. UVB rays trigger a reaction that produces vitamin D in the skin, which the liver and kidneys then convert into the active form the body uses. Between October and March in the UK, UVB levels fall too low for this process to work, which is why the winter supplement fills a gap that food alone can't reliably close.
The food side, and where it falls short
Food contributes to vitamin D levels, oily fish, egg yolks, fortified spreads, and liver are the main sources, but even a varied diet including oily fish a couple of times a week rarely meets the 10µg daily target through an indoor UK winter, once you factor in the drop in sunlight synthesis. That's the honest case for the supplement.
The supplement question
The NHS recommends 10µg of vitamin D daily through autumn and winter for everyone aged four and over, the only supplement it recommends for the general population without caveats. Some groups need a different dose year-round, and "fortified" on a food label doesn't always mean what it looks like it means. We've covered the winter supplement question in full, who needs a different dose, what to look for on labels, and what D2 versus D3 actually means in practice, in our piece on getting vitamin D through winter.
When to see your GP
A GP visit for vitamin D specifically makes sense in a narrower set of circumstances than people often assume. The NHS doesn't routinely offer blood tests for vitamin D levels in people without clinical symptoms. If the pattern above fits, the more practical first step is the 10µg supplement through autumn and winter, which is inexpensive and broadly recommended anyway.
See your GP if: symptoms persist after four to six weeks of taking the supplement consistently; you're experiencing significant bone pain or tenderness rather than a general background ache; you suspect a condition that affects nutrient absorption, such as Crohn's disease or coeliac disease; or you have a history of osteoporosis and want your vitamin D status properly assessed alongside it.
In those cases, a GP can arrange a blood test and read the result in context, which is more useful than a number on its own. If persistent low mood is the primary concern, a broader conversation with your GP about mood, sleep, and the season is more useful than treating vitamin D as the single answer.
A short, honest note
This page describes what the NHS associates with low vitamin D and what the evidence on food and supplementation says. It isn't a diagnosis, and it isn't a substitute for a clinical assessment. If the pattern here fits and things don't shift after following the NHS guidance, the right next step is a conversation with your GP, not more reading.