Low vitamin D and low vitamin B12 can both cause real, persistent tiredness, and both are correctable once a blood test confirms them. But they only explain fatigue when you are genuinely deficient. Topping up a level that is already normal does not create spare energy, so testing comes before supplementing.
Tiredness is the symptom people most often try to solve with a supplement, and it also has the longest list of possible causes. Poor sleep, an underactive thyroid, iron deficiency, low mood, sleep apnoea and simply not eating enough all sit on that list. Two nutrient shortfalls sit on it too, and unlike most of the others they are cheap to check.
That framing is the useful part. Vitamin D and vitamin B12 are not energy boosters. They are things that, when they run low, take energy away. The difference decides whether a supplement will do anything for you at all.
What does vitamin D actually do, and can low levels make you tired?
Vitamin D behaves more like a hormone your skin makes from sunlight than like a nutrient you eat. Most of what the body uses comes from ultraviolet light hitting bare skin, which is why levels drift down through autumn and winter at northern latitudes, and why the NHS advises everyone to consider a daily vitamin D supplement during those months. Adults are advised to get 10 micrograms, which is 400 IU, a day, and not to exceed 100 micrograms, or 4,000 IU, a day.
Its clearest job is calcium handling and bone health. Sustained severe deficiency causes rickets in children and osteomalacia, a painful softening of the bones, in adults. Osteomalacia produces aching legs and difficulty with stairs, and people describe that as tiredness long before anyone thinks about bones.
Outside that severe range the tiredness link is real but narrower than the internet suggests. In a double blind placebo controlled trial in adults whose vitamin D was low, supplementation reduced self reported fatigue compared with placebo. What that shows is that correcting a deficiency helps the people who have one. It does not show that extra vitamin D lifts energy in someone whose level is already adequate.
Claims about vitamin D and mitochondrial energy production, inflammation and mood sit on far weaker ground than the bone evidence. Laboratory and observational work exists behind each, but the human trials are mixed and none of it is settled enough to promise anything.
What does vitamin B12 do, and why does deficiency feel like exhaustion?
B12 has two jobs that matter here. It is needed to build red blood cells, and it maintains the myelin sheath that insulates nerves. Run short and both falter at once.
When red cell production stumbles, the cells that get made come out large and few. Less oxygen reaches muscle and brain, and the result is the flat, out of fuel tiredness people describe as being unable to get going. The NHS lists extreme tiredness, a lack of energy, muscle weakness, pins and needles, a sore red tongue, mouth ulcers and vision problems among the signs of B12 or folate deficiency anaemia, along with changes in memory, mood and judgement.
The nerve side is what separates B12 from most nutrient shortfalls. Prolonged deficiency can damage nerves in ways that may not fully reverse, which is the real argument for checking rather than guessing.
B12 comes almost entirely from animal foods: meat, fish, eggs, milk and cheese. Plants do not make it. Fortified products such as some breakfast cereals, plant milks and nutritional yeast are the exception, and only because the B12 has been added.
How do low vitamin D and low B12 compare?
They produce similar tiredness through completely different machinery, which is why one test cannot stand in for the other.
| Vitamin D | Vitamin B12 | |
|---|---|---|
| Main source | Sunlight on skin, with modest amounts from oily fish, egg yolk and fortified foods | Animal foods almost entirely, plus deliberately fortified products |
| Why running low is tiring | Muscle and bone symptoms in sustained deficiency; heavy, aching legs that read as fatigue | Fewer, larger red blood cells carry less oxygen, and nerve signalling slows |
| Typical reason for a low level | Little skin exposure to daylight, covered skin, darker skin in a low sun country, winter | Absorption failure more often than diet, though a vegan diet without supplements will do it |
| How it is confirmed | Blood test for 25-hydroxyvitamin D | Blood test, usually run alongside folate and a full blood count |
| How it is corrected | Oral supplement, with the dose set by how low the reading is | Injections first in many cases, then tablets or continuing injections |
| What it will not do | Add energy in someone whose level is already normal | Add energy in someone whose level is already normal |
Who is actually at higher risk of running low?
For vitamin D, the NHS advises a supplement all year for people who are rarely outdoors, who live in a care home, who usually cover their skin outdoors, or who have dark skin from an African, African-Caribbean or south Asian background, because melanin slows the skin’s production. Everyone else is advised to consider one through autumn and winter.
Circulating vitamin D also tends to be lower when body fat is higher, because vitamin D is fat soluble and distributes into fat tissue. That is why the topic keeps surfacing on weight loss sites, and the direction is worth stating plainly: a low reading is something to raise with a doctor, not a lever that shifts body weight. There is a fuller piece here on vitamin D and metabolism.
B12 risk is less about diet than most people assume. The most common cause of B12 deficiency in the UK is pernicious anaemia, an autoimmune condition in which the immune system attacks stomach cells and blocks absorption. Diet is a genuine cause for people eating a vegan diet without supplements or fortified foods. Some medicines reduce absorption too, including proton pump inhibitors and certain anticonvulsants. Age matters as well: B12 or folate deficiency anaemia affects around 1 in 10 people aged 75 and over, and around 1 in 20 of those aged 65 to 74.
How do you find out which one it is, if either?
Both are ordinary blood tests and a GP can arrange them. Vitamin D is measured as 25-hydroxyvitamin D. B12 is usually tested alongside folate and a full blood count, because the three together say more than any one of them alone.
Two things are worth knowing before a result comes back. Laboratories do not all use the same cut offs, and units differ between countries, so a number that looks alarming in one reporting format can be unremarkable in another. And a reading only means something inside the whole picture of symptoms, medicines and other tests, which makes it a conversation with whoever ordered it rather than a chart to match online.
Self dosing high amounts on a hunch is the thing to avoid. Vitamin D has a defined upper limit for good reason. And with B12, starting a high dose supplement before the test can lift a borderline reading and make the very thing you were trying to find harder to see.
What raises these levels in ordinary life?
Sunlight does most of the vitamin D work between late March and the end of September in the UK, and short, regular exposure of forearms and face is the practical version. You cannot overdose on it through sunlight, although you can burn. Food contributes modestly: oily fish such as salmon, sardines and mackerel, egg yolks, and fortified spreads and cereals. Through autumn and winter the 10 microgram supplement is standard advice, not an optional extra.
For B12, regular animal foods cover it comfortably for most people. Someone eating no animal products needs either a supplement or reliably fortified foods rather than occasional ones. And where the problem is absorption rather than intake, more food will not fix it. That is precisely the situation injections exist for.
What if your levels are normal and you are still tired?
This is the common outcome, and it is worth planning for rather than being disappointed by. Persistent tiredness has a long list of possible causes, from sleep debt and stress to iron deficiency, thyroid problems, coeliac disease and sleep apnoea. The NHS page on tiredness and fatigue is a reasonable starting point, and a GP is the right place to work through the rest.
There is also an unglamorous answer that applies to a lot of people reading a weight loss site. Eating too little, cutting carbohydrate sharply, sleeping badly and training more than you recover from will all produce fatigue that no vitamin will touch. If the tiredness arrived at the same time as a new diet or a new training block, that timing is information in itself, and when you train can matter as much as how hard.
Fatigue also quietly reduces how much you move across the day. That is part of why tiredness and a sluggish feeling metabolism arrive together, and why chasing the tiredness is the more productive of the two.
This article is general information, not medical advice, and is not a substitute for care from a qualified healthcare professional. Speak to your doctor about your own situation.
Common questions
Can vitamin D or B12 supplements give me more energy if my levels are normal?
No. Both correct a shortfall rather than adding capacity above normal. If your levels are in range and you are still tired, the cause lies somewhere else and is worth investigating properly.
Do I need a test, or can I just take both?
A test is the only way to know which, if either, applies to you. Vitamin D at the recommended daily amount is low risk, but taking B12 before testing can lift a borderline result and make the picture harder for a doctor to read.
How long does it take to feel different after a deficiency is corrected?
It varies with how low the level was and what caused it, and blood counts and nerve symptoms recover on different timescales. Your prescriber sets the plan and reviews it. Anyone offering a fixed number in advance is guessing.
Is a vegetarian diet enough for B12?
Usually yes, if it regularly includes dairy and eggs. A vegan diet is not, and needs either a supplement or consistently fortified foods rather than occasional ones.
Does carrying extra weight affect vitamin D levels?
Circulating vitamin D tends to be lower when body fat is higher, because vitamin D is fat soluble and distributes into fat tissue. That is a reason to discuss testing with a doctor, not a reason to expect a supplement to change body weight.
Can I get enough vitamin D from sunlight alone in winter?
In the UK, no. Between roughly October and March the sunlight is too weak at that latitude for skin to make useful amounts, which is why a daily supplement is advised through autumn and winter.
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