Vitamin D3 and B12: what happens when they're low?

The short answer

Low D3? Your body stops absorbing calcium properly and starts quietly taking it from your bones. You won't notice for a while. Later it's sore legs, weak thighs, a back that aches for no reason. Low B12 is a different story. It messes with your blood and your nerves. First you're tired all the time. Then your feet start to tingle, your balance feels off, you forget things. The good news: vitamin D levels usually bounce back fine with treatment. B12 is the one to take seriously, because nerve damage that's been going on for too long doesn't always heal completely.

Low D3 mainly hits

Bones and muscles

Low B12 mainly hits

Blood and nerves

Early warning

Usually nothing

Hardest to undo

Old B12 nerve damage

Tests

25(OH)D for D3, serum B12 for B12

In India

Most studies find 80–90% low in D

Why we always talk about these two together

Honestly? Mostly because they show up on the same blood report. Your doctor ticks both boxes, both come back low, and suddenly you've got two prescriptions and one big question: what were these things even doing?

Quite different jobs, it turns out.

So when D3 runs low, it feels like aches and weakness. When B12 runs low, it feels like exhaustion, plus odd sensations in your hands and feet. Two very different kinds of "not feeling right".

What happens when vitamin D3 is low

Here's something most people don't know. You could be having milk in your chai, a bowl of curd at lunch and paneer at dinner, and still be short on calcium. Why? Without enough vitamin D, your gut only takes in about 10–15% of the calcium you eat.¹ The rest just passes through.

Your body won't let the calcium in your blood drop, though. Your heart and nerves need it too much. So it finds a workaround, and that workaround costs you:

1. Less calcium comes in from food. The gut simply absorbs less.
2. Your parathyroid glands panic a little. They release more parathyroid hormone (PTH) to hold blood calcium steady. Doctors call it secondary hyperparathyroidism.²
3. PTH takes calcium from your bones. It's like withdrawing from savings to pay the monthly bills. Your blood test can look perfectly normal while this is happening.
4. Bones get thinner and weaker. In older people this speeds up bone loss and makes fractures more likely.²
5. New bone stays soft. If the deficiency is severe and goes on for long enough, adults get osteomalacia, which literally means soft bones, and children get rickets.¹

So what does it feel like?

For months, maybe nothing at all. That's what makes it sneaky. When it does show up, it's vague. A dull ache in the lower back or hips. Shins that hurt when you press them. Legs that feel heavy on the stairs. Getting up from the floor after sitting cross-legged becomes a bit of a project. Weakness in the hip and thigh muscles is a known sign of low vitamin D.¹ Most of us blame it on age, the office chair, or "bas thoda thakaan hai".

What happens when B12 is low

B12 deficiency takes its time. Your liver keeps a stash that can last years, so you can be eating too little of it for a long while before anything shows.⁴ When the stash finally runs out, two things go wrong.

Your blood

Your bone marrow makes red blood cells nonstop, and every new cell needs fresh DNA. B12 is part of how that DNA gets made. Short on B12, the marrow starts turning out fewer cells, and the ones it does make are oversized and half-finished. They don't carry oxygen well. The medical name is megaloblastic anaemia.³ You'd notice:

• Tiredness that a good night's sleep doesn't fix
• Getting out of breath on stairs you used to take two at a time
• Pale, slightly yellow-looking skin
• A sore, red, oddly smooth tongue, or mouth ulcers that keep coming back

Your nerves and brain

Now the part that actually worries doctors. B12 keeps myelin, the coating on your nerves, in good repair. Without it the nerves start to misfire, and the longest ones go first: the ones running down to your feet and hands.³ People describe it as:

• Pins and needles or numbness in the feet, later the hands
• Feeling unsteady, especially walking to the bathroom in the dark
• Forgetting things, feeling foggy, or being snappy and low for no clear reason

And this catches a lot of people out: your haemoglobin can be completely normal while the nerve symptoms are already there.³ "My blood test was fine" doesn't mean your B12 is fine. Anaemia tends to clear up quickly once B12 is replaced. Nerve damage that has been building for a long time may only get partly better.^3,4 That's why tingling feet shouldn't be left for "next month".

There's one more thing low B12 does. It lets a compound called homocysteine pile up in your blood. When researchers tested 441 men in and around Pune, 66% had low B12 and 40% had high homocysteine.⁶ Doctors keep an eye on homocysteine because high levels are linked to heart and blood vessel problems.

How it plays out over time

Neither of these hits you overnight. Roughly, if nothing changes, this is how it goes:

Who gets hit hardest

Women planning a baby, and pregnant women. B12 works hand in hand with folate while a baby's spine and brain are forming. In one large study, mothers with the lowest B12 levels had two to three times the odds of a pregnancy affected by a neural tube defect, compared with mothers who had the highest.⁵ The researchers' advice was to get B12 up before pregnancy, not during.

Our parents and grandparents. We absorb B12 less well as we age, and low vitamin D in older people speeds up bone loss and makes falls and fractures more likely.^2,4

Vegetarians, and especially vegans. B12 comes almost entirely from animal foods, so if you don't eat eggs, meat or fish and don't take a supplement, you're in the highest-risk group.⁴ Our B12 deficiency signs guide goes into who should supplement.

Anyone on certain medicines for years. Metformin for diabetes and acid-reducing tablets taken for two years or more are both linked to lower B12.^8,9 If that's you or someone at home, ask the doctor to check B12 at the next visit.

So what should you actually do?

Don't guess, and don't self-prescribe. A blood test settles it, it isn't expensive, and the right dose depends on how low you really are.

• For vitamin D, ask for a 25-hydroxy vitamin D test. On the report it's written 25(OH)D.
• For B12, ask for serum B12. If the number is borderline, your doctor may add homocysteine or methylmalonic acid (MMA) to see whether your cells are actually running short.

If you do turn out to be low, treatment is simple and it works. Part 2 of this series walks through symptoms, what your report numbers mean and how each one is treated. Until then, our pages on vitamin D3, methylcobalamin B12 and India's famous 60,000 IU weekly sachet have the detail on doses and forms.

Frequently asked questions

What happens if vitamin D3 is low for a long time?

Your gut takes in much less calcium, so your body pulls calcium out of your bones to keep blood levels normal. Over months or years that means thinner bones, bone pain, weak muscles and a higher chance of fractures. If it's severe and goes on long enough, adults can get osteomalacia (soft bones) and children can get rickets.

What happens if B12 is low for a long time?

Two things. Your blood: you can develop megaloblastic anaemia, which leaves you tired and breathless. Your nerves: tingling, numbness, poor balance and memory trouble. The anaemia usually clears up quickly with treatment. Nerve damage that has been going on for a long time may only partly recover.

Can low B12 cause nerve damage without anaemia?

Yes, and it's more common than people think. Tingling, numbness and balance problems can start while your haemoglobin is still normal, so a normal blood count doesn't rule B12 out. Ask for a serum B12 test, and if it comes back borderline, homocysteine or methylmalonic acid.

Is low vitamin D3 and B12 dangerous?

For most people it isn't an emergency, but ignoring it does real damage. Low D3 slowly weakens bones and muscles. Low B12 can cause anaemia and nerve damage that may become permanent if it's left untreated for long. The good part is that both are easy to test for and easy to treat.

Can you have low vitamin D3 and B12 at the same time?

Yes, and in India it's very common. Most people here are low in vitamin D, and B12 deficiency is widespread, especially among vegetarians. They cause different problems, so test for both instead of assuming one explains everything you're feeling.

This article is general information, not medical advice. If you have symptoms or take regular medication, talk to your doctor before starting any supplement.