Vitamin D and B12: Why Your Body Needs Both

It's November. The days are getting shorter, and somehow so is our energy. Many of us know the feeling and blame it on stress, the weather or too little sleep. Sometimes, though, there is something measurable behind it.

Vitamin D and vitamin B12 are among the most important micronutrients because they control basic functions: vitamin D acts like a hormone in the body and regulates, among other things, how much calcium reaches your bones and muscles. Vitamin B12 is needed for making blood cells, for cell division and for healthy nerves. Deficiency is common, often develops slowly and at first shows up as vague symptoms such as fatigue, muscle weakness or trouble concentrating ¹³. The good news: both can be measured, and in most cases they can be improved.

Why is vitamin D really more of a hormone?

Because the body makes most of its vitamin D itself, and it only becomes active after two conversion steps – just like a hormone that is produced in one place and sends signals to many others ¹.

Here is the route:

1. Skin: UVB rays from the sun turn a cholesterol-based precursor in the skin into vitamin D3 ¹.
2. Liver: It is converted into 25-hydroxyvitamin D, the storage and transport form. This is the value measured in a blood test when your vitamin D status is checked ¹.
3. Kidneys: Only here is the active form made – the actual hormone, calcitriol ¹.

Think of it as a warehouse and a key: the storage form is the stock in the warehouse, the active form is the key that actually unlocks something in your cells.

What's fascinating is how precisely the body controls this. The kidneys don't simply produce the active form non-stop, but according to need. Parathyroid hormone and the calcium and phosphate levels in your blood help regulate it, as does a messenger released by bone ¹. A built-in breakdown mechanism also makes sure that not too much active hormone builds up ¹. It's a small control system, constantly rebalancing.

Research has changed our view in another way, too: almost every tissue and cell in the body has a docking site (a receptor) for vitamin D, and some can even make the active form locally ¹. Vitamin D is far more than a "bone vitamin".

What does vitamin D do in the body?

Its main job is managing calcium and phosphate – and with that, keeping bones strong and muscles working. Without vitamin D, the gut absorbs only about 10 to 15 percent of the calcium in food; with enough vitamin D, it's around 30 to 40 percent ¹.

Beyond that:

• Muscles have their own vitamin D receptors; deficiency is linked to muscle weakness ¹.
• Immune cells use vitamin D to produce the body's own defence molecules against pathogens – shown mainly in laboratory and mechanistic studies ¹.
• Blood pressure and blood sugar: the active form can dampen a blood pressure–regulating system in the kidneys and stimulate insulin release in the pancreas ¹.

For a long time, people hoped vitamin D pills might also prevent cancer or heart attacks. One of the largest trials put this to the test: in healthy adults over 50, taking vitamin D daily for just over five years did not lower the risk of cancer or major cardiovascular events ⁵. That's not a setback but a valuable clarification: vitamin D is not a cure-all. It truly matters where there is a real deficiency – and that is exactly what needs to be detected.

Where do we get vitamin D – and why isn't winter sun enough?

The main source is sunlight, not food. Only a few foods contain meaningful amounts, mainly oily fish ¹.

The catch in Central Europe: above roughly 35 degrees north, the skin can make little or no vitamin D from November to February because the sun sits too low in the sky ¹. Vienna is at about 48 degrees. In winter, we live off the stores we built up over the summer.

How much your skin produces also depends on you [1]:

Factor	What happens
Age	At 70, the skin makes about 75% less vitamin D than in youth
Sunscreen	Even SPF 15 blocks production almost completely
Darker skin	More melanin filters out more UVB
Body fat	Vitamin D gets "parked" in fat tissue and is less available
Gut and kidney conditions	Absorption or activation can be impaired

How common low levels are is shown by a large European analysis of standardised blood values from more than 55,000 people: around 40 percent were below 50 nmol/l, a commonly used cut-off for deficiency, and clearly low levels (below 30 nmol/l) were about twice as common in winter as in summer ².

One important caveat: sunshine is no excuse for sunburn, and more vitamin D isn't automatically better. Very high levels can become toxic ¹.

What does the body need vitamin B12 for?

B12 is a helper molecule (a coenzyme) that key metabolic steps can't run without. Together with folate, it breaks down homocysteine, an intermediate product that would otherwise build up in the blood ³. Through this partnership, B12 is essential for cell division, for making new blood cells and for producing DNA ³. It's also involved in fat and protein metabolism ³ and in energy production in the cell's power plants, the mitochondria ⁴.

The nervous system is especially sensitive: B12 is needed for nerves and the brain to work properly. If it's missing for a long time, nerves can be damaged ³⁴.

How does vitamin B12 get from your plate into your cells?

By a surprisingly elaborate route – which is exactly why deficiency can occur even with a decent diet.

First, the source: neither plants nor animals can make B12 themselves; only microorganisms can. It enters the food chain and ends up in animal foods such as meat, fish, eggs and dairy ³. Algae or sauerkraut often contain only inactive B12-like compounds that can even make your status worse ³. For people eating a vegan diet, the German Nutrition Society therefore recommends taking a B12 supplement permanently ³.

Then, absorption: the stomach produces a carrier called intrinsic factor. Only when B12 is bound to it can the gut absorb it ³. Picture it as a ticket – without it, B12 isn't allowed through the gut wall. That's why stomach and bowel conditions, such as chronic gastritis, Crohn's disease or stomach surgery, can cause deficiency regardless of diet ³. The risk is also higher with older age and with certain long-term medications, such as acid blockers for heartburn or metformin for diabetes ⁴.

The good news: the body keeps large B12 stores. Deficiency usually appears only after years of low intake ³ – which gives us time to catch it early.

How much B12 is in common foods (reference intake for adults: 4 µg per day) [3]:

Food (portion)	Vitamin B12
Lean beef (100 g)	approx. 4.5 µg
Salmon (70 g)	approx. 2.9 µg
Mozzarella (100 g)	approx. 2.0 µg
Egg (1)	approx. 1.1 µg
Milk or yoghurt (150 g)	approx. 0.6 µg

What does vitamin D or B12 deficiency feel like?

Often, quiet. That's what makes both deficiencies so tricky: the symptoms are vague and easily put down to other causes.

With vitamin D, bones and muscles are at the centre. Deficiency can trigger or worsen osteoporosis, raise the risk of fractures and cause muscle weakness ¹. In more severe cases, bone softening can develop and show up as dull, aching bone or muscle pain – complaints that are easily mistaken for other conditions ¹.

With vitamin B12, the signals are more varied ³[4]:

• tiredness and exhaustion that sleep doesn't fix
• poor memory and difficulty concentrating
• low mood
• tingling, numbness or unsteadiness when walking, if nerves are affected
• anaemia with paleness and shortness of breath

Worth knowing: nerve symptoms from B12 deficiency can appear before any changes show up in a blood count ⁴. A normal blood count does not rule out B12 deficiency.

If you recognise yourself in some of these points, it doesn't automatically mean something is missing. These symptoms have many possible causes. But it's a good reason to check rather than guess.

Should you test vitamin D and B12 regularly?

This deserves an honest look at the evidence, because the answer is more nuanced than many marketing messages suggest.

What we know today: in its 2024 guideline, the US Endocrine Society advises against routinely testing vitamin D levels in healthy adults without a specific reason. The reason: studies have not yet identified a target level above which particular diseases become measurably less common ⁶. For B12, the German Nutrition Society recommends that at-risk groups, such as people on plant-based diets, have their status checked regularly by a doctor – ideally with a combination of status and function markers, because a single value on its own isn't always clear-cut ³.

In other words, testing is especially useful when risk factors are present. And those are more common than you might think: older age, little winter sun, sunscreen, a plant-based diet, digestive problems, metformin or acid blockers ¹³⁴.

What becomes possible: we believe prevention shouldn't stop at a single number. A vitamin D level in February tells you something different from one in August. A B12 level that slowly drifts down over years tells a different story from one that was borderline once. Only over time, and alongside other values such as your blood count, kidney function or inflammation markers, does it become a picture you can act on.

That's why vitamin D and B12 are a fixed part of the speak23 Core Health blood test in our base membership. Not as a stand-alone test answering "deficient or not", but as part of the micronutrient area, which we interpret together with your other body systems. If you want to go deeper, the add-on Comprehensive Vitamins & Trace Elements covers more values.

What can you do?

1. Check regularly instead of guessing once: with speak23, you measure your values 2 or 4 times a year – so you can see, for example, how much your vitamin D swings between summer and winter.
2. Know your personal trend: all values are recorded digitally and automatically. In the app, you can see whether your B12 or vitamin D is holding steady or slowly drifting in one direction.
3. Act with precision, not on a hunch: instead of taking supplements on suspicion, you get evidence-based, personalised recommendations tailored to your values and how they develop – on sunlight, diet and, where it makes sense, supplements.
4. See what works: at your next test, the app shows how your changes have affected your values. Advice becomes a change you can actually verify.
5. Have anything unusual checked by a doctor: clear deficiencies, nerve symptoms or an abnormal blood count belong in medical hands – speak23 does not make diagnoses. But you can bring your values and trends straight into that conversation.

Vitamin D and B12 show, on a small scale, what the medicine of the future is about on a large one: many complaints we accept as "normal" for the season or our age are measurable and changeable. In an ageing society, that's no side issue. It's a chance for more years in which we feel strong, clear-headed and like ourselves. Maybe it starts with a simple question: do you actually know how full your stores are?

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