Vitamin B12 is a good example of how the amount isn't always what matters in the body. We only need a few micrograms a day, yet a surprising amount depends on it: nerve function, energy, blood formation, and cell division.

Especially with a vegetarian or vegan diet, vitamin B12 intake quickly becomes an issue, because it naturally occurs primarily in animal-based foods.

But one mushroom in particular breaks this pattern: Shiitake can contain vitamin B12.

And that is precisely what makes it particularly interesting from a scientific and nutritional perspective.

Why B12 sometimes only becomes a problem late

Vitamin B12 is water-soluble. Nevertheless, our bodies can store unusually large reserves of it.

Approximately 1 to 5 milligrams are stored, primarily in the liver. This is roughly 1,000 to 2,000 times what we ingest daily.¹

This has a surprising effect:

Insufficient vitamin B12 intake can go unnoticed for a long time. Years can pass before stores are significantly depleted. This is precisely why vitamin B12 is not a vitamin you should rely solely on whether you feel tired or energetic.¹

Because a low B12 status doesn't just affect energy.

In cases of severe deficiency, fatigue and changes in blood count can occur, as well as tingling, numbness, and neurological changes.1

These neurological abnormalities can even occur without a typical anemia being present at the same time.1

This makes vitamin B12 significantly more interesting than its reputation as a classic "energy vitamin".

Our nerves are especially sensitive.

Vitamin B12 is needed, among other things, for the development and function of our nervous system and for the formation of myelin.1

Myelin is the insulating layer that surrounds many of our nerve fibers. You can think of it a bit like the insulation of an electrical cable: without it, electrical signals are no longer transmitted as efficiently.

A human study with 2,287 adults between 72 and 83 years of age investigated how a low vitamin B12 status can actually manifest itself in nerve function.²

7% of participants met the criteria for vitamin B12 deficiency, and a further 10.1% had low serum levels.

In people with vitamin B12 deficiency, the measured nerve conduction velocity was significantly lower: 42.3 instead of 43.5 meters per second.²

Furthermore, they often reacted worse to light touch stimuli.

This shows why vitamin B12 goes far beyond the topic of energy. The vitamin is closely linked to fundamental functions of our nervous system.

Who should pay particular attention to vitamin B12?

Vegans are a particular focus here.

A meta-analysis from 2024 evaluated 19 studies on the B12 status of vegan adults.³ Compared to omnivores, vegans had lower serum B12 levels and higher homocysteine ​​levels.

Even more interesting: Vegans who supplemented with B12 showed better values ​​for all B12 biomarkers examined than vegans without supplementation.³

But the issue doesn't end with vegans.

Vegetarians also tend to have lower B12 intake and lower B12 levels on average than omnivores.In a large systematic review of 141 studies, vitamin B12 was one of the nutrients whose intake and status tended to be lower in diets that were more plant-based.⁴

And even those who don't call themselves vegetarians, but rarely eat meat and fish and at the same time consume few eggs or dairy products, automatically reduce many of the classic B12 sources.

This does not mean that everyone with a predominantly plant-based diet will automatically develop a B12 deficiency. However, it illustrates why adequate B12 intake becomes increasingly important as traditional B12 sources are increasingly being phased out.

And of all things, shiitake.

This is where the story becomes unusual.

Vitamin B12 is produced by certain microorganisms. Therefore, our typical natural sources are primarily animal-based foods such as fish, meat, eggs, and dairy products.1

However, researchers have long been searching for non-animal foods that actually contain vitamin B12.

This is more difficult than it initially sounds. Some supposed plant-based sources of vitamin B12 contain predominantly so-called B12 analogs. These are molecules that are chemically similar to the vitamin but do not perform the same function in human metabolism.

It's a different story with shiitake mushrooms.

Vitamin B12 was indeed detected in studies of dried shiitake fruiting bodies.⁵ The average content was approximately 5.6 µg per 100 g of dry weight. However, the values ​​varied considerably between the mushrooms tested.⁵

This makes shiitake one of the unusual non-animal foods that are of particular scientific interest in connection with natural vitamin B12.

Why natural B12 levels can fluctuate

The fact that vitamin B12 has been detected in shiitake mushrooms does not automatically mean that every mushroom contains the same amount.

The investigations show significant variations between different samples.⁵

This point is particularly relevant from a nutritional science perspective. With natural foods, origin, growing conditions, and other factors can influence the actual nutrient content.

While shiitake mushrooms expand the range of possible natural sources of B12, the actual content cannot be determined solely from the fact that a food contains shiitake mushrooms.

Why high B12 doses can initially be surprising

Vitamin B12 has one of the more unusual absorption pathways among the vitamins.

Part of the absorption process works with the help of the so-called intrinsic factor. This protein, produced by the body, binds vitamin B12 and then enables its absorption in the final section of the small intestine.1

But this system is limited.

At very small amounts of vitamin B12, below approximately 1 to 2 µg, roughly 50% can still be absorbed. However, with increasing amounts, the percentage decreases drastically.

On average, only about 2% of 500 µg is absorbed. Of 1,000 µg, approximately 1.3%.¹

This explains why B12 supplements often contain dosages that seem very high at first glance.

The body does not need several hundred micrograms of vitamin B12 per day. However, only a small portion of a high single dose is actually absorbed.

Therefore, the microgram count alone does not tell the whole story, even with vitamin B12.

What Vitamin B12 means in everyday life

When people talk about vitamin B12, the word "energy" is usually the first one that comes up.

That's too simplistic.

Vitamin B12 is necessary for the formation of healthy red blood cells and for DNA synthesis. It is closely linked to homocysteine ​​metabolism and plays a central role in the development, myelination, and function of the nervous system.1

A pronounced deficiency can therefore manifest itself in very different ways: fatigue and weakness, changes in blood count, concentration problems, tingling or numbness in the hands and feet, and neurological complaints.1

This is precisely what makes B12 so special:

We only need very little of it. But this small amount is needed in places that are fundamental to our daily functioning.

People who regularly eat fish, meat, eggs, and dairy products obtain vitamin B12 from various foods. Those who reduce their intake of these foods or follow a vegetarian or vegan diet are more reliant on alternative sources.

And this is exactly where shiitake becomes interesting.

The mushroom expands the picture of a vitamin that we almost exclusively associate with animal foods, revealing it as an unusual natural source.

It is precisely this exception that makes shiitake particularly interesting from a scientific and nutritional perspective.

Sources

¹ National Institutes of Health, Office of Dietary Supplements. Vitamin B12 – Fact Sheet for Health Professionals. Describes, among other things, the importance of vitamin B12 for the nervous system, myelination, red blood cell formation and DNA synthesis, as well as body storage, deficiency symptoms and dose-dependent absorption.

² Leishear K, Boudreau RM, Studenski SA, et al. Relationship between vitamin B12 and sensory and motor peripheral nerve function in older adults. Journal of the American Geriatrics Society. 2012;60(6):1057–1063.

³ Niklewicz A, Hannibal L, Warren M, Ahmadi KR. A systematic review and meta-analysis of functional vitamin B12 status among adult vegans. Nutrition Bulletin. 2024;49(4):463–479.

⁴ Neufingerl N, Eilander A. Nutrient Intake and Status in Adults Consuming Plant-Based Diets Compared to Meat-Eaters: A Systematic Review. Nutrients. 2022;14(1):29.

⁵ Watanabe F, Yabuta Y, Bito T, Teng F. Vitamin B12-Containing Plant Food Sources for Vegetarians. Nutrients. 2014;6(5):1861–1873.