Vitamin B12 Deficiency: Nature Brooks No Greed

Executive Summary

Your body cannot absorb more than 2 µg of vitamin B₁₂ at a time. Megadoses of 500 to 1000 µg mostly go a waste.

It is better to take multiple doses of 5 to 10 µg vitamin B₁₂ in a day, except when medical medical disorders prevent its absorption.

The relevant science is intricate and vegetarians are advised to go through the article below.

Here is the full article.

Banquet desserts are my favourites, especially the ice creams. I can forego the main course to eat more gelato. But I have always regretted feeling full after a few bowls.

Alas, our body can’t take much more than what it needs. Greed has no place in Nature.

This is also true of nutrients; more is not better. One needs to be careful when taking them as supplements or medicines, e.g., Vitamin B₁₂ (cobalamin).

Vitamin B₁₂

Humans require 2.4 µg/day of vitamin B₁₂. Vegans and vegetarians are often deficient in it since it has almost no plant-based sources, unless fortified. The reason: Only microbes synthesise it, plants don’t.

The saving grace: The bacteria in our guts can manufacture some of our requirements. However, a diet of junk food can diminish their population, and a course of antibiotics can wipe them out completely.

Deficiency

A shortfall of vitamin B₁₂ leads to tiredness, nerve damage and heart disease. Prolonged deficit can cause permanent impairment, especially neurological issues like tingling, numbness, balance problems, dementia, psychosis and even paralysis.

To rectify vitamin B₁₂ deficiency, some people take megadoses of 500 to 1,000 µg tablets. Just like ice cream, the body can’t handle such large amounts.

Vegetarianism

Six per cent of people in the developed world have a vitamin B₁₂ deficiency, while in India, where diets are predominantly vegetarian, that number climbs to 47%.

A way to interpret this is that 6% of the population takes adequate vitamin B₁₂ but cannot absorb it in the gut due to medical disorders, whereas 41% people do not consume enough of it. (Mathematically, this number should be 44%, not 41%. Think about it!)

Absorption

Many studies have established that our intestines can absorb limited vitamin B₁₂, independent of the ingested quantity. A study found that consuming:

• One µg led to 0.5 µg absorption;
• Five µg raised it to 1 µg absorption; and
• Twenty-five µg increased it to just 1.4 µg.

Other studies have shown that even after taking 1,000 µg, the absorption is 1.5 to 2 µg, which seems to be the upper limit.

Reason

In simple terms, vitamin B₁₂ is absorbed in the intestines via certain receptors, which are like tiny hands. Limited in number, they cannot grab more than 2 µg of vitamin B₁₂ at a time.

The remaining vitamin B₁₂ is eliminated in faeces.

Solution

Opt for frequent doses in a day. For example, instead of one 1,000 µg tablet, consider five µg four times daily.

Note: This is not medical advice. The article is to help you understand, think and discuss with your healthcare advisor. If your doctor has prescribed a specific dose, follow it. There are situations when your doctor may choose to give a mega-dose, as explained below.

Severe Deficiency

This section is for medical professionals.

Vitamin B₁₂ is absorbed in two ways:

1. Active: Through the receptors, which is the main mechanism; and
2. Passive: Through a process called diffusion, directly from the gut into the blood.

The latter route can assimilate only 1.2% of the ingested amount, which is negligible compared to active absorption, except when a megadose of 500–1,000 µg is given.

The passive pathway is relevant only when the active one does not function properly, which is the case in vitamin B₁₂ absorption disorders.

Accordingly, in pernicious anaemia, vitamin B₁₂ malabsorption problem and intestinal surgery, megadoses are prescribed.

When insufficient intake of vitamin B₁₂ causes its deficiency, high doses should not be used as they risk an overdose.

Overdose

The liver stores 2,500 to 5,000 µg of vitamin B₁₂, enough to last 2 to 3 years of need.

Additionally, vitamin B₁₂ is water-soluble. So, even if an excess is absorbed, the body flushes it from the blood into urine.

However, in people with certain liver or kidney issues, blood vitamin B₁₂ levels can rise too high if the mega-dose continues for too long.

Taking vitamin B₁₂ injections can rapidly raise blood levels, which may cause side effects. In some situations, such injections are essential, but one must be aware of potential problems.

Read on this website: Vitamin B₁₂ Injections: Do You Really Need Them?

Luckily, consuming too much vitamin B₁₂ does not interfere with the absorption of other nutrients.

Summary

• Use megadoses of vitamin B₁₂ only if you have an absorption disorder (6% of the World population)
• Use small doses multiple times a day if you are a vegetarian.

Finally, there is one ‘nutrient’ your body can hoard in large excess: Calories, which are stored as fat. That is why it’s a good idea that I can’t eat unlimited dollops of ice cream.

To Read More

• National Institutes of Health: Vitamin B₁₂: Fact sheet for health professionals
• Linus Pauling Institute: Vitamin B₁₂
• On this website: Everything you want to know about B vitamins
• MSD Manual: Vitamin B₁₂ Deficiency
• Cleveland Clinic: Vitamin B₁₂ Deficiency
• Nutrients: The Oral Bioavailability of Vitamin B₁₂ at Different Doses in Healthy Indian Adults
• On this website: Are dementias and B-vitamins deficiency related?

First Published on: 22nd June 2025
Image Credit: A.I. generated on Freepik
Last Updated on: 25th June 2025