Many nutrients have a simple absorption system based on differences
between the concentration of the given nutrient in the gut and the
concentration of the same nutrient in the liquid (blood or lymph) which moves
the nutrient to body’s cells. The absorption of vitamin B12 is, however,
considerably more complicated.
According to Dr Shmeizer (2015), vitamin B12 which is found exclusively in food derived from
animals (meat, seafood and dairy products), must be released from the protein
food it is found in by the acid in the stomach. Vitamin B12 then binds to
so-called R factors (proteins) in the gastric juices and has to be released in
the duodenum from these proteins by the enzyme pepsin which is produced by the
pancreas. Vitamin B12 subsequently binds to a compound called the intrinsic
factor (IF) and this combination of vitamin and IF has to bind with a specific
receptor (cubulin) in the ileum of the gut to be absorbed into the circulation.
It stands to reason that the following factors must all be in place to
ensure that we obtain adequate supplies of vitamin B12:
- An adequate intake from foods such as
meat, fish and dairy or if your diet is deficient in vitamin B12, then from
supplements containing vitamin B12 (see below for the relative inefficiency of
- Sufficient gastric acid to release B12
from protein food. Elderly people often have a deficiency of stomach acid and
may suffer from a chronic vitamin B12 deficiency.
- Protease enzymes produced by the pancreas
to separate B12 from the R factors so that it can bind to IF – the intrinsic
- Adequate quantities of IF to ensure that
B12 can bind with the cubulin receptor that transports the vitamin into the
If any one of these above mentioned factors is absent for a longer
period of time, the individual in question may develop a vitamin B12 deficiency,
leading to pernicious or megaloblastic anaemia and neurological degeneration.
Why do we need Vitamin B12?
Vitamin B12 (also called cobalamin) acts as a co-factor (a helper) in
the synthesis of DNA in body cells that divide rapidly, such as the cells
producing red blood cells and enterocytes (cells that line the digestive
During DNA synthesis, a compound called methionine is formed which leads
to decreased homocysteine levels. If we have a methionine deficiency, it can
cause degeneration of the nerves (neuropathy) and brain. Raised homocysteine
levels have been implicated in heart disease. Vitamin B12, is therefore,
essential for normal DNA synthesis, normal blood production and nerve function.
Vitamin B12 deficiency
For most people who have an adequate intake of vitamin B12 from food
derived from animals such as meat, liver, fish, milk and dairy products, a
vitamin B12 deficiency is not really a problem unless they develop an
autoimmune disease that leads to pernicious anaemia.
Read: Should I supplement my diet?
Originally pernicious anaemia, which is caused by an inability to
produce IF or intrinsic factor for the absorption of vitamin B12, was regarded
as a purely genetic disease, but currently it is suspected that this anaemia
with its deficiency of IF which in turn leads to a deficiency of B12, is
probably an autoimmune disease.
Dr Shmeizer mentions that pernicious anaemia can also be associated with
other autoimmune diseases like thyroid disease, diabetes, vitiligo (a condition
characterised by loss of pigment in the skin), and Addison’s disease (a
condition that occurs when there is a loss of hormones produced by the adrenal
Other non-IF factors that can cause vitamin B12 deficiency and
pernicious anaemia are:
- Patients who have had a gastrectomy (removal of part or all of the
stomach), lack the stomach acid and pepsin enzyme required for the absorption
- Gastritis patients, particularly elderly
person and patients with Helicobacter pylori infection which is linked
- Malabsorption such as coeliac disease in
patients who are allergic to gluten
- A condition known as “blind loop syndrome”
where bacterial overgrowth in the gut is so great that it interferes with the
absorption of B12 and other nutrients
- Diseases/damage of the ileal part of the
gut such as Crohn’s disease, lymphoma, surgical resection, or deep X-ray
- Bariatric surgery for slimming purposes
which may remove those parts of the stomach or gut that are essential for B12
- Diseases of the pancreas which prevent
production of the enzymes required for the absorption of B12
- Tapeworm acquired from eating raw fish
(only eat Sushi at reputable restaurants!)
- Chronic alcoholism
- Certain medications - high doses of
proton pump inhibitors to treat ulcers and gastritis and metformin
(Glucophage), which is used to treat type 2 diabetes, may interfere with B12
- Hereditary tendency
- Inadequate dietary intake. Dr Shmeizer
(2015) emphasises that strict vegans and vegetarians, pregnant and lactating
women all need to ensure that their B12 intake is adequate to prevent pernicious
Symptoms of Vitamin B12 Deficiency
In keeping with its role in DNA synthesis mentioned above, a deficiency
of vitamin B12 can lead to the development of megaloblastic anaemia and
Read: Acid-reflux meds tied to B12 deficiency
Anaemia patients have a sallow appearance, prematurely grey hair, lack
of mental sharpness and a broad-based gait (walk with feet apart).
The neurological changes associated with a vitamin B12 deficiency
include the following:
- Degeneration of the spinal cord, which
particularly affects the legs more severely than the arms and is specific for
B12 deficiency. Patients suffer from abnormal sensations in the hands and feet
such as pins and needles, some loss of control of their movements, loss of
position and the ability to sense vibrations. If the deficiency is severe the
patient may become spastic, paraplegic and incontinent.
- Damage to the brain may manifest through
subtle memory loss developing to full dementia, irritability and damage to
sight because of the degeneration of the optic nerve.
The neurological fall-out caused by vitamin B12 deficiency is so severe
that we all need to guard against having low intakes of this vitamin. The old
habit of eating liver once a week was not a bad idea at all!
Sources of vitamin B12
The following foods are rich to adequate sources of B12 per 100g.
- Liver - 70.7 microgram (mcg)
- Clams - 98.9 mcg
- Tuna, canned in water - 3.00 mcg
- Beef, lean - 2.81 mcg
- Fish, baked - 2.56 mcg
- Milk, skim - 0,52 mg (but per 250 ml milk contains 1.30 mcg of B12)
- Yoghurt with fruit - 0,70 mcg
- Ready-to-eat- breakfast cereals - 1.8 to
2 mcg (per ½ cup or 50g, these cereals contribute 0,9 to 1 mcg)
(Note that in South Africa vitamin B12 is not part of the 8 nutrient
mixture added to fortified wheat bread and maize meal).
In this country, the Nutrient Reference Value (NRV) for vitamin B12 is 2,4
mcg/day for persons of 4 years and older. The USA RDA varies from about 2,4
mcg/day for adult men and women to 2,6 mcg/day during pregnancy and 2,8 mcg/day
for breastfeeding mothers.
If diagnosed with pernicious anaemia the doctor will probably treat you
with injections of 1 mg of vitamin B12 per day for a week, followed by once a
week for a month and then once a month for the rest of your life if the
condition does not clear up. In patients where the cause can be eliminated, the
injections can be stopped when body levels of vitamin B12 return to normal.
Read: First full image of vitamin B12 in action
b) Oral supplements
Because absorption of B12 from tablets and capsules can be poor, Dr
Shmeizer recommends the use of mega-dosses of 1-2 mg vitamin B12 a day and
cautions that if possible injections should be used instead or oral supplements.
Vitamin B12 may be one of the forgotten vitamins, but it is
important to keep in mind that we need a steady intake throughout our lives. Anyone
who does not eat foods derived from animals, the senior population, and
pregnant and lactating women, should check their intake and have tests done to
determine if they suffer from vitamin B12 deficiency.
To B12 or not B12?
Vitamin B12 tied to Alzheimer's
How much vitamin B12 do you need?
- Gov. Gazette (2010). Regulations Relating to the Labelling &
Advertising of Foodstuffs. No. R. 146. of the Foodstuffs, Cosmetics &
Disinfectants Act, 1972 (Act 54 of 1972). Published on 1 March 2010 in the
Government Gazette, No 32975. Gov. Printer, Pretoria.
- Mahan L K et al, (2012). Krause’s Food & the Nutrition Care Process.
13th Edition. Elsevier Publishers, USA.
- Shmeizer B (2015). Vitamin B12: An Overview. Published in July 17, 2016
in Gastroenterology & online in the Medical Chronicle.
Image: Vitamin B12 from Shutterstock