Nutrition Lecture 3: B12

Question 1

What are rich food sources of B12?

Answer 1

Animal products! Fortified products, marmite?

Question 2

What foods contain inactive forms of B12?

Answer 2

Tempe, mushrooms and spirulina

Question 3

Where does B12 come from?

Answer 3

Bacteria - animals eat or absorb this bacteria

Question 4

How is B12 commercially made?

Answer 4

Large scale production via microbial fermentation, using 3 strains of bacteria

Question 5

What is vitamin B12 the term for?

Answer 5

All corrinoids that have biological activity of “cynanocobalamin”

Question 6

What is the structure of B12?

Answer 6

Corrin ring (i.e. 4 pyrrole rings) with a cobalt in the centre and a dimethylbenzimidazole nucleotide

Question 7

What is the molecular weight of B12?

Answer 7

1355g/mol - Big!

Question 8

What is HC?

Answer 8

Haptocorrin (“R-binder”), an acid resistant glycoprotein

Question 9

What is HC produced by?

Answer 9

Salivary glands

Question 10

What is HC’s role in B12’s digestion?

Answer 10

HC binds to cobalamin in the stomach as soon as it is released from dietary proteins - protecting it

Question 11

How is cobalamin eventually split off from the HC?

Answer 11

Proteases in the duodenum free the B12

Question 12

What is IF?

Answer 12

Intrinsic factor

Question 13

What is the role of IF in B12’s digestion?

Answer 13

IF is used as a transporter to enable B12 to enter the ileal cell

Question 14

When does IF bind to B12?

Answer 14

After it has been released from HC in the neutralized gut

Question 15

Where does IF come from?

Answer 15

Released from parietal cells in the stomach

Question 16

Once bound to IF, what happens to B12?

Answer 16

It can absorbed via the intrinsic factor receptor into the ileal cell

Question 17

Is cobalamin water soluble?

Answer 17

Yes, therefore very easily recycled

Question 18

What bodily substance contains a lot of B12?

Answer 18

Bile

Question 19

How is B12 recycled in the body?

Answer 19

B12 from bile is pumped out from the liver and then binds to the IF to be absorbed

Question 20

What are the two active (co-factor) forms of B12?

Answer 20

1. Methylcobalamin
2. Deoxyadenosylcobalamin

Question 21

What two enzymes are B12 dependent?

Answer 21

1. Methylmalonyl-CoA mutase
2. Methionine synthase

Question 22

What is the role of methylmalonyl-CoA?

Answer 22

The conversion of methylmalonyl CoA into succinyl CoA

Question 23

What is the role of methionine synthase?

Answer 23

To transfer methyl group from 5MTHF to homocysteine to form methionine - this regenerates the THF

Question 24

A decrease in B12 causes a decrease in what enzymes activity?

Answer 24

MS = methionine synthase

Question 25

What is the “methyl folate trap”?

Answer 25

The negative effects resulting from the stopped activity of the enzyme MS (methionine synthase)

Question 26

Why is it called the “methyl folate trap”?

Answer 26

Because the folate is being stuck in the form of 5-methyl THF - cannot be regenerated into THF and methyl group cannot be used to make methionine

Question 27

What are the negative effects of the “methyl folate trap”?

Answer 27

1. 5-methyl THF levels increase
2. THF levels decrease
3. 5,10 methylene THF decrease
4. DNA synthesis decreases

Question 28

What are ways to develop B12 deficiency? (5)

Answer 28

1. Lack of IF
2. Lack of stomach acid
3. GI disorders/surgery
4. Metabolic defects
5. Vegan/vegetarianism

Question 29

What disease is caused by a lack of intrinsic factor?

Answer 29

Pernicious anaemia

Question 30

What is pernicious anaemia?

Answer 30

A decrease in red blood cells when the body can’t absorb enough vitamin B12.

Question 31

What can a lack of intrinsic factor be caused by?

Answer 31

Autoimmune disorder that attacks parietal cells - limits of eliminates the production of IF

Question 32

What age group often produce less IF?

Answer 32

Elderly

Question 33

How is a lack of intrinsic factor treated?

Answer 33

Intramuscular injection

Question 34

A lack of stomach acid means there is limited or absent production of what?

Answer 34

HCl

Question 35

What age group does a lack of stomach acid most often affect?

Answer 35

The elderly

Question 36

What condition does a lack of stomach acid cause?

Answer 36

Atrophic gastritis - chronic inflammation of stomach mucosa

Question 37

A lack of stomach acid lessens the release of what?

Answer 37

B12 from animal protein

Question 38

What GI disease causes reduced B12 absorption?

Answer 38

Crohn’s disease - a type of inflammatory bowel disease

Question 39

What GI surgery causes reduced B12 absorption?

Answer 39

Gastric bypass - may reduce the stomach area secreting stomach acid

Question 40

What do inherited metabolic defects affect?

Answer 40

• Intestinal absorption
• Transport in blood
• Cellular uptake
• Intracellular metabolism

Question 41

What must vegans do?

Answer 41

Take vitamin B12 supplements or B12 fortified foods - breastfed infants of vegan mothers are particularly vulnerable

Question 42

What type of anaemia does B12 deficiency cause?

Answer 42

Megaloblastic anaemia = tiredness and breathlessness

Question 43

What neural condition can B12 deficiency cause?

Answer 43

Peripheral neuropathy

Question 44

What are the symptoms of peripheral neuropathy?

Answer 44

• Loss of position sense
• Spasms and weakness
• Cognitive changes

Question 45

How is peripheral neuropathy caused?

Answer 45

Because of the impaired methylation of myelin basic protein due to deficient methionine synthase activity

Question 46

What is the upper limit of B12?

Answer 46

“not possible to set” - there is no evidence of harm

Question 47

What is the EAR for B12?

Answer 47

2.0ug/day

Question 48

What is the RDI for B12?

Answer 48

2.4ug/day