Vitamin B12 and Folate Flashcards
What is vitamin B12?
Bacterial product that is ingested and stored in animals
What are good sources of B12?
Meat, salmon, cod, milk, cheese and eggs
Who suffers from vitamin B12 deficiency?
Vegans and due to malabsorption
What is the daily need for vitamin B12?
1.5-3 micrograms
What are the hepatic stores of vitamin B12 like?
2-5mg (lasts for several years)
What is vitamin B12 required for?
DNA synthesis
Integrity of nervous system
What is folic acid?
Thermoliable water soluble vitamin found in leafy green vegetablrs
What causes folic acid deficiency?
Dietary
Impaired absorption
Increased requirement
What is the requirement for folate?
400-600mcg
In terms of how quickly you run out, how does folate compare to vitamin B12?
Run out much quicker of folate
What is folic acid required for?
DNA synthesis
Homocysteine metabolism
What happens to B12 once it initially enters the stomach?
It binds to transcobalamin 1 (aka R protein)
What do the gastric parietal cells at the bottom of the stomach produce?
Intrinsic factor
What happens to B12 when it leaves the stomach and enters the duodenum?
Pancreatic enzymes displace it from transcobalamin and then free B12 binds to intrinsic factor
What happens to the B12-intrinsic factor complex after formation?
It travels all the way to the terminal ileum where there is a specific receptor that binds to the complex and allows it enter the cell
What happens after the B12-intrinsic factor complex enters the cell in the terminal ileum?
It enters the portal circulation
What happens after the B12-intrinsic factor complex enters the portal circulation?
The B12 binds to transcobalamin 2 making it active B12
What happens to folate when it enters the stomach?
It is hydrolysed to monoglutamates at acid pH then absorbed as pteroglutamates which can be methylated in the luminal cells to tetrahydrofolates
What is the link between folate and B12 in DNA synthesis?
Both needed for the synthesis of deoxythymidine (dTMP) which is a crucial building block in DNA synthesis
What is deoxythymidine made from?
Deoxyuridine
What happens to dietary folate during DNA synthesis?
It is converted to methyl tetrahydrofolate (Methyl-THF) as it’s absorbed into the luminal cells
What does B12 act as during DNA synthesis?
It acts as a cofactor for methionine synthetase for conversion of homocysteine to methionine
What does the formation of methionine release?
Methyl groups which go on to methylate building blocks of DNA
What are the clinical features of B12 and folate deficiency?
Anaemia- macrocytic, megaloblastic Weak, tired and short of breath Jaundice Ineffective erythropoiesis Glossitis- red, raw tongue Angular cheilosis- soreness in corner of mouth Weight loss, change of bowel habit Sterility
What causes macrocytic anaemia?
Vitamin B12/folate deficiency Liver disease or alcohol Hypothyroidism Haematological disorders- myelodysplasia, reticulocytosis and aplastic anaemia Drugs that interfere with DNA synthesis Prolonged nitrous oxide anaesthesia
What is the difference between macrocytic anaemia caused by B12/folate deficiency and liver disease?
B12/folate- oval macrocytes
Liver- round macrocytes
What is myelodysplasia?
Group of disorders in which production of any one or all types of blood cells by the bone marrow is disrupted
What is aplastic anaemia characterised by?
Failure of blood cell production resulting in pancytopenia and reduced bone marrow cellularity
Name some drugs that interfere with DNA synthesis?
Azathioprine
Zidovudine
Hydroxycarbamide
Methotrexate
What does the term megaloblastic describe?
Morphological change in red cell precursors within the bone marrow
What effects how blue or pink a reticulocyte is?
More blue= More DNA
More pink= More haemoglobin
What is the nucleus of a proerythroblast like?
It has a lot of space in it- this is chromatin which looks very open in the proerythroblast
As a proerythroblast becomes more mature, what happens?
It becomes tighter and just before red cell is about to spit out its nucleus, it appears very dark
To determine whether the cells in the lineage are normal, what do you have to look at?
Chromatin and how open it is
Colour of cytoplasm and how blue it is
What should a normal big precursor cell be like?
Open chromatin and blue cytoplasm
Why are big precursors not like this in megaloblastic anaemia?
There is asynchronous maturation of the nucleus and cytoplasm in the erythroid=
Nucleus doesn’t mature but cytoplasm does so you get immature open nuclear chromatin with normal mature haemoglobinisation of the cytoplasm- unable to synthesise DNA properly
Which people are at increased risk of decreased folate intake?
Elderly, sick, eating disorders and alcoholics
What does a decrease in folate lead to?
Inability to methylate nucleotides and affects DNA synthesis
Homocysteine will accumulate because it can’t be converted to methionine without folate
What causes increased demand of folate?
Physiological:
Pregnancy, adolescence and premature babies
Pathological:
Malignancy, erythroderma and haemolytic anaemia
In what condition does folate malabsorption and iron deficiency occur?
Coeliac disease
What happens in coeliac disease?
Sensitivity to gliadin in wheat leads to subtotal villous atrophy with crypt hyperplasia in the duodenum
What can be used to diagnose coeliac disease?
Anti-gliadin (transglutaminase) antibodies or duodenal biopsy
What interferes with normal absorption?
Surgery
Inflammatory bowel disease
Drugs
Where does Crohn’s mainly affect and what does this lead to normally?
Terminal ileum so B12/folate deficiency
What tests would you use to establish folate deficiency?
FBC Blood film Blood folate level Serum folate- screening test Red cell folate- confirmatory test
What are the consequences of folate deficiency?
Megaloblastic anaemia
Neural tube defects
Increased risk of VT
What neural tube defects occur?
Spine- Spina bifida
Brain- Anencephaly
What are very high levels of homocysteine (caused by lack of folate) associated with?
Atherosclerosis
Premature vascular disease
What are mildly elevated homocysteine levels associated with?
Cardiovascular disease
Probably arterial and venous thrombosis
What can affect normal B12 deficiency?
Autoimmune- Pernicious anaemia
Surgery- Partial/total gastrectomy, gastric bypass, resection of ileum
Inflammatory disease- Crohn’s disease, chronic pancreatitis, bacterial overgrowth and parasitic infection
What is pernicious anaemia?
Lack of intrinsic factor
What are the consequences of B12 deficiency?
Macrocytic and megaloblastic anaemia (reversible)
Neurological problems due to demyelination
Subacute combined degeneration of spinal cord
Cognitive impairment due to loss of white matter in central nervous system
Optic atrophy
What is the relationship between Hb level and neurology in B12 deficiency?
Inverse- higher Hb more likely to have neurological findings
In subacute combined degeneration of spinal cord, where is there demyelination?
Posterior and lateral columns- they are pale
What does subacute combined degeneration of the cord result in?
Loss of joint position sense and vibration sense
How would a patient with subacute combined degeneration of the cord present?
Wide based gait and sometimes pain
What are the symptoms of B12 deficiency?
Weak, tired, lethargic Symmetrical paraesthesiae/numbness Muscle weakness Difficulty walking and loss of balance Visual impairment Memory impairment Psychiatric disturbance
What are the signs of B12 deficiency?
Anaemia and jaundice- lemon-yellow tinge
Neurology- loss of vibration and joint position sense but also cutaneous sensation lost and absent reflexes and up-going plantar responses in legs
What level of serum B12 is normal?
> 180ng/l
How accurate is serum B12 levels as a test for B12 deficiency?
Not very- poor positive predictive value
You can have healthy people with a low level
Falsely high levels in B12 deficient patients due to high levels of antibodies interfering with commercial assays
Why is there a problem with measuring serum B12 levels?
You measure total B12 levels- bound to transcobalamin 1, 2 and 3
What tests are used to establish B12 deficiency?
Serum B12
Plasma homocysteine- easy and reliable (high with folate as well)
Serum methylmalonic acid levels- expensive
Holotranscobalamin levels (transcobalamin II)- automated immunoassay that measures active B12 rather than total
What was the Schilling test used to test?
B12 absorption
What is the schilling test?
Give B12 injection to replenish B12 stores then two B12 capsules orally (different radioisotopes):
Capsule 1= B12 alone
Capsule 2= B12 and intrinsic factor complex
Then you collect urine for 24 hours after administration and measure presence and relative proportion of each isotope
What would you expect to find in the Schilling test in urine of a normal person, someone with pernicious anaemia and inflammatory disease of terminal ileum?
Normal- both isotopes excreted
Pernicious anaemia- Only B12 isotope complexed with IF
Inflammatory disease of terminal ileum- Neither isotope excreted
What is pernicious anaemia?
Autoimmune atrophic gastritis with loss of intrinsic factor
What does pernicious anaemia result in?
Macrocytic/megaloblastic anaemia +/- neurological damage
How do you diagnose pernicious anaemia?
You look at the auto-antibodies:
Intrinsic factor antibodies- (occasionally found in other conditions) 40-60% of people with PA have them
Parietal cell antibodies- 80-90% of people with PA
Increased in relatives of patients with PA
