Vitamin B6

Question 1

List the vitamers of B6

Answer 1

Pyridoxine, pyridoxine phosphates
Pyridoxal, pyridoxal phosphate
Pyridoxamine, pyridoxamine phosphate

Question 2

List dietary sources of B6

Answer 2

Pyridoxine and pyridoxine phosphate are found exclusively in plant foods

Pyridoxal phosphate and pyrodoxamine phosphate found primarily in animal products

Good sources: fish, bananas, poultry, whole grains, nuts, potatoes
Cereals are fortified with B6

Question 3

Describe the digestion of B6

Answer 3

Dietary phosphorylated vitamers are dephosphorylated at the intestinal brush border

Question 4

Describe the absorption of B6

Answer 4

Primarily absorbed in jejunum by passive diffusion

• dietary B6 dephosphorylated vitamers are rapidly absorbed
• supplemental B6 phosphorylated vitamers ingested in high concentrations can be absorbed directly

Question 5

Describe the transport of B6

Answer 5

Dephosphorylated vitamers are released into the portal blood which are then taken up by the liver

Most are converted to pyridoxal phosphate (PLP) in the liver and most vitamin B6 is found in systemic circulation as PLP bound to albumin

Question 6

Describe the storage of B6

Answer 6

Only dephosphorylated forms of B6 are taken up by cells

Once inside the cell pyridoxal is converted to PLP which is then trapped inside the cell

Muscles store 75-80% (as PLP bound to glycogen phosphorylase)

Liver stores 5-10%

Question 7

Describe the function and mechanism of action of B6

Answer 7

Coenzyme functions (always in the form PLP):
Transaminase - transforming a.a.
Decarboxylation
- formation of GABA from glutamate
- formation of serotonin from tryptophan
- formation of histidine from histidine 
- formation of dopamine from L-dopa

Other enzymatic reactions

• synthesis of heme
• synthesis of niacin
• synthesis of carnitine
• synthesis of taurine (important for cardiac function)
• glycogenolysis

Question 8

List interactions with other nutrients of B6

Answer 8

Mg - B6 increases intracellular uptake of Mg & vice versa
Folic acid - B6 may increase folate requirements & vice versa
Zinc - B6 may increase zinc requirements
L-tryptophan - B6 may increase tryptophan uptake into brain
L-tyrosine - B6 may interfere with the supplementation of tyrosine in order to increase dopamine levels by shunting metabolism toward tryamine formation instead

*mass amount of flax can inhibit absorption of B6

Question 9

Describe the metabolism and excretion of B6

Answer 9

Pyridoxine oxidase and amino transferase requires FMN (dependent on B2), therefore B6 metabolism is dependent on adequate riboflavin status

Primarily excreted in the urine as its metabolite 4-pyridoxic acid
If large amounts are ingested (over 100mg) pyridoxine will be excreted intact in the urine

Question 10

Describe consequences and symptoms of deficiency

Answer 10

Symptoms:
Adults
- fatigue, depression, anxiety, insomnia, irritability and confusion
- cheilosis/stomatitis, glossitis and dermatitis (face and mouth specifically)

Other Consequences:
Anemia - hypochromic, microcytic, sideroblastic, hypoferremic
Impaired niacin synthesis from tryptophan
Homocysteinemia

Question 11

Describe consequences and symptoms of toxicity of B6

Answer 11

UL = 100mg

Symptoms:
Acneform rash
Unsteady gait
Impaired deep tendon reflex
Peripheral neuropathy and paresthesias
- degeneration of dorsal root ganglia in the spinal cord
- loss of myelination
- degeneration of sensory fibers in peripheral nerves
Lowest dose to clearly cause neurotoxicity is 500mg
Patients prescribed >200mg/d should be advised to watch for signs of toxicity and monitored

Other possible side effects:
Adult
- nervousness, insomnia and “feeling wired”
- >50mg/d
- can be prevented with 200-400mg/d Mg
Children
- irritability, hyperactivity, sensitivity to sound and enuresis
- can be prevented with Mg supplementation

Question 12

Describe the assessment of nutrient status of B6

Answer 12

Plasma PLP concentration
Erythrocyte transaminase activity
- large increase in activity with addition of B6 indicates deficiency
Xanthurenic acid excretion in urine following tryptophan loading
- elevated indicates B6 deficiency
Urinary B6 and 4-pyridoxic acid
- only indicates recent intake
Plasma homocysteine
- deficiency in folate, B6 or B12 will result in increased levels

Question 13

Discuss some of the therapeutic uses (clinical indications) of B6

Answer 13

Autism - use with Mg
Carpal tunnel syndrome - combine with other B vitamins, like B2
Arteriosclerosis, heart disease and stroke - B6 is essential in conversion of homocysteine to cystethionine
Nausea, vomiting (especially during pregnancy)
PMS
Dysmenorrhea