Vitamin B1 - Thiamin

Question 1

List dietary sources of thiamin

Answer 1

Widely distributed in foods, but particularly high in:

• meat, especially pork
• legumes
• whole, fortified or enriched grain products and wheat germ
• yeast
• soy milk

Question 2

Describe the digestion of thiamin

Answer 2

Occurs in the small intestine, primarily the jejunum
Two mechanisms
1. Passive diffusion
2. Active transport - sodium dependent and independent
Transport of thiamin across the basolateral membrane of enterocytes with thiamin/H+ antiporter system
- alcohol interferes with the function of this antiporter

Question 3

Describe the transport of thiamin

Answer 3

90% of thiamin present in the blood is present within the red blood cell

Question 4

Describe the storage of thiamin

Answer 4

Thiamin is not stored in large quantities in tissues, thus a continuous supply is necessary

Question 5

Describe the function and mechanism of action of thiamin

Answer 5

Coenzyme role:
Energy transformation
- TDP is required as a coenzyme for the decarboxylation of: pyruvate, alpha-ketogluterate,branched chain amino acids (leucine, isoleucine & valine)
Synthesis of pentoses and NADPH
- transketolase, the key enzyme in the hexose monophosphate shunt, requires thiamin as a coenzyme

Non-coenzyme roles:
Membrane and nerve conduction
- TTP plays an important role in membrane conduction - activate chloride ion transport in nerve membranes
- TTP also involved in nerve pulse transmission - regulates sodium channels and acetyl-choline receptors

Question 6

List interactions with other nutrients with thiamin

Answer 6

Thiamin supplementation may not be effective if there is a concurrent magnesium deficiency
Alpha lipoid acid may be toxic in thiamin deficiency

Question 7

Describe the metabolism and excretion of thiamin

Answer 7

Taken up by liver and is phosphorylated to thiamin do phosphate (TDP or thiamin pyrophosphate - TTP) - this reaction require ATP and Mg
TDP is coenzyme form of thiamin
80% of thiamin in body exists as TDP (10% TTP)
Thiamin monophosphate (TMP) is the final inactive metabolite

Excretion
Excreted in the urine as free thiamin, TDP and TMP
Also catabolized to various metabolites which are also excreted in the urine

Question 8

Describe consequences and symptoms of deficiency of thiamin

Beriberi

Answer 8

Beriberi - 3 types
1. Dry - wasting and paralysis, nerve/neurological disorders
In older adults
Secondary to chronic low thiamin intake
Especially with high carbohydrate intake
Symptoms
- muscle pain, weakness and wasting (especially in lower extremities)
- peripheral neuropathy: symmetrical, bit sensory and motor nerves impaired

2. Wet
   More cardiovascular involvement with cardiomegaly and tachycardia due to right sided heart failure leading to pulmonary and peripheral edema
   Peripheral neuropathies also seen
3. Acute or infantile
   Seen in breast fed infants of malnourished mothers
   **crying, but not loudly and without tears
   Anorexia, vomiting
   If untreated can be fatal within 24 hours

Question 9

Describe consequences and symptoms of deficiency of thiamin

Wernicke-Korsakoff syndrome

Answer 9

Severe thiamin deficiency usually secondary to alcohol abuse
Several factors in the alcoholic leading to thiamin deficiency
- decreased intake from food
- increased requirement
- liver damage impairs TDP formation
- thiamin is required for ETOH metabolism
- decreased absorption

Symptoms:

• confusion
• ataxia
• visual changes
• amnesia
• confabulation
• hallucinations

Question 10

Describe consequences and symptoms of deficiency of thiamin

Mild deficiency

Answer 10

Anorexia, nausea, constipation
Loss of reflexes in knees and feet, peripheral neuropathy, muscle pain
Mental changes: depression, hysteria

Question 11

Describe consequences and symptoms of toxicity of thiamin

Answer 11

Large doses can exacerbate Mg deficiency
Some people are sensitive to the odor
No UL established
Anaphylaxis –> very rare, more common with IM and IV administration

**high dose of B1 prolongs effects of LSD

Question 12

Describe the assessment of nutrient status of thiamin

Answer 12

Serum thiamin, erythrocyte thiamin, urinary thiamin are not reliable tests
Erythrocyte transketolase activity
- increased activity with addition of TDP to incubation medium indicated deficiency
Clinical response to supplementation

Question 13

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

Answer 13

Neurological:
Peripheral neuropathy
Neuralgias - neuropathic pains (sciatica, trigeminal neuralgia)

Or insect repellent