30 Questions
Which vitamin mentioned in the text is absorbed by carrier mediated processes when ingested in small amounts?
Thiamin
What is the main form of thiamin mentioned in the text?
TTP - Thiamine triphosphate
Which source of thiamin is not mentioned in the text?
Tea
What factor can affect the absorption of thiamin from foods, according to the text?
Polyhydroxyphenols
Which type of enzymes are responsible for destroying thiamin in raw fish, according to the text?
Thiaminases
What happens to water-soluble vitamins when blood levels exceed the renal threshold?
They are excreted in urine.
What facilitates the destruction of the vitamin according to the text?
Divalent minerals like Ca and Mg
Which process prevents the destruction of the vitamin?
Reducing compounds like vit C and citric acid
Where does most of the free thiamin get phosphorylated to TDP?
Within liver
What is the main form in which thiamin exists in erythrocytes?
ThDP form
Where is small amounts of thiamin stored in the body?
Kidney and muscles
How is thiamin dephosphorylated in the body?
By phosphatases
What is the role of thiamin in nerve membranes?
Activates chloride transport
What vitamin is released in response to nervous system stimulation?
Thiamin
Which of the following is a symptom of dry beriberi?
Muscle weakness and wasting in lower extremities
What is a symptom associated with wet beriberi?
Altered heart rate and cardiomegaly
What can cause acute thiamin deficiency in infants?
Excessive carbohydrate intake
How does thiamin deficiency affect nerve conduction in the limbs?
Symmetrical sensory and motor nerve conduction issues
What is the function of TDP in the hexose monophosphate shunt?
Generating pentoses and NADPH
Which complex does TDP function as a coenzyme for?
Pyruvate dehydrogenase complex
In which reactions do TPP-dependent α-keto acid dehydrogenases play a role?
Oxidative decarboxylation of α-keto-acids
What is the consequence of a deficiency in branched-chain α-keto acid dehydrogenase complex?
Maple syrup urine disease
Which enzyme is responsible for deamination of leucine, isoleucine, and valine?
Branched-chain α-ketoacid dehydrogenase
What is the main product of the Pentose Phosphate Pathway (hexose monophosphate shunt)?
$eta$-Methyl Valeric acid
What is the main cause of B1 deficiency due to alcoholism?
Increased requirements due to liver damage impairing thiamin phosphorylation
Which neurologic disorder is often associated with B1 deficiency due to alcoholism?
Wernicke’s encephalopathy
What symptoms are associated with Wernicke’s encephalopathy resulting from B1 deficiency?
Ophtalmoplegia, nystagmus, ataxia, loss of recent memory
What might excessive intake of B1 (100 x greater than recommendations) lead to?
Headache, convulsions, cardiac arrhythmias
What assessment method can be used to measure the activity of a B1-dependent enzyme?
Erythrocyte transketolase activity testing
Why might thiamin deficiency be prevalent in congestive heart failure?
Low thiamin intake and increased urinary losses due to diuretic use
Study Notes
Vitamin B1 (Thiamine)
- Vitamin B1 is destroyed by divalent minerals like Ca and Mg, but protected by reducing compounds like vit C and citric acid.
- Absorption of thiamine occurs in the jejunum and ileum through active transport (ThTr1 and ThTr2) and passive diffusion.
- Thiamine is phosphorylated within intestinal cells and stored in the liver as thiamine diphosphate (TDP).
Digestion, Absorption, Transport, and Storage
- Thiamine is transported in plasma, bound to albumin or TMP, and 90% is stored in erythrocytes as ThDP.
- Cellular uptake occurs through facilitated diffusion into RBCs and active transport in other tissues.
- The body stores approximately 30 mg of thiamine, primarily in the liver, muscles, heart, and brain, with the muscles containing about half of the body's thiamine.
Functions and Mechanisms of Action
- Thiamine plays coenzyme roles in E transformation, synthesis of pentoses and NADPH, and membrane and nerve conduction.
- It functions as a coenzyme in the pyruvate dehydrogenase complex, α-ketoglutarate dehydrogenase complex, and branched-chain α-keto acid dehydrogenase complex.
- Thiamine is also involved in the synthesis of pentoses and NADPH through the pentose phosphate pathway.
Sources and Destruction
- Thiamine is widely distributed in foods, with rich sources including pork, salmon, legumes, and grain products.
- It is destroyed in alkaline environments and by heat, and can be affected by antithiamin factors like thiaminases and polyhydroxyphenols.
Deficiency: Beriberi
- Chronic low thiamine intake with high CHO intake can cause dry beriberi, characterized by muscle weakness, peripheral neuropathy, and symmetrical foot drop.
- Wet beriberi results in more extensive cardiovascular system involvement, including cardiomegaly and peripheral edema.
- Acute beriberi can occur in infants, causing anorexia, vomiting, and lactic acidosis.
Deficiency Risk Factors
- Alcoholism is a main cause of thiamine deficiency due to reduced absorption, low food intake, and increased requirements.
- Elderly individuals and those with congestive heart failure are also at risk of thiamine deficiency.
Toxicity
- There is no UL for thiamine, and side effects are rare, but high doses (> 500 mg/d) may cause headache, convulsions, cardiac arrhythmias, and anaphylactic shock.
Test your knowledge on water soluble vitamins which are easily absorbed by simple diffusion when ingested in large amounts. Learn about the sources and forms of Vitamin B1 (Thiamin) and understand their importance in human nutrition.
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