The document discusses thiamine and niacin, their sources, absorption, metabolism, and biochemical roles in the body. Thiamine occurs in plant seeds and is crucial for converting pyruvate to acetyl CoA, while niacin is important for metabolic coenzymes NAD and NADP, involved in various enzymatic reactions. Both vitamins are vital for proper bodily functions, and deficiencies can lead to significant health issues, including symptoms like dermatitis and neurological effects.

2. THIAMINE

3. Occurrence
• Thiamine occurs in the outer
coats (Aluerone) of the seeds
of many plants including the
cereal grains-
Whole wheat flour
Unpolished hand pound
rice
• Yeast is also a very good
source
• In animal tissues it occurs
primarily as the coenzyme
thiamine pyrophosphate or
cocarboxylase.

4. Conversion of Thiamine to TPP
Thiamine is then converted to its active co-enzyme
form, TPP or Cocarboxylase, by addition of two
phosphate groups, with the help of ATP.
Catalysed by Thiamine pyrophosphotransferase.

5. Absorption & Metabolism
• Most absorption of thiamine occurs in jejunum and
ileum of the small intestine.
• If consumed in small quantity- thiamine absorbed by
sodium dependent active transport mechanism.
• If consumed in large quantity- absorption carried out by
passive diffusion.
• TPP within food must be dephosphorylated to thiamine
before it can be absorbed by humans. Except in RBCs,
where TPP can cross its cell membrane.

6. • About 80% of thiamine in the body is in the form of TPP.
• Half of the body’s thiamine is in the muscle tissues.
• The body contains no specific storage site for thiamine.
• Thiamine is excreted from the body in the form of
thiamine acetic acid and as various other metabolites
produced by its degradation.

7. Biochemical Role of TPP
• TPP is needed for the oxidative decarboxylation of
pyruvate (pyruvic acid) to form the acetyl coenzyme A
that enters the TCA cycle.
• It is also used for the decarboxylation of α –
ketogluterate to succinyl coA and CO2 within the TCA
cycle.
(Pyruvic acid and α – ketogluterate accumulate in
those with thiamine deficiency)

8. • It is a co-enzyme of the enzyme transketolase which is
required for the metabolism of glucose via the hexose
monophosphate shunt (HMP) pathway.
(HMP pathway is the only way by which the body can
produce the ribose and deoxy ribose sugars needed for the
synthesis of ribonucleic acid and deoxy ribonucleic acid)

9. • Thiamine is involved in the transmission of high
frequency impulses across nerve synapses, the junctions
between neighboring nerve cells, across which the signals
are carried by chemical neurotransmitters.
• Thiamine is also known to be involved in the conversion
of the amino acid tryptophan to the vitamin Niacin and
in the metabolism of the branched- chain amino acids
leucine, isoleucin and valine.

11. Glucose is incompletely metabolised &
pyruvic and lactic acids accumulates in
the tissues and body fluids.
Inability to use glucose efficiently.
Dilates peripheral blood vessels. If
vasodilatation is extreme, fluid leaks out
of the capillaries, producing edema.
High cardiac output- myocardium over
worked.
Edema of legs, trunk and face are themain
features. Palpitation, breathlessness and
distended neck is also observed.

12. Nutritional background is
same as wet beriberi.
Degeneration and
demyelination of both sensory
and motor nerves, resulting in
severewasting of muscles.
Symptoms include numbness
in legs, irritability, vague
uneasiness, disorderly
thinking and nausea.

13. It often presents acutely
in alcoholics. But
sometimes seen in
people with persistent
vomiting.
The patient seems quite
confused. The most valuable
signs are- bilateral,
symmetrical opthalmoplegia-paralysis
of eye muscles, jerky
repetition movement of eye
balls.
If left untreated, psychosis is
observed. May also lead to
death.

14. Occurs in infants between 2
and 5 months of age.
Beriberi occurs more often in
breast fed because a lactating
mother’s intake of thiamine
may be too low.
The affected baby develops
cyanosis, accumulation of
CO2 or lack of O2 in the blood
vessels - skin turns blue,
tachycardia is observed.

15. Recommended Daily Allowance
of Thiamine -

16. NIACIN

17. Occurrence
• Niacin are widely distributed
in plant and animal tissues.
• Meat products are an
excellent source of the
vitamin.
• The coenzyme forms of the
vitamin are the nicotinamide
nucleotide coenzymes;
namely, Nicotinamide
adenine dinucleotide
(NAD+) and Nicotinamide
adenine dinucleotide
phosphate (NADP+).

18. Absorption & Metabolism
• Niacin is readily absorbed from the stomach and the
small intestine.
• It is converted into the coenzymes NAD and NADP within
the cells and limited stores of these coenzymes are held in
the kidney, liver and brain.
• Any excess niacin is excreted in urine in the forms of
methyl nicotinamide and methyl carboxamido –
pyridone.

19. Niacin & Tryptophan interrelationship
• About 97% molecules of tryptophan are metabolised in
the major pathway. About 3% molecules are diverted at the
level of 3- hydroxy anthranilic acid, to form NAD+ .
• The enzyme, Quinolinate phosphoribosyl transferase
(QPRT) is the rate- limiting step.
• About 60 mg of tryptophan will be equivalent to 1 mg of
nicotinic acid.

21. Biochemical role of Niacin
• Its major biochemical role is to form part of the
coenzymes NAD and NADP.
• These coenzymes are required by many of the key
pathways of metabolism with NAD being primarily
involved in catabolic reactions and NADP functions
mainly in anabolic reactions.
• NAD and NADP act in a large number of reversible
oxidation reduction reactions.
• NAD and NADP participate in more than 200 enzyme
reactions.

22. NAD+ dependent enzymes
Lactate dehydrogenase (lactate  pyruvate)
Glyceraldehyde-3-phosphate dehydrogenase
(glyceraldehyde-3-phosphate 1,3-bisphosphoglycerate)
Pyruvate dehydrogenase (pyruvate  acetyl CoA)
Alpha keto glutarate dehydrogenase ( alpha keto gluterate 
succinyl CoA)
Glutamate dehydrogenase (Glutamate  alpha keto
glutarate)

23. Niacin Deficiency

24. It is an Italian word, meaning “rough skin”.
The symptoms are-
Dermatitis: in early stages, bright red
erythema occurs, especially in the feet,
ankles and face. Increased pigmentation
around neck is known as Casal’s necklace.
Diarrhea: It may be mild or severe with
blood and mucus. This may lead to weight
loss. Nausea and vomiting may also be
present.
Dementia: It is frequently seen in chronic
cases. Irritability, inability to concentrate
and poor memory are more common in mild
cases.

25. Recommended Daily Allowance of
Niacin-

26. References
• Nutrition Science (fourth revised edition)
▫ B. Srilakshmi
• Textbook of Biochemistry for Medical Students
▫ D.M. Vasudevan
▫ Srikumari S.
• Outlines of biochemistry (5th edition)
▫ Eric E. Conn
▫ Paul k. Stumpf
▫ G. Bruening
▫ Roy H. Doi