niacin

Question 1

Structure of niacin

Answer 1

Niacin is a general name for the nicotinic acid and
nicotinamide, either of which may act as a source of the
vitamin in the diet. Niacin is a simple derivative of pyridine.

Question 2

Active Form

Answer 2

• Nicotinamide adenine dinucleotide (NAD+)
• Nicotinamide adenine dinucleotide phosphate
  (NADP+)

Question 3

Sources

Answer 3

• Yeast, liver, legumes and meats are major sources
  of niacin.
• Limited quantities of niacin can also be obtained from
  the metabolism of tryptophan. For every 60 mg of tryptophan, 1 mg equivalent of niacin can be
  generated.

Question 4

Functions

Answer 4

• Niacin is a precursor of coenzymes, nicotinamide
  adenine dinucleotide (NAD+) and nicotinamide
  adenine dinucleotide phosphate (NADP+).
• NAD+ and NADP+ are involved in various oxidation
  and reduction reactions catalyzed by dehydrogenases
  in metabolism.
• They are, therefore involved in many metabolic
  pathways of carbohydrate, lipid and protein.
  Generally, NAD+ linked dehydrogenases catalyze
  oxidation-reduction reactions in oxidative pathways, e.g. citric acid cycle and glycolysis.
• Whereas NADP+ linked dehydrogenases or reductases are often found in pathways concerned with
  reductive synthesis, e.g. synthesis of cholesterol, fatty
  acid and pentose phosphate pathways.

Question 5

Nutritional Requirement

Answer 5

• The RDA for niacin is 15 to 20 mg.
• Tryptophan can only provide about 10% of the total
  niacin requirement.

Question 6

Deficiency Manifestation

Answer 6

Pellagra
* Deficiency of niacin in human causes pellagra, a
disease involving the:
*Skin
*Gastrointestinal tract
*Central nervous system.
* The symptoms of pellagra are characterized by
three ‘Ds’:
1. Dermatitis
2. Diarrhea
3. Dementia and if not treated death.
Dermatitis: Skin inflammation is seen in any area
exposed to direct sunlight.
Diarrhea: Frequent diarrhea nausea, vomiting, anorexia
are the disorders of GI tract.
Dementia: Dementia (loss of memory) is associated with
degeneration of nervous tissues.

Question 7

How does niacin deficiency occur?

Answer 7

• To produce niacin deficiency, diet must be poor in
  both available niacin and tryptophan. Niacin
  deficiency occurs in:
  – Population dependent on maize (corn) or
  sorghum (jowar) as the staple food.
  – Deficiency of vitamin B6 (pyridoxal phosphate) leads
  to niacin deficiency as it is involved as a coenzyme
  in the pathway of synthesis of niacin from
  tryptophan.
  – Malignant carcinoid syndrome in which
  tryptophan metabolism is diverted to formation
  of serotonin.
  – In Hartnup disease, a genetic disorder in which
  tryptophan absorption and transportation is
  impaired.

Question 8

Therapeutic Uses of Niacin

Answer 8

Nicotinic acid (not nicotinamide), used at high doses
(1–2 gm/day), has been shown to lower total cholesterol,
LDL cholesterol and VLDL triglyceride in patients with
hyperlipoproteinemias.