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The USMLE frequently touches on the concept of pellagra pathogenesis and an understanding of the biochemical causation and the full list is required to answer related questions.

Niacin is also called Vitamin B3 is important for the formation of NAD which in turn is needed in several intermediary biochemical reactions.
Niacin can be endogenously synthesized from Tryptophan via a reaction that needs vitamin B6 as a cofactor.

Causes of deficiency (Pellagra):

  • Dietary deficiency mainly arise in geographic areas where consumption of corn is the main source of staple rather than wheat. Niacin in corn in nonabsorbable. This is specially seen in African countries.
  • Hartnup disease, because Tryptophan being a large neutral amino acid will be deficient and thence Niacin will also be deficient.
  • Isoniazid treatment as it inhibits vitamin B6 needed for endogenous synthesis as mentioned above.
  • Alcoholics as the over active dehydrogenases will consume all the NAD available.
  • HIV patients
 

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Niacin deficiency

The answer provided is comprehensive.

Niacin deficiency known as Pellagra primarily occurs in individuals whose diets are deficient in both Niacin and Tryptophan or in conditions in which Tryptophan is lost in the urine (Hartnup's disease ) or excessively utilized (e.g. carcinoid syndrome).

1. Individuals who consume corn-based diets are particularly prone to pellagra since maize protein has a low tryptophan content and niacin is in a bound form that can not be reabsorbed (treating corn with lime or calcium carbonate , releases bound Niacin).

2. Hartnup's disease is an autosomal recessive disease with a defect in the intestinal and renal re-absorption of neutral amino acids (e.g Tryptophan )

3. In Carcinoid syndrome tryptophan is used to synthesize serotonin which produces the flushing and diarrhea associated with the syndrome .
Hopefully this will clarify further .
 
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