< raw fish contains a thiaminase. That's an enzyme which purges thiamin
(vit.B1).>

well pointed out.  here's something interesting i found:

Thiaminase can create a problem for people who consume raw fish. The enzyme
is present in many bacteria and in fish and was first recognised in 1949
when domestic animals fed raw fish heads developed paralysis.
...Thiamin is combined with allicin from garlic to produce allithiamin which
is resistant to thiaminase. Consequently the biochemist who recognised this
has made his fortune selling multivitamin pills in Japan containing
allithiamin instead of thiamin, and Japanese continue to eat raw fish

http://www-biol.paisley.ac.uk/Courses/Enzymes/glossary/Nutrient.htm

andrew