Ben,

It appears that there is animal evidence in support of the concept of
micronutrient-induced alterations in the capacity for urea synthesis.
Rates of urea synthesis have been observed to depend on, but may not be
limited to, vitamin A [1], biotin [2] and, in diseased-states, carnitine
[3].

Interestingly, with regard to biotin and carnitine at least, effects are
noted at the level of gene expression for urea cycle enzymes.  As vitamin A
is quite active at the gene level, a change in gene expression (although
not reported in the study cited) may indeed explain its affects on urea
synthesis, as well.

It would be insightful to measure the differential
 intake of these 3
regarding organ meats vs "muscle meats" (and whether this "difference"
would be predicted to significantly alter rates of urea synthesis in the
population at large).

Ed Thompson

References:
[1] John A, Sivakumar B. Effect of vitamin A deficiency on nitrogen balance
and hepatic urea cycle enzymes and intermediates in rats. J Nutr 1989
Jan;119(1):29-35

[2] Maeda Y, Kawata S, Inui Y, Fukuda K, Igura T, Matsuzawa Y. Biotin
deficiency decreases ornithine transcarbamylase activity and mRNA in rat
liver. J Nutr 1996 Jan;126(1):61-6

[3] Horiuchi M, Kobayashi K, Tomomura M, Kuwajima M, Imamura Y, Koizumi T,
Nikaido H, Hayakawa J, Saheki T. Carnitine administration to juvenile
visceral steatosis mice corrects the suppressed expression of urea cycle
enzymes by normalizing their transcription. J Biol Chem 1992 Mar 15;267