Why is histidine an essential amino acid for children but not for adults ? What changes in the body occur which lead to the formation of histidine in adults but not in children ? What causes these changes ?
UPDATE : Source : Campbell Biology
If an amino acid is essential it means that we cannot synthesise it de novo from central metabolites, or from another amino acid. Since histidine isn't related structurally to any other amino acid we can assume that in this case we would require a pathway for de novo biosynthesis.
The biosynthetic pathway enzymes for histidine in yeast (Saccharomyces cerevisiae) are encoded by the genes HIS1 - HIS7. In a BLAST search using yeast His4p against Arabidopsis (plants can make histidine) there is a significant match to a histidinol dehydrogenase (the penultimate step in the pathway). In a similar search against humans there is no significant hit. Expanding the search to all mammals there is only one significant match, which is to the Tibetan antelope, Pantholops hodgsonii. This puzzled me, but when I searched with the antelope sequence as probe I got lots of perfect matches to bacterial histidinol dehydrogenases, so presumably the antelope sequence is a contaminant.
I also searched for human matches to His1p (encodes ATP phosphoribosyltransferase - the first step in the pathway) and His3p (encodes imidazoleglycerol-phosphate dehydratase - the sixth step in the pathway) and found no significant matches.
I conclude that humans have no capacity for histidine biosynthesis and thus histidine must be essential in the diet.
There are many articles in health literature debating on the question. Everytime the experimental setup seem to me quite basic and I am affraid that one cannot infer many things from these experimental designs. I guess the ethics is limiting much of the medical experimentation (which is good!). Usually it is just the report of observations of what happen to the global health of a patient when one amino acid is missing in their food. Therefore, these articles does not explain much about the metabolism reasons of these needs (which was your question) but only try to define what are the needs. Moreover statistics seem sometimes a bit fuzzy (or absent) to me.
Anyway here is a review of these results and there is a whole paragraph on the question of histidine. The best is that you read it yourself. To make it short: Histidine doesn't seem to be more important to the children than other amino acid. Moreover there is big storage of histidine in our body suggesting that one that is histidine deprived for a while might use these stored histidines.