Not sure why people would get their panties in a twist? Sounds like quite a reasonable question to me, actually, and one I hadn't considered.
Most common and well-studied conditions have epidemiological (i.e. social statistics) information about them. Unfortunately, because TMAU is such a rare condition it's unlikely there's been enough research to give a clear answer.
I found this on PubMed where all the medical journal articles are published:
https://www.ncbi.nlm.nih.gov/pubmed/9110361. It says that regular members of white British populations, on average, convert TMA into TMA N-Oxide a bit better than people in Jordan, Ecuador and New Guinea - so you could be right, although it was an extremely small study.
FYI, I am a mixture of caucasian ethnicities and
very white, but I have TMAU.
Hope that helps...