Many thanks for all the interesting suggestions and links.
> However - as you suggest, in some locations vegan diets could never
> be locally-based.
It might then have to be a personal decision as to whether
the relative harm (to the environment) of transporting staple vegan
foods to a population is outweighted by the relative harm (to people,
animals and the environment) of using animal foods.
I'm bothered by conflicting evidence about the health effects
of soy products (e.g. see http://www.power-surge.com/transcripts/tabor-shomon.htm). Commercial
soy (Glycine max) and traditional soy (Glycine soja) may have
important nutritional differences, but I've not found studies
comparing health of 'traditional' and 'commercial' soy eaters. There
is evidence that (at least) commercial soy can cause thyroid damage,
particularly in infants.
Of course, one answer to this is that heavy reliance on _any_
one food is likely to cause human health (and other) problems.
Soy grows poorly in the current UK climate, in any case.
Vicia faba (broad, or fava beans) grow well, but lack some of the
trace nutrients found in soy. Ah, well, the search continues :-)