This question about protein deficiency does have some RW analogs.
Archaeological evidence proves that health deteriorated with the advent
of agriculture. The diets of the Egyptions, Babylonians, Mayans, etc..
all show an increase in diet related diseases after the onset of
agriculture (except for the rulers, of course). Why, then, should
agriculture be such a prominant and non-reversable invention? It turns
out that you can feed ten times as many people on agriculture compared to
hunting&gathering per unit area. Ten malnourished soldiers can still beat
1 hunter gatherer. Thus the hunter gatherers were pushed to the areas
where agriculture will not work.
How does this relate to Glorantha? That is up to you to decide. It was
not until recently that these major health problems were conquered in the
real world. Maybe they do not exist in Glorantha. The presence of magic
can compensate, as can the presence of greater powers. Take your choice,
or come up with your own alternative.