I found this article on zinc and yeast acivity.
http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1987.tb04478.x/pdf
Here is an exert:
"In the second paper of the session Mr F. B. Martens
(Heineken Technisch Beheer BV, Zoeterwoude, The
Netherlands) discussed The Effect of Zinc on Yeast Be
haviour: Continuous Culture Studies. The trace element zinc
plays an important role in yeast metabolism. Important glycolytic
enzymes, such as glyceraldehyde-3-phosphate dehydrogenase,
aldolase and alcoholdehydrogenase require zinc for
catalytic activity. Experimental data taken from batch fer
mentations suggest that a low level of zinc in the wort has a
negative effect on yeast behaviour. A chemostat is a perfect
instrument to study the influence of individual substrate
components. Thus, a continuous yeast culture system has
been operated and brewers' yeast metabolism monitored as a
function of the zinc concentration and dilution rate with
respect to activity of zinc-dependent glycolytic enzymes. A
synthetic medium was developed in order to study brewing
yeast metabolism under zinc-limiting conditions."
Given that many use RO/Distilled water, it would probably be wise to use some kind of zinc supplement towards the end of the boil. Wyeast Nutrient supplies this and other "goodies".