My hypothesis is that what you eat has an impact on brain function.  Basically if you are eating a lot of meat it implies that your group are hunters.  For hunting one needs a nimble mind (and body for that matter.)  That will naturally develop to help the group operate effectively in their environment.

On the other hand, if you eat a lot of fruits and roots and what-not, your group are probably a gatherers or at least that is the most effective food source at the present time.  Not much brain is necessary for that, and excess brain wastes energy.

If you eat a lot of grain your group are probably farmers.  It's much better to be a mindless dolt who is content to work the fields dawn to dust.  A well working brain not only wastes energy but interferes with the life's work.  Double-negative.

I'd say that the 'leadership' understands this.  The food pyramid and diet offered by the powers that be favor grains and more recently high fructose corn surup.  This is mainly because they plan to avoid any more revolutions going forward and diet is one of the tools that they use to keep the peeps in a state of perpetual dopiness.  The 'war on beef' which is a thing among the eco-crowd is another face of the same basic coin.

Again, the socialists have a fascination with the concept of getting the kids early and keeping them as much as possible.  Mostly is that they understand that this is key to 'revolutionary change', and to enduring rule once revolution is accomplished.