I'm certain it is but I'm not sure it's explicit. There's the bit in Case of Denzel where his friend gets offended because Denzel makes a joke about eating rats "like a real slum kid" and the friend explains that the rats are too polluted to eat. Although this seems to point to them not eating the rats, I'll give it that.
The take home point I think is that if they're not starving, they're eating the world's most unhealthy shit to survive (which IMO is still starving).
With a significant portion of the slum dwellers being explicitly poor as shit -- living in hovels made of literal junkyard scrap or in what appears to be out-of-service collection system piping -- it stands to reason malnutrition at the very least is a rampant problem.
To Lex's point about "Episode: Denzel," that does at least suggest something about the common understanding of people on the plate with regard to their groundward neighbors.