The r/K thing is a teensy bit different. That is more to do with offspring quality - with many vs few offspring.

The idea (from dietary energy restriction) is that organisms face resource-investment tradeoffs between self-maintenance and reproduction - and that circumstances and diet can affect where that tradeoff is made. If there isn't enough dietary energy to support reproduction, what resources are available are devoted to maintenance - so the organism can live to reproduce another day.

It is a bit like K-selection taken to an extreme where no babies are produced at all - and all resources get invested in personal survival.

I have a page all about this general topic: http://cr.timtyler.org/why/