>>/69365/
As per Yoshinori Ohsumi's research, the body has a bunch of immune/health mechanisms that depend on humans (and animals in general) having periods of hunger to happen. The cells go into autophagy and eat up the weak/sick ones and repair DNA and do all sorts of shit. De-aging even.
This been a long debate since the first notable perception of this in relation to the Biosphere 1 and 2 experiments.

Your friend seem to be referring to a ketogenic diet, which is a bit outdated by now. Just fasting is more effective.