It's possible that parent is referring to the mild acidosis that can accompany ketogenic diets or fasting. At some point along the process of using fat, rather than glycogen, for fuel, your body's compensatory mechanisms (bicarbonate buffering, renal excretion, and a few others) can't keep up with the acid in the blood stream, causing a lower pH.

I'm not going to take a guess at 'toxins.' When I hear that terminology, I think of a blindfolded kid trying to hit a pinata with a bat, or carpet-bombing: they'll get there eventually if only by the imprecision of their targeting, but it's not pretty.