If you supplement minerals and listen to you body response, then you can fast as long as you have fat stores in your body.

Its only dangerous if you don't know what you are doing or ignoring signals from your body. Lightheadedness is common thing people report or /r/fasting/ and almost always they don't take salt while fasting.

But I am not an expert, it feels like I have done more reading about fasting then actual fasting. So don't take advice from strangers (especially /r/fasting or worse dryfasting nutcases).