This reminds me of a doctor back in the 30s named Dr. Linda Hazzard. (easy name to remember when you know what she did.)
She came up with a revolutionary new way to cure all disease. You eat nothing but broth until your are cured. She ran a clinic where people could go and get better. Needless to say, not one of her patients checked out alive. All of them starved to death before the treatment worked.
However, she honestly believed in her fasting treatment, so when she fell ill, she administered the treatment to herself. Within 2 months she died of starvation.
Fasting with water allowed will actually pep you up after about a week, when you body runs out of energy and starts to consume the fatty tissue. However, your condition will quickly deteriorate and you will die.
Without water, it would be much much worse.
Some animals can go for a very long time without eating. Polar bears for example can go up to 6 months without food. Of course, they don't get nutrition from some mystic force, or whatever Breatharians claim.
Plants can draw energy from the sun and the soil, which is pretty close the the claim. However, I would suspect if photosynthesis were happening in these Breatharians, they would have green skin.