Now many skeptics are in debate about breatharianism. The debate of course isn't about whether it works or not, as all agree that it doesn't. The debate is about whether it one of the dumbest things to come from the New Age Movement, or if it's one of the craziest. The one thing that all skeptics will agree upon is that it is definitely one of the most dangerous.
No organism on the planet can live off of sunlight and air alone, be it either animal, or plant, or microbe. All lifeforms on this planet require at the very least water and nutrients in order to live. In fact for many lifeforms sunlight and air isn't even required in order to live, just water and nutrients.
Now we of course need air to live (although sunlight isn't really needed, it is a good thing to get, so as soon as you finish reading this, I recommend you go outside and get some sun) we can not live on air alone, because air does not contain any nutrients and other substances we need to live other than oxygen. Sunlight has the same problem with not having any nutrients and substances either, and thus cannot be used as a source of energy for the human body. The only the way to get nutrients and substances into our bodies that can be converted into energy is through food.
The amount of energy we can get from food (via digesting it) is measured in calories. The amount of calories a person needs is on average is around 2400 (this of course varies from person to person, depending on the person's size, how active they are, and their metabolic rate). Of course what you eat is also important as well, as a well balanced diet is necessary for a healthy life (getting 2400 calories a day in just cake and ice cream is not healthy no matter what). Air and sunlight does not contain anything that can be measured as calories, thus it cannot be converted into energy inside the human body.
Of course there is the claim that air and sunlight can be converted into prana, but there is a major problem with this claim: There's no evidence what so ever that humans can somehow convert air and sunlight into prana (or any other type of energy necessary to live). In fact there's no evidence at all that prana even exists, more or less that we humans can live off of just prana alone.
Some of you might be asking "if breatharianism is impossible, then how come some many people claim to be successful at it?" The answer is simple: they are lying, and they eat as much as we do.
Several high profile members of the breatharian movement have in fact been caught cheating (i.e. eating). Probably one of the most famous breatharians to get caught cheating is Wiley Brooks, who does admit to occasionally having a Double Quarter Pounder with cheese from McDonalds, and a Diet Coke.
One high profile member, by the name of Ellen Greve, a.k.a. Jasmuheen, actually did try to prove that a person could live off of air and sunlight alone, and agreed to do so under controlled conditions (i.e. she was being watched 24/7) after accepting a challenge from the Australian version of 60 Minutes (whom also filmed her) back in 1999. The test was stopped after four days after she began experiencing life threatening conditions due to dehydration.
Of course Ellen Greve got lucky. Many people who have tried to live the breatherian lifestyle (including followers of Greve) have either given up because they found out pretty quickly that it can't be done, or they died from severe malnutrition and/or dehydration. This of course is why so many skeptics (and doctors and scientists) consider breatharianism to be so dangerous.
People simply can not live off of air and sunlight alone. Everyone needs food in order to survive, including immobile people in vegetative states (which is why they are often times hooked up to feeding tubes). If you try to live off of air and sunlight, you are going to die.