All beings eat.
All races need to eat.
All the races that visit Earth eat plants.
They eat plant foods native to their own unique worlds.
They may be put into another form but are still pure.
Almost all other races of people and planetary species intake just what is needed for sustenance of the body and elimination is enough through the skin and breath.
They only eat what their bodies absorb.
So they don't have bladders or intestines.
They exhaust small amounts of waste through skin and breath, the same as we do. 
Earth humans eliminate waste largely in a rare way, through the bladder and bowels, which is strictly a planetary life characteristic.
Normally, beings eat just what they need to eat and have no need for bowels or bladder to exit waste in quantity. 
All people need to eat food native to their home worlds.
One race that visits Earth is human - that is they have the same DNA we have.
Our race on Earth came from the DNA of their race.
This human race is alien to us now and we cannot share atmosphere.
They are also much further (longer) evolved and technologically advanced than we are.
We humans on Earth eat great quantities and varieties of species of foods compared to almost all other races/worlds of people anywhere.
(I only say "almost all other races" because there is so much life at such a diversity in the universes that it's difficult for anyone to know or say one thing for all.)
None of our visiting races eat as much as we do, and none of it is unnatural to their bodies.
All of them eat just about as much pure plant foods and water as their bodies absorb, so they do not urinate or defecate.
They do not have the need to discharge bulk quantities of waste, but they do eliminate small amounts through the breath and sweat as we do.
Urinating and defecating is a unique planetary characteristic of life on Earth, as is bodily milk production for mammal young. 
We were brought up on processed foods, now we are sick.
The other races don't eat processed foods.
They don't pay for food and don't have advertising either.
We make weapons instead of feeding all our people.
We already have a way to feed everybody.
But we don't.
Our leadership is maintaining human suffering.