Given the absence of teeth, the narrowness and rigidity of the esophagus, the absence of a true stomach and the absence of defecation, it is strongly believed that EBOs can only consume food in liquid form. It is assumed that, given the high metabolic needs of their brains, this food would have a high carbohydrate concentration. In order to meet other metabolic needs, there must also be a high protein content in the food consumed. These two statements are supported by the type of enzyme secreted by the digestive organ. It is therefore speculated that the food consumed is a sort of broth rich in sugar and protein, which probably also has a high copper content. Given the strict limitations on the type of food that they can consume, it's unlikely that this type of creature could survive in our biosphere without technological support.

In the cattle mutilation phenomenon, livestock is found, usually a single bovine, missing it's blood and specific organs, all removed with surgical precision. Not a drop of blood seems to be spilled in the process. This amounts to around 4 gallons/16 liters of blood harvested, depending on the size of the animal. If the EBO/Grey digestive system description above is true, then blood seems to qualify as food, as it is a liquid that contains protein, amino acids (the building blocks of protein), and sugar (glucose). Perhaps the other organs harvested also contain nutrients that they require, which are combined with the blood. We expect that they usually synthesize their food somehow, but we can speculate about why they would need to occasionally harvest cattle for this purpose. Consider the following points:

According to many reports, their culture seems to be hyper-obsessed with efficiency. Everything is designed with the absolute minimal specifications for the intended purpose. There is little safety factor. There are no provisions for the comfort of the crew. There are no storage spaces for supplies. Things just do what they are designed to do, exactly and precisely, nothing more.

They don't seem to have any concern for the well-being of any particular individual. There is never any recovery of crash victims, alive or dead, for example. The Greys seem to be manufactured clones and their masters can just make more.

Say a certain mission takes an unexpected turn, and has to stay away from home base for longer than planned. For the reasons given above, the craft has no more food than is required for the original mission. But there are thousands of cattle, full of gallons of Grey food, and the entities have the ability to harvest it with little effort. Perhaps the Greys themselves are designed with a similar design philosophy as the craft, with little safety factor in the form of energy storage.

(http://www.autoadmit.com/thread.php?thread_id=5856491&forum_id=2...id.#49813325)