The Food Web
"We are what we eat." Only in as far as we need a balanced diet to be healthy. There are good records to show that eating certain foods is beneficial for health (herbalism, homeopathy, chinese medicine etc) but generally eating dog (as is customary in some parts of the world) is not going to make us more virile (although that is the reason given for doing so) and eating cow is not going to make us moo! We eat carbohydrates, proteins, fats, vitamins and minerals and water and the body does its best with what we give it. (Check out Health).
We are part of the food web. Basically this is a map of energy flows between living organisms. With each link there is a loss of energy as molecules change structure to satisfy the demands of the organism using them. Here's a simplified look at it:
Notice how humans eat everything. We barely even make a contribution to bacteria because we are usually buried deep in containers (coffins) below where most bacteria live or we are incinerated.
Here are a couple more illustrations showing various trophic levels.
Here you can see that another level of consumption by
meat-eaters uses 5-10 times as much energy as a vegetarian simply
because teh energy is being reduced at each level. Human
non-vegetarians are above the pyramid, while vegatrians would be at the
primary consumer level.
And here's The Meatrix - a short cartoon about industrial livestock production.