Aliens never eat. How do they grow?

Although Aliens is a remarkable invention, with a very interesting life cycle, there is one BIG problem I see in its biology : Aliens don't eat.

They grow from an embryo to a chestburster, and then, with remarkable speed, adulthood without ever eating. This can't be, all that body mass has to come from somewhere.

During the embryo stage, it has to feed off its host in some way. The only way for it to grow inside its host without harming him, is if he grows inside the stummach. (Sorry, English is my second language) This would make sence, because the environment in the stummach would compliment its acidic biology.

However, the biggest problem is between the chestburster stage and the adult. That remarkable growth needs to be fueled by something, he can't grow all that much without ever eating. Aliens often kill potencial hosts without actually eating them. That doesn't make sence. Why kill a potensial host other than for food or defence?

Which brings me to the second question. Has anyone ever seen an Alien actually USING those vicious looking teeth?