The author talks about how we can't really just live off of caffeine because it doesn't have everything we need to grow and help the process of digestion without more of our daily needs.

We and the caffeine loving bacterium, need nutrients like vitamins and minerals to get certain calories, which caffeine doesn't have.

Cellulose is the primary component of plant cell walls and accounts for the fiber in fruits and vegetables. we humans can't break down cellulose, so it just passes on through the body intact. But other creatures can extract calories from it, because they have bacteria living in their guts.

The specific reason caffeine isn't an energy source for us, thus, is that we don't have the gear to digest derivatives further. We can do other things with them-usually, we pop an oxygen on xanthine to make uric acid and pee it out-but taking them apart isn't possible.

I drink caffeine almost everyday. I'm drinking coffee every morning, and also I drink soda all day long and it taste good and I feel good. But if I don't drink milk or water once in awhile, I don't think it would be a very good idea.

So you can't just live on caffeine all your life, it's not a smart choice. If you don't get your usual daily nutrients in you will get bigger and bigger and you might die depending on what you do in your life.