It's not quite that simple:

The Human Balance Sheet

You are what you eat.

Minus what you excrete.

Plus what you inhale.

Minus what to exhale,

including what you wheeze,

not to forget that sneeze.

Minus what you secrete, not

to forget tears, sweat, and snot.

Minus that which you pare,

By which I mean nails and hair.

Lastly, there's the thing

about shedding skin.

Did I yet mention burping?

Or worse belching?

I guess that covers it.

Wait! I forgot: vomit!