Why is the following sentence
“Ich habe zugenommen”
“I took on weight.”
with haben instead of sein?
To me it is an obvious change of state and there is no direct object, so I would have thought it would be with Sein.
(originally asked by Lisa at http://www.german-is-easy.com)
Ask about German 
Hi Lisa, this question had me thinking for a while.
You are right. When you think about it it feels very much like a change of state. However, that is not the focus.
The focus is that you take something to yourself… but that would be a poor explanation as it merely expresses my native perception of the word which could very well be due the the very fact that we use haben…. so I started thinking if there were a fail prove answer.
I could say, that nehmen goes with haben so all prefixed nehmens will use haben as well, but there might be an exception somewhere (yet to be found), so this is not good enough.
But I finally realized that zunehmen can take an object…
Ich nehme 10 Kilo zu.
I gain 10 Kilos.
10 Kilo is an accusative object. It is not mandatory but neither is coffee for trinken or pizza for essen.
So zunehmen can take an object, with or without object the word meaning is the same and thus it is using haben to build the spoken past.
Ich vermute, es geht ebenfalls mit “leben”. (Das eigene Leben geleben hat)