Debian :: Why $http_proxy Only Works For Root User In Apt-get

I just setup a debian OS(in emulator) and trying to use apt-get update. When I log in as root and do:

export http_proxy=http://proxy.com:9090
apt-get update   works

If I use another user and ssh to this debian, sudo apt-get update will fail to work because it don't use the proxy. I try to do the export http_proxy stuff again but still not working. echo $http_proxy showing it already set correctly...

p/s:
I have a workaround by adding this lines in the apt.conf
ACQUIRE {
http::proxy "http://proxy.com:9090"
}

but I really don't want this solutions because I want to easily disable the http_proxy in command prompt (by unset it).