syntax error when setting variable in front of "while"

[Ralf Goertz]

Bash Version: 4.0
Patch Level: 35
Release Status: release

Description:

I am used to setting variables in front of a command like in 

> LANG=C man mplayer

However, I get a "bash: syntax error near unexpected token `do'" error
when trying: 

> FOO=BAR while read a b; do echo $a $b; done

Why? The similar 

> FOO=BAR read a b && echo $a $b

works fine. (The problem occurred when I tried to temporarily set IFS to
the tab character for a "read").