diff --git a/bin/brew b/bin/brew
index be14c51257..5a0aed1331 100755
--- a/bin/brew
+++ b/bin/brew
@@ -118,8 +118,8 @@ export_homebrew_env_file() {
   [[ -r "${env_file}" ]] || return 0
   while read -r line
   do
-    # only load HOMEBREW_* lines
-    [[ "${line}" = "HOMEBREW_"* ]] || continue
+    # only load variables defined in env_config.rb
+    [[ "${line}" =~ ^(HOMEBREW_|SUDO_ASKPASS=|(all|no|ftp|https?)_proxy=) ]] || continue
 
     # forbid overriding variables that are set in this file
     local invalid_variable