pre-process functions are overwritten

I am trying to add some global variables in the preproccess function as follows:

function zeropoint_preprocess_node(&$vars) {
$GLOBALS['user']->x = $x;
}

initially, it appears this works as I think it should, however one inside a given node, the new variable dissapears from the object. This also happens when I set a new variable:

function zeropoint_preprocess_node(&$vars) {
$GLOBALS['newobject']->x = $x;
}

Any help would be appreciated.