I need to reference a custom field I created in User Profile
Use the $user object to access the logged in user.   

 global $user;
 $account = user_load($user-&gt;uid);


Now you can use $account-&gt;field_iweight['und'][0]['value'] (just an example) to access the weight or any other field from the user profile.

Updated code: 

&lt;?php
global $user;
$account = user_load($user-&gt;uid);
$entity_field[0]['value'] = $account-&gt;field_iweight['und'][0]['value'] - $entity-&gt;field_current_weight['und'][0]['value'];
?&gt;