Getting custom body class from node field
function TEMPLATE_preprocess_html(&amp;$vars) {
    $node = menu_get_object();
    if ($node &amp;&amp; isset($node-&gt;nid)) {
        $field_body_class = field_get_items('node', $node, 'field_body_class');

        if (isset($field_body_class[0]['value'])) {
            $vars['classes_array'][] = $field_body_class[0]['value'];
        }
    }
}


This is how it would look today in Drupal 7, assuming your field name is what it is in the example. (The difference with the other answer being instead of $vars['attributes_array']['class'][] using $vars['classes_array'][])