Is there any alternative of node_save to update database fields in iteration?
you can use entity module. This module proves entity_matadata_wrapper API which allows you to load and save fields of nodes. The following code is an example of altering a node's title. 

$nodeWrapper = entity_metadata_wrapper('node', node_load($node-&gt;nid));
$nodeWrapper-&gt;title-&gt;set(t("Saved by Entity Save"));
$nodeWrapper-&gt;save();


entity_matadata_wrapper is not limited to nodes. As you can see in it's signature you must specify the element which in your case it is node.