Dynamic Menus with hidden fields

I have a table full of drop mens, all populated from various tables in a
database. Each table has fields for the item_id, itemname, itemfat, itemcals,
itemprotein & itemcarbs. The itemname displays as the label and I need a way to
do some maths on the other fields (fat etc) which are a mixture of decimals and
INTs.
I would like to add the values of each of these 4 fields, for each selected
option, into new text fields when a user clicks a "Calculate" button.

How do I go about this please?