BuddyPress.org

Create a template overload of this file:
buddypress\bp-templates\bp-legacy\buddypress\members\single\profile\profile-loop.php

Look at bp_the_profile_field_name() and bp_the_profile_field_value() – they display the value.

You’ll need to write a conditional using
bp_get_the_profile_field_name and bp_get_the_profile_field_value() so that you can use the values without displaying them.

For example:

if( bp_get_the_profile_field_name() == 'current_weight' )
  $current_weight = bp_get_the_profile_field_value(); 

if( bp_get_the_profile_field_name() == 'starting_weight' )
  $starting_weight = bp_get_the_profile_field_value(); 

$weight_lost = $starting_weight - $current_weight; 

echo '<br/>weight lost so far: ' . $weight_lost;