I have a spreadsheet for
weight loss for a group of people if the begginging weight is in cell
A1 and the next week's weight is in B1 and week after that is in C1
can I have the subtraction "follow" the numbers to show weight lost
each week? :

A1 B1 Formula
196 194 2

next week would be

A1 B1 C1 Formula
196 194 190 4

One way:
=OFFSET($A$1,0,COUNTA($A$1:$J$1)-2)-OFFSET($A$1,0,COUNTA($A$1:$J$1)-1)

Adjust to suit.