First  a vet visit  is required when a furball suddenly starts losing weight
for no reason.  Just like a human it is a major sign of something wrong.
Cats are notorious for hiding problems.

Have you noticed any problems in the litter box  or  if the cat is starting
to drink more?

This is a quote from Phil site
http://www.maxshouse.com/PreventativeHealth.htm

Weight loss
This sign often goes unnoticed, especially in longhaired cats. Owners who
regularly groom their cats may notice the ribs and backbone becoming more
prominent. Those who regularly weigh their cats are sure to see a change. A
sudden loss of one pound in a cat that normally weighs ten pounds is cause
for concern.  Subtle weight gains and losses are difficult to notice in a
cat you see every day - especially in long-haired cats.   Sudden weight loss
is almost always a certain sign of water loss and dehydration which are
early symptons of feline diabetes and chronic renal failure, especially if
the cat eats primarily dry food. A human pediatric scale is one of the best
investments you can make in your cat's health care program