We almost lost our oldest cat due to hepatic lipidosis (fatty liver) and we originally traced it back to her "allergies" acting up. (She suffered miserably through pneumonia as a young cat and ever since shows classic hayfever signs every year.) That was our first thoughts on it, anyway.
To make a long story short, her liver basically shut down. The first signs were vomiting, lowered appetite, and elevated and depleted liver counts (just everything out of whack.) After months of very intensive home nursing, she's her same old sweet adorable tyrant again. We pieced together that she started her decline about the same time our landlord chose new lawn and outdoor pesticide companies who used "new" and rather aggressive treatments to "make a showcase lawn." (Arrgghh). She Never goes outside, but they do so love to sit by a ground level open window. We had also tried a new carpet cleaner inside the house as well. Best we can put together is she was highly intolerant of at least one or maybe a combination of the "new" chemicals in her environment.
Between her vomiting, and not smelling her food (animals will usually not eat what they can't smell) and the toxic overload of chemicals (which we now feel aggravated her "allergies), she finally stopped eating on her own, lost a lot of weight, and turned a very scary bright yellow (jaundice from the liver issues) before we got her back to healing.