very low hepatic enzymes

Hello, During the past months I noticed that my 10 y.o. cat was loosing weight --although he has never lost his appetite, so I took him to the vet for a check-up. I opted for testing him for Fiv and Felv too, which were negative; all the blood tests were in range (thyroid levels included) except for ALT and ALP, which were very low, much more than the normal levels. The vet prescribed a supplementation of the vitamin Bs for 3 weeks, followed by 3 weeks of pause before repeating the blood tests. This time also ALP was in the normal range, only ALP was very low once again. We opted for an ultrasound too, to check liver and small intestine, and it was all okay. He would ruled out a lymphoma, and as said the liver looks fine, the thyroid too… Is there anything that very low levels for those hepatic enzymes make you think of? I forgot to mention: it would look like malabsorption but as said the small intestine looked okay; and I would ruled out some dietary deficiency too because my cats all eat the same food (for the most complete and balanced wet food, plus some grain free, dry food) and he is the only one who lost weight. There are no other clear symptoms, signs of discomfort or illness, he is a very active, 'young-looking-and-behaving' cat (neutered when he was 8 months old, always vaccinated according to latest protocols). Thank you in advance if you can share what this 'picture' make you think of.

ps: forgot to add, the vet ruled out kidney-related issues too. also, at the moment he decided to wait for other 5-6 months, during this time give the cat vit Bs supplementation again for 1 week/month, then repeat the blood tests and in case a more throughout ultrasound..