[pharrlax]

>The doctor wanted to prescribe an appetite enhancing drug but my father explained he can not simply gain weight by eating more. It's as if his digestive system is already at a maximum threshold.

It's more likely that his weight set point has been lowered, and now his hormonal hunger indicators are defending that set point.

He needs to power through it and eat more, even though it's going to feel like he's uncomfortably full all the time. I went through the same thing. It's all mental. It has nothing to do with the digestive system.

[taneq]

Note that he doesn't need to eat much more. A few hundred kilojoules a day will do the trick over time - he could reverse such a slow trend by something as small as switching from low fat to full cream milk, or having a couple of Tim Tams with his morning coffee.