The Hunger Artist is physically isolated from the world via his cage, but he is also isolated from the world in that only he can understand the seriousness and the importance of his "art." Although his art is to be displayed to the public, he has no personal life other than his own isolation. This is a symbolic but also a literal image of the "starving artist" - the idea that the artist must suffer for his/her work, and that the artist does it for the sake of art/not for money; thus, with no money, the artist may starve. With the Hunger Artist, his starvation and dedication to his art is mostly self-imposed, just as any artist is when he/she chooses to be an artist. But the Hunger Artist also notes that he fasts because he has never found a food he likes, implying that he was literally seeking nourishment and found none; so he doesn't find material nourishment. And he doesn't find public nourishment because no one understands him. And he doesn't find spritiual or mystical nourishment (until the end) because he is never alowed to really push the limits and flirt with death: fast beyond 40 days.
The fleeting nature of fame is a theme here as well. When the audience starts to ignore him, his isolation is of course increased, which makes him even more desperate in his fasting and eventually leads to his death.
Other than the final fast, the hunger artist must also stop after 40 days because that's when people lose interest. When he did stop every 40 days, he "sells out" by ensuring that he will eat and live to fast and sell out again. So, during his last fast, he is completely isolated in the fact that he is not understood and now he is not part of the economic institution of art. So, he is out of contract and no longer even noticed, hidden under the straw. I would call this total isolation, but the real complete isolation is his death.
In the end, he achieved no material (food), public (understanding), or artistic (spriritual) nourishment.
Check out the second link below for other interpretations of isolation of the artist in the modern world.