SOLVED. Overweight girl runs away and lives in the wild, with a twist at the end **SPOILER in comments!** [s]

[Yvonne]

1970's, or more likely 1980's.

**SPOILER**

At the end of the book, the girl has lost so much weight that her family doesn't recognize her when she tries to go home

[Tab]

The Island Keeper by Harry Mazer fits.

Here is a description from Kirkus:
Rich, fat, Cleo, 16, is desolate when her (view spoiler)[younger sister is killed in a boating accident. Their mother died in a car accident years earlier, their father is preoccupied with business, and their critical grandmother has shunted them off to various boarding schools and camps (hide spoiler)]. Now Cleo, mournfully alone, decides to duck camp and spend the summer, secretly, on an island her father owns in Canada. (view spoiler)[When Cleo finds the cabin on the island burned down, she sets up her sleeping bag in a cave. A raccoon gets into her provisions, so she stretches out what's left and supplements it with fish and wild plants. She lingers when summer turns to fall, and then a storm destroys her canoe and she has to stay on until the lake freezes and she can walk to the nearest mainland town. By then Cleo has enlarged her cave shelter with an outside wall and roof, killed the raccoon and then a deer (for its meat and hide), lost all her fat, and gained the strength to go on with her life independent of her father's and grandmother's expectations.(view spoiler)[ (hide spoiler)] (hide spoiler)]

[Yvonne]

Oh my gosh - that's it! Thank you, Tab!!

[Tab]

You're welcome :)