I was reading through a book titled 'A Buddhist Bible' and while reading a chapter on the life of Milarepa I read about how he entered into 'Dagkar Taso' cave where he performed austerities causing his body to become very emaciated.
That was because he didn't always have a lot of food to eat, especially in winter. He was not emaciated because of an imposed fast. Tradition holds that he ate a lot of nettle soup. This gave his skin a greenish color (perhaps because of the Chlorophyl in the nettles). He's often depicted that way in Thangkas
Concerned relatives would try to get him to refrain from the strenuous austerities but failed to do so.I recall that Shakyamuni Buddha exhorted his followers to refrain from excessive austerity and to instead follow a 'Middle Path'. Was Milarepa Buddhist ?
Perhaps the author of the book didn't provide an accurate excerpt of the story.
Milarepa was a Yogin. He's often referred to as "Lord of the Yogins" because of his extended retreat practice. The austerities, as another pointed out were not the purpose of Milarepa's retreat. The austerities were a side-effect. He was also a Buddhist.
You should read Thrangu Rinpoche's biography of Milarepa. You should also read and meditate on the 100,000 songs of realization that Milarepa composed. This would be especially good to do if you have any interest in the Kagyu lineage or Mahamudra practice.