It is highly unlikely that fasting will have any effect on PTSD as this illness is caused by a terribly frightening, life-threatening, or otherwise highly unsafe experience.
PTSD sufferers re-experience their traumatic event or events in some way, and then tend to avoid places, people, or other things that remind them of that event.
In contrast, fasting will be highly beneficial in such illnesses as Schizophrenia/depression, bi-polar, or any psychotic/neurotic condition that has its roots in biochemistry.
Fasting was used with extreme benefit for schizophrenics at the Moscow Psychiatric Institute where they were fasted on water only for anywhere up to 30 days and beyond and supervised by Dr Yuri Nikolayev and reported on to the West by Dr Allan Cott MD..................... http://books.google.co.uk/books?id=jMdZPlcx07UC&pg=PA34&lpg=PA34&dq=Dr+Yuri+N...