Can yellow stools be caused by dietary fasting ?
Yes, it is possible, although unlikely and it depends on the nature of the fast. More common causes should first be considered, one of which is the intake of foods that may stain the stool abnormally. Then bile related conditions and rapid transit of food through the gut (like with frequent bowel movements or even diarrhea). Ideally a stool test would be helpful to isolate the changes in the stool and provide a more definitive answer.
If you are noticing any other symptoms then you should speak to your doctor immediately.