Good luck with you exam. Living in the world we live in it is nearly impossible to not have any toxins in our body. Obviously the food we intake is a huge factor, but also the air we breath and the chemicals we are exposed to and do not even know it.
It takes more disciple for me to do fasts for three and four days in the city then a two week one in the desert. In the desert there were no distractios or food to tempt me. I felt very energized and clear during and after the fast. I felt "alive" in the truest sense. I was more focused there then at home where temptation is everywhere. But once I find that will power within myself, it is so empowering.
Wheat products are one of the biggest addtictions/source of food allergies. Funny cause we are encouraged to eat more wheat for its "health" benefits. Instead I have substituted rye and amaranth- which I find tasty without the side effects of wheat.
Anyway, if your practitioner suggests a fast , start small. He/she may just have you eliminate the foods that are not good for you and subsitute them with ones that work for you. But if you fast, you might be surprised that if you can let go of the thoughts of "I can't" and approach it as a new and plausible experience, you'll have no problems.