todotoday,
if you suspect cancer, you should run to your doctor and find out for sure. Your chances to beat it at an early stage are very good. I believe that regular fasting can _prevent_ cancer from developing, but I have not seen yet one case of already existing cancer (other than brain) cured by fasting. Yes, tumors shrink but then usually return --and quite aggressively!-- upon refeeding. So please do not delay checking it out if you suspect it.
But, as trimnut2 pointed out, there may be many reasons for the breath soreness and cancer is the least likely among them.
Regarding your preparedness, you plan sounds good. Some people prefer to simply take a plunge without any preparation. Whatever works for you. It helps a lot to purge the content of the GI tract at the onset of a fast.
Re your "distilled spring water", that's a contradiction in terms. Spring water usually contains some minerals while distilled water has none whatsoever. I would not recommend you to drink distilled water on a fast lasting longer than a week. With sodium running low you will certainly develop electrolyte imbalance, which will make you weak, nauseous, lightheaded and it will strain your heart. If you want the details, you may want to check out a post I contributed a while ago in the distilled water thread.
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As for your main question, the bad breath is due to the metabolites of bacteria that proliferate during a fast. Most often, the culprit is P. gingivalis, which colonizes both mouth (causing gingivitis) and the GI tract. This bug is very common and, proliferating during a fast, it results in very bad breath and aggravation of gingivitis or even gum disease (if it went already that far).
What you can do is take a course of
Antibiotics and fast immediately after (actually you can start fasting on the last day-two of
Antibiotics , before ketosis starts). I know that
Antibiotics is the anathema in curezone lol but that's the only sure thing that works for these bugs.
Without antibiotics, the other method is to go through a series of fasts, like 10 days on, 10 days off, etc. This sort of fast is reportedly better for a steady weight loss -- and to loose weight is why you mainly want to fast, right? This sort of fast will also teach you how to eat right in between, which is the most important measure for maintaining the right weight.
The bugs that proliferate during a fast can usually be dealt with quite successfuly by the immune sys during the refeeding stage (that's why bad breath promptly goes away then). By going through a series of shorter fasts (provided your eat right in between! especially essential amino acids -- the immune sys will get rid of those bugs naturally and eventually you will be able to fast with a sweet breath.
Ketones themselves actually have a fruity smell, like bruised apples, with a peachy undertone. Maybe not roses, but it's not bad at all.
Good luck!