howdy,
i fast usually twice every years for about two weeks. i do this for health reasons primarily but the dulling effects of food on mental processes are well known.
there are lots of different kinds of fasts but if you do any fast which includes any kind of intestinal clearing the mental effects will be almost immediately noticable. even if a normal person is a few hours out from their last meal the effects of 'hunger' are noticable. crankiness, irritation etc. are common.
once one intentionally fasts however, some of the stress generally falls away. usually after day two there is no 'hunger' present and the mind starts becoming clear. without intestinal clearing, though, many people will begin having headaches and hunger due to the detoxification process which happens automatically. as soon as your intestines are genuinly empty and 'resting', they begin to clear out the genuinely toxic material that has built up over time. we all have this stuff to varying degrees but the typical american diet is a toxic soup.
in any case, when this cleansing process begins some of these toxins can be reabsorbed and cause the headaches etc. one usually notice elevated body and breath odor for the same reason. it is a good thing but it is very important to clear the pipes when you fast.
the two methods for emptying your intestines are either enema or ingestion of a high specific gravity drink like salt water, bitter salt water, sauerkraut juice etc. if done properly the negative effects (discomfort) of fasting can be reduced dramatically and the mental clarity it brings you is notable.
there is a lot of info out there and one should inform oneself well before one fasts. i don't consider the precepts eating model as fasting but that alone will show results in mental clarity pretty quickly.
hope this helps
