The original Master Cleanse did not make any provision for the P&B shakes. Many purists believe it is best to not put any kind of solid into the system while doing the MC. This is a form of fasting, and fasting needs to be liquids only to be the most effective in deep cleansing. Remember that The Master Cleanse is more than a colon cleanse,(in fact I believe that far too much stress is put on keeping the colon flushing out with irritating substances) done correctly the Master-Cleanse cleanses the entire body. The digestive system is effectively shut down, except for the colon flushing, and the energy generally required for digestion is diverted to healing the body. The liver, kidneys and skin must process the released toxins at first and might seem stressed; but as the cleanse goes on, and as the body grows cleaner these vital organs will begin to recover from the stress of overwork and begin to work better and more efficiently. The skin will begin to glow, veins and arteries will be cleaned out, decades of accumulated mineral deposits will be flushed out of stiff joints. A fasting cleanse is a near miracle in healing the body. However, it is important to remember that one cleanse will rarely be enough and for the majority of people it is much better to begin with a series of short 10 days cleanses than one very long one. The key is to spend the time between cleanses rebuilding the body with healthy nutrient dense foods.
For those who chose to add the P&B shakes, The Master Cleanse basically becomes a colon cleanse. While most of the fiber in psyllium is non-soluble, there is enough soluble fiber to activate the digestive system and energy is once again diverted to the cause of digestion. The soluble fiber is important and can aid in clearing clogged arteries, but that is done at the price of effectively ending the deep cleansing action of a true fast. The Bentonite clay must be used carefully, some people actually can wind up in worse shape because too much Bentonite can be very constipating. A tablespoon or two of the liquid Bentonite a day is enough to attract and adsorb toxins in the gut.
For those who wish to combine Master-Cleanse and P&Bs, I suggest beginning with a plain Master Cleanse for a week to ten days, this will give the system time to begin a good deal of detoxification and to soften the hardened gunk in the bowel. After this time you can begin to add the P&Bs slowly to the cleanse. Start with one shake a day, then gradually increase. A sudden influx of high fiber is not good for the system and will most likely cause trouble for the individual. Shakes must be added slowly and be accompanied with an increased amount of water. Do not do the P&Bs for more than two weeks with a pause. After a while the psyllium can begin to abrade the tender villi that line the small intestine. Its the villi that absorb the digested foods into the bloodstream where it is then transported to the trillions of cells in the body and utilized for energy and rebuilding. Damage the villi and you risk serious problems for the entire body. Always be cautious when cleansing and avoid extremes. Its far better to take things slowly so the body can utilize them than try to accomplish everything at once.
Try at least one "by the book" Master Cleanse. After that you can begin to make subtle changes to meet your specific needs at any given time.