By the end of four weeks, patients on the experimental diet had significantly fewer swollen, tender joints, less pain, better grip strength, and improved lab-test results. Those volunteers who stayed off the regime didn't fare nearly as well. Others speculate that a vegetarian diet may alter the bacterial population in the large intestine, somehow relieving the disease. Prolonged, modified fasts should be avoided by all but the seriously obese, and should be undertaken only under the supervision of a physician who has plenty of experience and training in this kind of treatment. Experts also advise that even with adequate supervision, a modified fast should not extend beyond 16 weeks.
Shorter fasts are safer, of course, but for many people it's wise to avoid even a three-day fast.
Pregnant mothers, for instance, risk depriving their baby of much needed proteins, fats, and other nutrients whenever they fast. Similarly, nursing mothers compromise the quality of their breast milk. Others who should avoid any sort of fast are those with ulcers, diabetes, or diseases of the liver, heart, kidneys, and lungs, as well as anyone with advanced cancer. Children are not very good candidates for even a one-day fast. In one study, children who fasted from dinner time until noon the next day had poorer scores on tests that measured memory and the ability to recognize various types of visual stimuli.