The bowels may stop functioning during fasting and this is not a concern. In long fasts if there is a history of a toxic bowel an enema or colonic irrigation may be suggested, although this is seldom necessary.
Possible (rare) Side Effects During Long Fasts
If serious symptoms occur on a long fast, such as a sudden drop in blood pressure, or a feeling of extreme cold which persists, or a prolonged, rapid and weak pulse, or extreme weakness, or difficulty in breathing then the fast should be stopped. These symptoms are unlikely to occur on short fasts, but are possible on long-fasts which highlights the need for supervision and the regular checking of vital signs.
If vomiting and/or diarrhoea occur and are persistent then expert advice should be sought. It is essential to maintain liquid intake at an optimum level.
If acute anxiety and emotional distress are experienced then the fast should be carefully broken (vegetable broth, yogurt, steamed fruit etc).
If there are signs of hepatic or renal problems the fast should be terminated.
Uric acid levels in the bloodstream rise during a fast and if there is a history of gout caution is required. High levels of fluid intake can ensure that gout is an unlikely outcome of fasting even if uric acid levels become relatively high.
Lab Tests During Long-term Fasting (more than 4 days)
Salloum, Burton and Fuhrman as well as many other experts suggest that during a supervised long fast there should be daily assessment of vital signs (heart, blood pressure etc) and weekly evaluation of electrolyte levels and reserves (which should be repeated if vomiting and/or diarrhoea or sudden weakness are noted)
Despite this apparently lengthy list of possible dangers and side-effects
Before a long fast is started liver and kidney tests should be performed. A long fast should be ruled out if the liver or kidneys are in a distressed state.
No specific patterns of laboratory results are predictable during a fast, with individual characteristics being evident depending upon overall health status and any concurrent medical problems.
Liver enzyme levels may rise, with or without liver disease being present.
Cholesterol and triglyceride levels usually rise as fat stores are mobilised and uric acid levels rise (they should all fall after the fast).
Blood glucose levels decline in most fasting individuals (and normalise subsequent to the fast).
Erythrocyte Sedimentation Rate (ESR) usually decreases during a fast while most aspects of complete blood count remain stable if hydration is adequate.
Increased specific gravity of urine usually indicates inadequate hydration. A variety of unusual products are commonly found in urine during a long fast.
Insulin and thyroid hormone levels usually drop during a fast, while growth hormone increases (except in obese patients). Other increases usually include serum melatonin (assisting sleep and stress reduction), glucagon, cortisol, plasma norepinephrine.
Blood pressure is likely to drop as is weight and pulse rate.
A marked improvement occurs in immune function, especially during the first 36 hours of a fast. There may be raised levels of T-lymphocytes and lymphokines, decreased complement factors, decreased antigen-antibody complexes, increased immunoglobulin levels, enhanced natural killer cell activity, heightened monocyte killing and bactericidal activity and marked increase in resistance to infection in the post-fast period.
fasting is safe, and short term fasting is almost totally safe.