People have used fasting for many centuries as a therapeutic treatment, and some also practice it for religious and spiritual reasons. There are many degrees of fasting, from the meatless days of Catholicism to ascetic, water-only fasts that run for several days. The initial discomfort of fasting passes in a few days, with heartburn as the most notable result. However, any significant fast has potential long-term health effects, and it's important to re-introduce foods carefully. Check with your doctor before beginning a fast.
The Physiology of Fasting
Our bodies have well-defined processes for dealing with the transient unavailability of food. This is a survival characteristic, hard-coded into our genes. After the food in our digestive system is exhausted, the body draws on reserves of glucose stored in our liver, and in muscle tissues throughout the body. The pancreas supports the switch to glucose as fuel by shutting down production of insulin, which can be problematic for diabetics. When glucose stores are exhausted the body begins burning fat stores, and eventually will begin to consume protein from muscles. At this stage fasting becomes debilitating and potentially fatal.
Paradoxically, although the body produces less stomach acid while fasting, heartburn is a commonly-reported complaint during short fasts and the early stages of longer fasts. Ordinarily the body would produce extra digestive juices when you eat, but it can be misled by your brain. Thinking about food or smelling it can give your body the erroneous impression that a meal is on the way, causing it to generate acidity. It is safe to take an over-the-counter antacid to counter this. If you routinely take a more powerful acid blocker, consult your physician for advice on whether to use it while fasting.
Breaking Short Fasts
Short-term fasts generally have no lasting physiological effects, but you might experience some discomfort if you eat too much too soon, or begin with unsuitable foods. Begin with small amounts of foods high in fiber and complex carbohydrates, such as whole grains, fruits and vegetables. Until normal digestion is restored, avoid fried or fatty foods. Your body will tolerate them poorly, to begin with. Bear in mind that sweet treats such as cake and donuts are high in fats, as well. Avoid highly processed foods for the first few days, because they digest quickly and can cause spikes in blood sugar.
Breaking Longer Fasts
Fasts exceeding two weeks, unless undertaken with medical supervision, can cause long-lasting damage to the liver and other internal organs, and can impair overall health for a lifetime. Re-establishing normal digestion is especially crucial after a long-term fast, because of the potential for damaging overconsumption, or refeeding syndrome. Usually the first step is to replenish fluids and electrolytes intravenously, then follow with intravenous glucose and vitamins to help re-establish body functions. Small amounts of solid food can be added after this, building up over a period of 10 to 14 days to regular meals. Again, fatty foods should be among the last ones re-introduced.