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Most religions use periods of fasting as a means of demonstrating faith or penitence, and an opportunity for spiritual reflection. Fasting has also historically been a means to express political views and a form of protest.
Though it may not be the most practical -- or safest -- diet, some people use fasting as a way to lose weight or to cleanse the body of toxins, although some experts say our bodies are perfectly equipped with organs that already do the job. How fasting is used for weight loss varies by diet. Some fasting diets involve drinking nothing but water or eating only raw foods for a period of one or more days, while others restrict food on alternate days. Certain fasting diets only allow liquids like water, juice, or tea, while others dramatically cut calories but do not eliminate food altogether.
Does Fasting Help You Lose Weight?
When you fast, your body is forced to dip into energy stores to get the fuel it needs to keep going, so you will lose weight. The big question is how long you will keep that weight off. Because food was often scarce for our ancestors, our bodies have been genetically programmed to combat the effects of fasting. When you eat less food, your metabolism slows down to conserve energy. Then, when you go back to your usual diet, your lowered metabolism may cause you to store more energy, meaning that you will probably gain back the weight you lost and possibly even put on more weight when eating the same calories you did before the fast.
As you fast, your body will adjust by reducing your appetite, so you will initially feel less hungry. But once you have stopped fasting, your appetite hormones will kick back into gear and you may actually feel hungrier and be more likely to binge.
Research has shown that fasting on alternate days can help people lose weight, but not for long. In one study, people who followed an alternate-day fasting diet shed weight, even when they ate all they wanted on the nonfasting days. However, they could not maintain the weight loss over time.
Can Fasting Detoxify the Body?
Some fasting diets claim that they can cleanse the body of impurities. However, there is no evidence that fasting detoxifies your body, or that your body even needs to be detoxified. It is naturally designed to remove toxins through the skin (by sweating), liver, colon, and kidneys.
Could Fasting Help You Live Longer?
Studies of fasting in both rodents and humans appear to indicate a connection between calorie restriction and longevity. In one study of overweight men and women, a calorie-restricted diet improved markers of aging, such as insulin level and body temperature.
Fasting might also improve longevity by delaying the onset of age-related diseases including Alzheimer's, heart disease, and diabetes. One study showed that skipping meals once a month, as members of the Mormon religious group do, reduces the risk of clogged arteries (the build-up of plaque that can lead to heart attacks and strokes). However, it is not clear from this research whether fasting alone or the Mormons' generally healthier lifestyle (they also abstain from coffee, alcohol, and smoking) is responsible for the improved heart health.
Researchers do not yet know whether the effects of fasting translate into an actual increase in lifespan, because they have not followed people for long enough periods of time. However the concept of intermittent fasting, such as skipping a meal purposefully on an intermittent basis, is gaining attention. It appears to be a relatively safe way to reduce caloric intake that is easier for some people to do. The data however is not there yet nor is this an accepted practice.
Is Fasting Safe?
Fasting for a day or two probably won't hurt people who are generally healthy, provided they maintain an adequate fluid intake. However, fasting entirely for long periods of time can be harmful. Your body needs a variety of vitamins, minerals, and other nutrients from food to stay healthy. Not getting enough of these nutrients during fasting diets can lead to symptoms such as fatigue, dizziness, constipation, dehydration, gallstones, and cold intolerance. It is possible to die if you fast too long.
Even short-term fasting is not recommended for people with type I diabetes, because it can lead to dangerous dips and spikes in blood sugar. Women who are pregnant or breastfeeding, or anyone with a chronic disease, should not fast.
Before you go on any type of new diet, particularly one that involves fasting, talk to your doctor to find out whether it is safe and appropriate for you. Also ask your doctor to refer you to a registered dietitian, who can show you how to design a healthy eating plan.