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Diet Myth or Truth: Fasting Is Effective for Weight Loss

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WebMD Expert Column

Fasting is an age-old practice, often done for religious reasons, but fasting for weight loss is still capturing the public imagination. You can find dozens of do-it-yourself plans touting the unproven benefits of fasting, ranging from flushing "poisons" from the body to purging 30 pounds of fat in 30 days. 

It's true that fasting -- that is, eating little to no food -- will result in weight loss, at least in the short term. But the risks far outweigh any benefits, and ultimately, fasting can cause more harm than good.

Typical Fasting Weight Loss Plans

Fasting regimens vary, but the basic premise usually starts with a strict regimen allowing only water, juice and/or some kind of laxative concoction.  Some plans allow a few solid foods, but are still called fasts because they provide so few calories.

Not all fasts are created equal. Some can be perfectly safe, such as medical fasts supervised by a physician. Religious and cultural fasts are typically undertaken as an act of devotion, last from 24-48 hours, and are not intended to promote weight loss.

Fasts lasting a day or two are unlikely to be dangerous for most healthy adults. But high-risk people, the elderly, anyone with a chronic disease, pregnant women, and children are advised against any type of fasting.

The real danger lies in staying on the fast for prolonged periods, anywhere from three days to a month.

Dangers of Fasting for Weight Loss

When you dramatically reduce your calorie intake, you will lose weight. But it can also cause all kinds of health problems, including muscle loss. Further, when you start fasting, your body goes into conservation mode, burning calories more slowly. 

Keep in mind that the initial weight lost on a fast is primarily fluid or "water weight," not fat. And when you go back to eating, any lost weight usually gets a return ticket back. Not only do most people regain weight lost on a fast, they tend to add a few extra pounds because a slower metabolism makes it easier to gain weight. Worse, the weight that is regained is likely to be all fat -- lost muscle has to be added back at the gym.

Side effects of fasting include dizziness, headaches, low blood sugar, muscle aches, weakness, and fatigue.  Prolonged fasting can lead to anemia, a weakened immune system, liver and kidney problems, and irregular heartbeat. Fasting can also result in vitamin and mineral deficiencies, muscle breakdown, and diarrhea. When you drink laxative concoctions during a fast, there is an increased risk of fluid imbalance and dehydration.

The risks get more complicated and severe the longer you stay on a fast, or if you repeatedly go on fasts.

Why You Don’t Need a Detox Diet

It sounds logical that fasting could cleanse your body of harmful substances that could cause a host of ailments like obesity, fatigue, and headaches. But there is no scientific evidence that you need to fast to "cleanse" your body or remove toxins.

Your body - specifically, the kidneys, liver, lungs, colon, and skin -- is perfectly capable of removing toxins itself.

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