Ritual fasting has been part of religious traditions for thousands of years, from Muslims who fast during daylight hours in the month of Ramadan to Mormons who take a regular break from food the first Sunday of each month. But a recent growing body of research shows that abstaining from food intermittently may have physical as well as spiritual benefits — the latest, a study from Utah researchers that found that occasional fasts (defined as extended periods of time in which people generally abstain from all food and drink except for water) may reduce the risk of heart disease and diabetes.
Google “fasting for health” and you’ll get more than 7 million hits, ranging from doctors who recommend it in their practices to treat a range of diseases, spas that promise detoxifying food-free vacations, and message board postings from devotees who say that fasting makes them feel clearer mentally and more fit. “I fast whenever my body feels like it needs a reboot,” says Yoli Ouiya, 31, a New York City blogger who writes about eco-friendly living. She fasts once every few months.
But is fasting a good idea for your health? Possibly, says David Katz, MD, MPH, director of the Prevention Research Center at Yale University. Every day, organs such as the liver, kidney, and spleen work to remove and neutralize toxins from the body to keep our cells healthy. “When you fast, you eliminate input of additional toxins from food,” says Dr. Katz, “and there is a potential biological benefit to that.”
Leading researchers and experts share the details you need to know before you forgo food:
Your Body on a Fast
Thanks to our history as hunter-gatherers, human bodies are equipped to handle periods of not eating, says Benjamin D. Horne, PhD, MD, author of the Utah study and director of cardiovascular and genetic epidemiology at Intermountain Medical Center in Salt Lake City. And since the ancestors who made it through those lean times are the ones who survived, Horne suggests that our DNA may actually be coded to receive a benefit from fasting.
Here’s how your body reacts when you stop feeding it:

When you eat, your digestive system breaks down carbohydrates into the sugar glucose, the body’s major source of energy. Glucose is absorbed from the digestive tract into the blood, which then travels to your body’s cells to provide them with fuel.

If you haven’t eaten recently, the supply of glucose in your blood drops and your body turns to stored glucose, called glycogen, for energy.

Once the glycogen is used up, your body begins to burn fat and muscle stores to make its own glucose to fuel your cells.

After a few days without eating (which experts don’t recommend) your body kicks into ketosis mode, meaning you burn fat as the primary source of fuel, in order to spare muscle. You will lose weight in the form of body fat. However, ketosis also makes your blood will also become more acidic, and can cause bad breath, fatigue, and other unpleasant symptoms; long-term, it can lead to kidney and liver damage.

What Fasting Can and Can't Do for Your Health

Fasting may help your heart.

Fasting for a day once a month may prevent heart disease and insulin resistance, the precursor to diabetes, according to two studies from Utah’s Intermountain Medical Center presented recently at the American College of Cardiology’s scientific sessions. When researchers looked at the habits of 200 men and women, they found that those who fasted once a month were 58 percent less likely to have heart disease than those who didn’t (after they controlled for factors such as age, smoking status, and high blood pressure). They then set out to understand why.
In a smaller study, the scientists measured various blood levels in 30 healthy adults after one day when they fasted and one day when they ate normally. After they fasted, participants had huge increases in human growth hormone (HGH) — 13-fold in women and 20-fold in men, among other changes. HGH protects lean muscle mass and encourages the body to burn fat stores instead. “During fasting, your fat cells are being metabolized and used as fuel,” says Horne. “If fat is being used for fuel, in the long run you have fewer fat cells in your body.” This may mean less insulin resistance and a lower risk of heart disease later in life.

There’s a chance fasting can cut cancer risk.

Periods of fasting did slow the rate of cell division (a measure of cancer risk) in mice, according to an American Journal of Physiology study. The researchers aren’t sure why, but say it may result from a decrease in growth factors that results from nutrient deprivation. But since the science is preliminary, you shouldn’t fast solely for cancer-prevention purposes until there is more definitive research on humans, says study author Marc Hellerstein, PhD, MD, professor of human nutrition at University of California, Berkeley

The jury's still out on fasting for other ailments.

While there isn't much research on many of the diseases proponents claim fasting helps treat - multiple sclerosis, rheumatoid arthritis, and allergies among them — doctors say that if you’re medically able to fast safely (which rules out those with more serious health issues), there's no research against doing it either.
In fact, one small Iranian study of 40 adults with multiple sclerosis found that there were no negative effects from fasting during the month of Ramadan compared with a group who didn’t fast. "If you’re not on prescription medicine, generally in good health, and want to fast periodically because you feel you get a health benefit from it, we don't have evidence that this would be harmful," says Katz.

Fasting won’t help you lose — and keep off — weight.

“Fasting for weight loss is just another form of yo-yo dieting,” says Joel Fuhrman, MD, board certified family physician specializing in nutritional lifestyle medicine and author of Fasting and Eating for Health. While you may see a small drop in the scale, don’t expect the weight loss to last.
“The pounds that come off on a short-term fast are mainly water and stored carbohydrates, which will come back as soon as you start eating again,” says Cynthia Sass, MPH, RD, author of the bestselling book Cinch! Conquer Cravings, Drop Pounds, and Lose Inches.
And if you’re tempted to fast one day as a green light to eat whatever you want the next, think again. “Weight loss is about energy balance — if you have consume fewer calories than you burn, you will lose weight,” Katz. “On the days you fast you have a calorie deficit. But what really comes into play is what you do on the other days.” In other words, you can negate the potential health effects of a fast by binging afterward.
Bottom line: True weight-loss success involves healthy eating (along with exercise habits) that you are committed to and can maintain over time.

Fasting can’t take the place of a healthy diet.

While there may be health perks to intermittent fasting, the research is still preliminary. Horne’s lab is currently working on studies that will evaluate how often and for how long people need to fast to see health benefits. One thing we know for sure about health: Eating well every day plays a major role in preventing heart disease, diabetes, and certain types of cancer. “Focusing on consistently eating enough nutrient-rich whole foods like fruits, vegetables, and whole grains snowballs into proven powerful benefits over time,” says Sass.
Another important thing to keep in mind: Just as fasting gives your body a break from toxins, it also saps your body of vital nutrients like vitamin D, calcium, and omega-3 fatty acids. “With fasting, you risk throwing out the baby with the bathwater,” says Katz. “As you create nutrient deficits on fast days, it may be difficult to compensate on the days you do eat.”
So if you choose to fast, you have to pack your diet with nutritious foods. “Fasting is not a way of fixing an otherwise broken diet,” says Katz. “It should be used only as a way of helping you establish a healthy way of eating, rebooting your body to focus on what’s important.”

Talk to your doctor before you start a fasting regimen

While fasting may have the potential to play a role in wellness, it’s definitely not for everyone. Specifically, if you have a history of eating disorders, diabetes, low blood pressure, anemia, are pregnant or nursing, or are on prescription medication, as well as other conditions/circumstances, fasting is not safe for you.
“If someone wants to use fasting as part of a medical intervention, they should discuss it with their doctor first,” says Horne. While there’s no evidence that fasting one to two nonconsecutive days per week is harmful for a healthy person, it could be dangerous for others, says Katz. See your doctor for recommendations that suit your needs.
When you talk to your doctor, make sure he’s aware of every drug you take, including over-the-counter meds and dietary supplements. A seemingly benign medication like acetaminophen can be harmful on an empty stomach.
Even if your doctor gives the go-ahead, don’t jump head first into fasting — it’s crucial that you understand how to do so safely, and in the context of a healthy lifestyle.
For instance, treat a fast day as a time of physical and emotional rest, not simply a day of not eating, says Fuhrman. You may feel fatigued and grumpy and face a higher risk of fainting, making fasting ideal for weekends and holidays as opposed to hectic workdays.
Talking to your doctor and educating yourself on fasting before you attempt it can mean the difference between a potentially health-boosting habit and a trip to the emergency room.
By Rachel Meltzer Warren, MS, RD