What a Fasting Diet Can and Can’t Do for Your Health
More research needs to be done, but some studies suggest abstaining or limiting food consumption for a stretch of time can have a variety of health benefits.
Every day, organs such as the liver, the kidney, and the spleen work to remove and neutralize toxins from the body to keep our cells healthy, says David Katz, MD, MPH, director of the Yale-Griffin Prevention Research Center at Yale University in New Haven, Connecticut. “When you fast, you eliminate input of additional toxins from food,” Dr. Katz says — “and there is a potential biological benefit to that.”
“Potential” is the key word. While a growing body of research suggests intermittent fasting may have health benefits, much of the evidence is far from conclusive, and there are still a lot of unknowns about how a fasting diet or intermittent fasting diet might affect our bodies, particularly over the long-term.
How Intermittent Fasting Diets Work
Fasting, or intermittent fasting diets, shift the focus from what you’re eating to when you’re eating. It’s not about restricting calories for days on end, but rather it’s about eating for a prescribed number of hours a day, or a certain number of days per week, and then abstaining or limiting food consumption for another period of time.
For example, the Fast Diet, also known as the 5:2 diet, advocates for eating what you want for five days of the week (without too much thought to calorie intake), and restricting calorie intake to 500 calories for women and 600 for men (about one-quarter of the diet’s “rule of thumb” calorie intake for non-fasting days) for the other two days of the week.
Any time you restrict calories, your body reacts. 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. When you don’t eat:
- The supply of glucose in your blood starts to drop, and your body eventually 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, to spare muscle.
- In ketosis mode, you will lose weight as you burn body fat. Note that ketosis also makes your blood more acidic and can cause bad breath, fatigue, and other unpleasant symptoms. Longer fasts can lead to kidney and liver damage.
While so-called fasting diets have become increasingly popular, fasting itself is nothing new. Ritual fasting has long been part of religious traditions, and being able to survive periods of going without food is thought to be an essential part of human evolution. Thanks to our history as hunter-gatherers, human bodies are equipped to handle periods of not eating, says Benjamin D. Horne, PhD, MPH, director of cardiovascular and genetic epidemiology at the Intermountain Medical Center Heart Institute in Salt Lake City. And because our ancestors who made it through those lean times are the ones who survived, that suggests that our DNA may actually be coded to receive a benefit from fasting, Dr. Horne says.
What Beginners Should Know Before Trying Intermittent Fasting
Horne and other experts suggest anyone who’s considering trying a fasting diet should check with their doctor before doing so. Things like having a history of eating disorders, diabetes, low blood pressure (hypotension), or anemia; being pregnant or nursing; taking some prescription drugs; and other conditions may not marry well with fasting diets, and could be harmful.
When you talk to your doctor, also make sure they’re aware of every drug you take, including over-the-counter meds and dietary supplements. A seemingly benign medication like Tylenol (acetaminophen) can be harmful on an empty stomach, Katz adds.
And finally, read the fine print before you start, explains Joel Fuhrman, MD, a family physician specializing in nutritional lifestyle medicine in Flemington, New Jersey, and a New York Times best-selling author, whose books include Fasting and Eating for Health. You may need to adjust your activity level and workload on days you’re severely restricting calories, he adds, as you may feel fatigued and grumpy, and face a higher risk of fainting. You may want to schedule fasts for weekends and holidays, as opposed to hectic workdays, he notes.
Doctors Still Aren’t Sure Fasting Is the Best Way to Lose Weight
“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 bestseller S.A.S.S. Yourself Slim, as well as and Slim Down Now and other books. Intermittent fasting — repeated cycles of short-term fasting and regular eating — may be more effective if followed correctly. But even short periods of fasting can lead to “rebound” overeating.
To get the health benefits of any diet, Sass suggests making sure the foods you are eating are mostly whole, nutrient-rich foods — and your food choices are balanced. Even if you restrict eating to an eight-hour window, you may not benefit much if you’re eating processed, low nutrient food, she says.
While research suggests there may be many health perks to intermittent fasting, the research is still preliminary and mostly inconclusive in people.
One thing experts do know for sure about health: Eating well every day plays a major role in preventing heart disease, diabetes, and certain types of cancer, Sass says. “Focusing on consistently eating enough nutrient-rich whole foods, like fruits, vegetables, and whole grains, snowballs into proven powerful benefits over time.”
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. “As you create nutrient deficits on fast days, it may be difficult to compensate on the days you do eat,” .
