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.iStock.com
Google “fasting for health” and you’ll get more than 6.3 million hits, ranging from doctors who recommend it for treating a range of diseases, to spas that promise detoxifying food-free vacations, to bloggers who say that fasting makes them feel clearer mentally and more fit — and, increasingly, to fitness professionals touting diets that incorporate fasting as a method for weight loss. But does medical research actually support those claims?
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
Before we elaborate on the possible benefits of fasting, let’s review.
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.
What Are the Touted Health Benefits of Intermittent Fasting?
Now for all the health-boosting claims that get touted with intermittent fasting, here’s what the evidence says:
Fasting May Help Your Heart
Fasting for a day once a month may prevent clinical diagnoses of heart disease and diabetes, according to two studies from Horne’s team of researchers at Intermountain Medical Center. One study published in June 2012 in The American Journal of Cardiology looked at the habits of 200 men and women, and found that those who fasted once a month were 58 percent less likely to have heart disease than those who didn’t — after controlling for factors such as age, smoking status, and high blood pressure.
It’s important to note that there’s also conflicting research. A study published in July 2017 in the journal JAMA Internal Medicine compared alternate-day fasting with a caloric restriction regimen for weight loss and cardiovascular disease indicators. Researchers found that alternate-day fasting reduced weight about the same amount as daily calorie restriction, but that it was harder to adhere to. They also found that LDL (“bad”) cholesterol increased in the fasting group.
Horne’s lab is currently working on studies that will evaluate how frequency and length of fasting affects health outcomes, and more specifically what mechanisms explain why intermittent fasting might be good for the heart — including looking at how it affects cholesterol levels.
Fasting May Reduce Diabetes
In a smaller study published in November 2013 in the journal Nutrition, Metabolism & Cardiovascular Diseases that was authored by Horne’s team, 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 increases in human growth hormone (HGH) — 4.7-fold in women and 13.6-fold in men, among other changes. HGH protects lean muscle mass and encourages the body to burn excess fat instead.
“During fasting, the fat stored within your fat cells is being metabolized and used as fuel,” says Horne. Over time that means you have less fat in your body, which could lead to less insulin resistance and a lower risk of heart disease later in life, he says.
Another potential benefit of fasting, Horne adds, is that normal cells in the body go into a self-protection mode during fasting: They actually become resistant to outside sources of injury, and internally they optimize functioning in an effort to try to survive until the fasting period ends.
That theory is based in part on animal studies, including some conducted by Valter Longo, PhD, director of the Longevity Institute at the USC Leonard Davis School of Gerontology in Los Angeles. In a study published in February 2017 in the journal Cell, Dr. Longo’s team of researchers found that a fasting-like diet promotes the growth of new insulin-producing pancreatic cells that reduce symptoms of type 1 diabetes and type 2 diabetes in mice.
There’s a Chance Fasting Can Fight Cancer
In a rodent study published in July 2016 in the journal Cancer Cell, Longo’s team found that a fasting-mimicking diet, combined with chemotherapy, helped the immune system recognize and attack cancer cells. Mice on the diet had smaller tumors than mice receiving only chemotherapy.
Periods of fasting have also been shown to slow the rate of cell division (a measure of cancer risk) in mice, according to research published in the Journal of Applied Physiology. But researchers aren’t sure why, exactly; it may result from a decrease in certain growth factors, specifically insulin-like growth factor-1 (IGF-1), associated with cancer.
“IGF-1 is highly correlated with prostate cancer and colon cancer, so it makes sense that if caloric restriction reduces IGF-1 (which it does) that that could be a player in reducing cancer. But again, until we show that that’s the case — that it does reduce cancer in humans — it’s speculation,” notes study author Marc Hellerstein, MD, PhD, professor of nutritional science and toxicology at University of California in Berkeley. This research is preliminary and is still limited to animal models, so more data on humans is necessary before fasting solely for cancer prevention is recommended.
Fasting Might Protect the Brain
Researchers have also looked into connections between caloric restriction and brain function. Mark Mattson, PhD, chief of the Laboratory of Neurosciences at the National Institute on Aging in Baltimore, co-authored a review published in February 2014 in the journal Cell Metabolism with Longo, which concluded there was convincing evidence in animal studies that intermittent-fasting diets were linked to less neuronal dysfunction and degeneration, as well as fewer clinical symptoms of Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease in the rodents in those studies.
Again, the same conclusions have not yet been proven in people, but Dr. Mattson and Longo conclude it’s an important opportunity for more research.
One explanation is that during periods of fasting and exercise certain brain activity actually increases, as does blood flow in certain areas, helping protect the brain from all of these other problems. That theory is supported by multiple brain imaging studies in people, Mattson explains in another review article published in December 2012 in the journal Cell Metabolism.
Fasting Diets Haven’t Been Linked to Health Problems, if Followed Safely — At Least Not Yet
While the research suggests fasting can help a number of diseases — from multiple sclerosis to allergies — 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. “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,” Katz says.
A review article published in March 2017 in the journal Behavioral Sciences concluded that there were no clear harms linked to intermittent fasting diets, according to studies on the topic. Still the authors of that review also note that long-term data on the safety of intermittent fasting hasn’t been collected yet, so how such diets affect people over the long-term is yet to be determined.
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.
Doctors Still Aren’t Sure if the Benefits of Fasting Are the Same as a Healthy Diet
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,” Katz says.