Google “fasting for health,” and you’ll find more than 6.3 million results, ranging from doctors who recommend it for a variety of ailments to spas that promise detoxifying food-free vacations to bloggers who claim that fasting helps them feel more mentally clear and physically fit — and, increasingly, fitness professionals touting diets that incorporate fasting as a method of weight loss. However, does medical research substantiate these assertions?
Organs like the liver, kidney, and spleen work every day to remove and neutralize toxins from the body to keep our cells healthy. You can remove additional toxins from your body by fasting. The operative word is “potential.” While an increasing body of research indicates that intermittent fasting may have health benefits, much of the evidence is inconclusive. There are still many unknowns about how a fasting or intermittent fasting diet may affect our bodies, particularly in the long term.
The Intermittent Fasting Diet’s Mechanism of Action
Fasting, or intermittent fasting, shifts the emphasis away from what you eat and toward when you eat. This does not include calorie restrictions for days to come, but a certain number of hours to eat per day or a certain number of days a week, and then refrains from or restricts food intake for another timespan. For example, the Fast Diet, also known as the 5:2 diet, advocates for eating whatever you want five days a week (without regard for calorie intake) and restricting calorie intake to 500 calories for women and 600 calories for men on the other two days of the week (approximately one-quarter of the diet’s “rule of thumb” calorie intake on non-fasting days).
When you restrict your calorie intake, your body reacts. When you consume carbohydrates, your digestive system converts them to glucose, the body’s primary energy source. Glucose is absorbed from the digestive tract into the bloodstream and then transported to the body’s cells to provide fuel. When you do not consume food:
As the supply of glucose in your blood decreases, your body eventually uses stored glucose, called glycogen, as an energy source. Once glycogen is depleted, your body begins burning fat and muscle to generate glucose for your cells.
After a few days without food (which experts do not recommend), your body enters ketosis, which means it uses fat as its primary fuel source to conserve muscle. While in ketosis, you will lose weight as your body fat is burned. Keep in mind that ketosis also increases blood acidity, resulting in bad breath, fatigue, and other unpleasant symptoms. Prolonged fasts can be harmful to the kidneys and liver.
While so-called fasting diets have grown in popularity, fasting is not new. Fasting has long been a part of religious traditions. The ability to go without food for extended periods is believed to be a necessary component of human evolution. Human bodies are equipped to handle periods of fasting as a result of our hunter-gatherer ancestors, according to Benjamin D. Horne, Ph.D., MPH, director of cardiovascular and genetic epidemiology at the Intermountain Medical Center Heart Institute in Salt Lake City. And because our forefathers and mothers survived those lean times, this suggests that our DNA may be programmed to benefit from fasting, Dr. Horne says.
What Intermittent Fasting Beginners Should Know Before Trying It
Before we delve into the potential benefits of fasting, it’s worth reviewing.
Horne and other experts advise anyone considering a fasting diet to consult their physician first. Conditions such as a history of eating disorders, diabetes, low blood pressure (hypotension), or anemia; pregnancy or nursing; taking certain prescription medications; and other conditions may not work well with fasting diets and may be harmful.
When speaking with your doctor, ensure that they know all medications you are taking, including over-the-counter medicines and dietary supplements. Katz adds that even seemingly benign medication such as Tylenol (acetaminophen) can be harmful on an empty stomach.
Finally, read the fine print before you begin, explains Joel Fuhrman, MD, a family physician in Flemington, New Jersey, specializing in nutritional lifestyle medicine and is a New York Times bestselling author of books Fasting and Eating for Health. On days when you’re severely restricting calories, you may need to adjust your activity level and workload, he adds, as you may feel tired and grumpy and run a higher risk of fainting. You may wish to schedule fasts for weekends and holidays rather than crammed workdays, he observes.
What Are Health Benefits Asserted for Intermittent Fasting?
Now, for all the health benefits ascribed to intermittent fasting, here is what the evidence indicates:
Fasting May Be Beneficial to Your Heart
According to two studies conducted by Horne’s team of researchers at Intermountain Medical Center, fasting for one day once a month may help prevent clinical diagnoses of heart disease and diabetes. After adjusting for factors such as age, smoking status, and high blood pressure, one study published in June 2012 in The American Journal of Cardiology examined the habits of 200 men and women and discovered that those who fasted once a month were 58 percent less likely to have heart disease than those who did not.
It’s critical to keep in mind that there is also conflicting research. In a July 2017 article published in the journal JAMA Internal Medicine, researchers compared alternate-day fasting to a caloric restriction regimen in weight loss and cardiovascular disease indicators. (2) Researchers discovered that alternate-day fasting had a similar weight-loss effect to daily calorie restriction but was more challenging to maintain. Additionally, they found an increase in LDL (“bad”) cholesterol in the fasting group.
Horne’s lab is currently researching how fasting frequency and duration affect health outcomes. More precisely, what mechanisms explain why intermittent fasting may be beneficial to the heart — including how it affects cholesterol levels.
Fasting May Help Prevent Diabetes
Horne’s team published a smaller study in November 2013 in the journal Nutrition, Metabolism, and Cardiovascular Diseases. They measured various blood levels in 30 healthy adults after one day of fasting and normal eating. Following the fast, participants’ human growth hormone (HGH) levels increased 4.7-fold in women and 13.6-fold in men, among other changes. HGH maintains lean muscle mass while encouraging the body to burn excess fat.
“While you are fasting, the fat stored in your fat cells is metabolized and used as fuel,” Horne explains. This results in a decrease in body fat over time, resulting in decreased insulin resistance and a decreased risk of heart disease later in life, he says.
There Is a Possibility That Fasting Can Help Fight Cancer
Longo’s team discovered that a fasting-like diet combined with chemotherapy improved the immune system’s recognition and attack of cancer cells in a rodent study published in July 2016 in the journal Cancer Cell. Mice fed a diet had significantly smaller tumors than mice treated only with chemotherapy.
According to research published in the Journal of Applied Physiology, fasting periods have also been shown to slow cell division in mice (a marker of cancer risk). However, researchers are not sure why; it could be due to a decrease in specific growth factors, specifically insulin-like growth factor-1 (IGF-1), linked to cancer.
“Because IGF-1 is strongly correlated with prostate and colon cancer, it makes sense that if caloric restriction decreases IGF-1 (which it does), this could be a factor in cancer prevention. However, until we demonstrate that this is the case — that it does indeed reduce cancer in humans — all of this is speculation. This research is preliminary and limited to animal models; therefore, additional data on humans are required before fasting exclusively for cancer prevention is recommended.
Researchers have also examined the relationship between caloric restriction and brain function. Mark Mattson and Longo co-author of a review published in February 2014 in The Cell Metabolism Journal concluded that animal studies have demonstrated convincing evidence that intermittent fasting diets are related to minor neuronal dysfunctions and degeneration as well as lesser clinical symptoms of aging. Again, the same conclusions have not been validated in humans, but Drs. Mattson and Longo conclude that this is an essential area for additional research. One explanation is that particular brain activity and blood flow in specific regions increase during fasting and exercise periods, thereby helping to protect the brain from all of these other problems. As Mattson explains in another review article published in December 2012 in the journal Cell Metabolism, multiple human brain imaging studies corroborate this theory.