What Can and Cannot a Fasting Diet Do for Your Health?

Google “fasting for health” and you’ll get over 6.3 million results, from doctors recommending it for a variety of ailments to spas offering detoxifying food-free vacations to bloggers who claim it makes them more fit and mentally clear. But is there evidence to back up those claims?

In order to keep cells healthy, organs like the liver, kidney, and spleen remove and neutralize toxins from the body. “When you fast, you eliminate additional toxins from food,” Dr. Katz explains.

The word is “potential”. There is a growing body of research that suggests intermittent fasting may be beneficial for our health, but much of the evidence is still inconclusive.

Is Fasting Effective?

Intermittent fasting shifts the focus from what you eat to when you eat. It involves eating for a certain number of hours per day, or days per week, and then abstaining or limiting food consumption for another period of time.

In the 5:2 diet, for example, you eat whatever you want for five days of the week, and then for two days of the week you eat only 500 calories for women and 600 calories for men (roughly one-quarter of the diet’s “rule of thumb” calorie intake for non-fasting days).

Your body reacts to calorie restriction. When you eat, your digestive system turns carbohydrates into glucose, the body’s main energy source. Glucose is absorbed from the gut into the bloodstream, where it fuels your cells. You don’t eat:

• As your blood glucose level drops, your body begins to use stored glucose (glycogen) for energy.
• Once the glycogen is gone, your body starts burning fat and muscles to make its own glucose.
• After a few days without food (which experts advise against), your body enters ketosis mode, burning fat for fuel instead of muscle.
• In ketosis, you lose weight by burning fat. Ketosis increases blood acidity, causing bad breath, fatigue, and other unpleasant symptoms. Longer fasts can harm the kidneys and liver.

While “fasting diets” have grown in popularity, fasting itself is nothing new. Fasting has long been a part of religious rituals, and the ability to go without food is thought to be essential to human evolution. People’s bodies are designed to handle periods of not eating, says Benjamin D. Horne, PhD, MPH, director of cardiovascular and genetic epidemiology at Salt Lake City’s Intermountain Medical Center Heart Institute. Dr. Horne believes that because our ancestors survived those hard times, our DNA may be coded to benefit from fasting.

What Newbies Should Know Before Trying IIF

Let’s review before diving into the benefits of fasting.

Horne and other experts advise anyone considering a fasting diet to consult their doctor first. Being pregnant or nursing; taking certain prescription drugs; and other conditions may not be compatible with fasting diets.

When you visit your doctor, make sure they know about all your medications, including OTC and dietary supplements. Like Tylenol (acetaminophen) on an empty stomach, Katz says.

Finally, read the fine print before you start, advises Joel Fuhrman, MD, a Flemington, New Jersey, family physician who specializes in nutritional lifestyle medicine and the author of Fasting and Eating for Health. On days when you severely restrict calories, you may feel fatigued, grumpy, and at risk of fainting, he adds. Fasts should be planned for weekends and holidays, he advises.

What Are Intermittent Fasting’s Health Benefits?

Regarding the health benefits of intermittent fasting, the evidence shows:

Fasting and the Heart

Two studies by Horne’s team at Intermountain Medical Center suggest that monthly fasting may help prevent heart disease and diabetes. After adjusting for age, smoking status, and high blood pressure, one study published in June 2012 in The American Journal of Cardiology found that those who fasted once a month were 58% less likely to develop heart disease. 

Notably, there is conflicting research. A 2017 JAMA Internal Medicine study compared alternate-day fasting to calorie restriction for weight loss and cardiovascular disease indicators. The same amount of weight loss was found with alternate-day fasting, but it was more difficult to maintain. The fasting group’s LDL (“bad”) cholesterol increased.

Horne’s lab is currently investigating how intermittent fasting affects health outcomes, specifically how it affects cholesterol levels.

Diabetes and Fasting

An earlier study by Horne’s team measured various blood levels in 30 healthy adults after one day of fasting and one day of normal eating. After fasting, participants’ HGH levels increased 4.7-fold in women and 13.6-fold in men. HGH protects lean muscle mass and promotes fat burning.

Fat stored in your fat cells is metabolized and used as fuel while fasting, says Horne. Eventually, less fat means less insulin resistance and less risk of heart disease, he says.

Horne adds that normal cells in the body go into self-protection mode during fasting, becoming resistant to external sources of injury and internally optimizing functioning to try to survive until the fasting period ends.

Some of Valter Longo’s research at the USC Leonard Davis School of Gerontology in Los Angeles supports this theory. Dr. Longo’s team discovered in a 2017 Cell study that a fasting-like diet promotes the growth of new insulin-producing pancreatic cells that reduce symptoms of type 1 and type 2 diabetes in mice.

Fasting May Help Fight Cancer

Oncologists found that a fasting-like diet combined with chemotherapy helped the immune system recognize and attack cancer cells in a 2016 rodent study published in Cancer Cell. Diet-fed mice had smaller tumors than chemotherapy-only mice.

Fasting also slows cell division in mice, according to research published in the Journal of Applied Physiology. Researchers aren’t sure why, but it could be due to a drop in growth factors, specifically insulin-like growth factor-1 (IGF-1).

Because IGF-1 is linked to prostate and colon cancer, it makes sense that caloric restriction would reduce IGF-1, which it does. But until we show that it works in humans, it’s just speculation, “says study author Marc Hellerstein, MD, PhD, professor of nutritional science and toxicology at UC Berkeley. This research is preliminary and limited to animal models, so more human data is required before recommending fasting for cancer prevention.

Caloric restriction and brain function have also been studied. An animal study linked intermittent-fasting diets to less neuronal dysfunction and degeneration, as well as fewer clinical symptoms of Alzheimer’s, concluded Mark Mattson, PhD, chief of the Laboratory of Neurosciences at the National Institute on Aging in Baltimore, with Longo.

Dr. Mattson and Dr. Longo agree that more research is needed before concluding the same in humans.

A possible explanation is that certain brain activities and blood flow increase during periods of fasting and exercise, thereby protecting the brain from other issues. In another review article published in December 2012 in the journal Cell Metabolism, Mattson explains how human brain imaging studies back up his theory. If followed properly, fasting diets haven’t been linked to health issues yet.

Doctors say that if you’re medically able to fast safely (which excludes those with more serious health issues), there’s no evidence that it’s harmful to you. There is no evidence that intermittent fasting is harmful for people who are not on prescription medications and are otherwise in good health, according to Dr. Katz.

Studies on the topic concluded that intermittent fasting diets had no clear negative effects, according to a March 2017 review article in Behavioral Sciences. Intermittent fasting is safe, according to the reviewers, but its long-term effects on people are unknown.

Still Unsure If Fasting Is the Best Way to Lose Weight?

Weight loss from a short-term fast is mostly water and stored carbohydrates, which return once you resume eating, says Cynthia Sass, MPH, RD. Following intermittent fasting correctly may be more effective. In fact, even brief fasts can trigger “rebound” overeating.

Whatever diet you choose, make sure it’s a balanced one with lots of whole, nutrient-dense foods. Foods low in nutrients and processed may not benefit from a strict eight-hour eating window.

Fasting Diet May Have the Same Benefits as a Healthy Diet, Doctors Say.

Intermittent fasting appears to have many health benefits, but research on humans is still preliminary.

Experts agree that eating well every day helps prevent heart disease, diabetes, and some cancers, according to Sass and other experts. “Eating nutrient-dense whole foods like fruits, vegetables, and whole grains consistently has proven to have powerful benefits.”

While fasting removes toxins from the body, it depletes vital nutrients like vitamin D, calcium, and omega-3 fatty acids. As a result of the nutrient deficits created by fasting, compensating on eating days may be difficult.