What Fasting Can Do for You…and Can't Do
In 1993, Charles Barkley received the MVP award in the National Basketball Association. Almost 30 years later, during an NBA broadcast on TNT, the player once known as the "Round Mound of Rebound" revealed that he had started intermittent fasting.
Moments later, the show's secret "Snack Cam" showed him munching in the green room outside of his usual time frame for calorie consumption.
Barkley is far from alone in his decision to try time-restricted eating. Former National Football League linebacker and actor Terry Crews said that for five years, he limited his eating to between 2 p.m. and 10 p.m. every day, fasting the other 16 hours.
Once primarily a religious practice—for example, fasting in the Jewish tradition and the monthlong fast Muslims observe every year during Ramadan—celebrities such as Halle Berry and Hugh Jackman have helped propel fasting into the mainstream. Twitter co-founder Jack Dorsey famously adopted the "OMAD" or one meal a day diet, a version of intermittent fasting previously popularized two decades ago as the "Warrior Diet." Two-day fasting has been called "the Hollywood 48-Hour Miracle Diet."
Mark Mattson, M.D., author of "The Intermittent Fasting Revolution: The Science of Optimizing Health and Enhancing Performance," published in February 2022, said he's noticed an increased interest in fasting among the general public in recent years. He said the trend comes on the heels of several decades of relevant research: first, through animal studies focused mainly on rats and mice, and then human research starting a little more than a decade ago.
Mattson mentioned the work of Michelle Harvie, a research dietitian in the United Kingdom who published research on energy restriction in relation to weight loss, metabolic risk markers and breast cancer. The BBC got word about this emerging research and subsequently released a documentary about the science behind fasting about 10 years ago.
Mattson, who for almost 20 years led the Neuroscience Research Laboratory at the National Institute on Aging at Baltimore, said a "snowballing effect" ensued. Mainstream medicine has since continued to engage with fasting as a form of dietary intervention.
Fasting and health
In a 2017 paper, Mattson, Harvie and Valter Longo, a biologist at the University of Southern California, reviewed existing literature and reported a number of documented health indicator improvements—such as reduced insulin, reduced body fat, lowered resting heart rate and lowered blood pressure—in lab rats and mice on intermittent fasting diets.
"We know from animals for sure that intermittent fasting can suppress tumor growth," Mattson said. "There's a lot of reasons to believe that might be the case in humans. There are also data in animals showing that fasting can enhance the ability of chemotherapeutic drugs and radiation to kill cancer cells."
The 2017 review also cites human studies indicating that following intermittent fasting protocols for two to four months or more can help in the prevention of diabetes and cardiovascular disease, and the authors referenced small trials involving people with cancer and multiple sclerosis that have produced promising results.
While complete fasting involves not eating or cessation of energy intake, intermittent fasting entails regular, short fasting periods of more than 12 hours, Mattson noted. For instance, a time-restricted eating pattern characteristic of intermittent fasting could mean eating only during a six- or eight-hour window and withholding sustenance for 16 or 18 hours.
Given that different types of fasting and restricted eating might induce different effects, Peter Attia, M.D., has argued for greater precision in defining and using terms associated with abstention from energy consumption. Attia is a physician focused on the applied science of longevity and has a medical practice with offices in California and New York City.
Intermittent fasting involves intervals of limiting food intake for extended periods of time, explained Zhaoping Li, M.D., Ph.D., professor of medicine and chief of the division of clinical nutrition at UCLA. In a paper published in October 2021 in JAMA, Li and co-author David Heber, M.D., recognized that after eight to 12 hours of not eating, a person's liver produces ketones, and those ketone bodies—made from the breakdown of fatty acids—serve as an alternative fuel source. They further noted studies have indicated reductions in blood inflammatory markers when subjects use intermittent fasting and said the practice has also been shown to improve cardiovascular risk factors within two to four weeks of implementation.
On the other hand, they wrote that studies lasting from three months to a year have not shown differences in weight loss when intermittent fasting is compared to daily calorie-restricted eating—though weight loss can occur with either approach.
Li said fasting is a strong behavior tool to control calorie intake, suggesting humans appear to have evolved some ability to tolerate and perhaps respond favorably to a lack of food. As Mattson confirmed, sample studies indicate overweight people who change from eating three meals per day to daily time-restricted eating or a single meal a couple of days a week (eating regularly the rest of the week) all lose weight. He qualified, however, if overweight individuals do not reduce the overall amount of calories consumed over the course of weeks or months, weight loss won't happen.
Fasting and longevity
While doctors believe fasting can aid weight loss, the question remains whether fasting can benefit a person of normal weight who's in decent health. Can intermittent fasting, say, extend an individual's health span and/or life span?
Researchers have looked at the role of fasting in producing autophagy, a process by which the body eliminates damaged cells—including those that could become cancerous—suggesting a key mechanism potentially useful for life extension.
In 2014, Mattson and Longo co-wrote a review in the journal Cell Metabolism to explore intermittent fasting's possible clinical applications. The review referenced research indicating that in rodents, the results of alternate-day fasting could range from a negative effect to a 30 percent increase in life span, depending on the type and age of the animal. Additionally, the review referenced a study that found twice-weekly 24-hour fasts significantly increased the life span of black-hooded rats. Dietary energy restrictions have also been shown to protect the brain against aging.
The 2014 review also noted the need for a much better understanding of the type of fasting that can maximize its longevity effects and the mechanisms responsible for the detrimental effects that may be counterbalancing antiaging effects.
The lack of clinical studies in humans renders it unclear as to whether fasting produces longevity.
"We don't know yet whether intermittent fasting can extend the life span of people with a normal weight," Mattson explained.
He added certain research that administered food in intermittent-fasting fashion also reported delayed onset of cancer in lab animals. Studies in humans have indicated a link between fasting and decreases in both free radical damage to molecules and systemic inflammation—both biomarkers associated with fatal chronic disease. Since fasting can mimic the positive effects of exercise, it conceivably could extend health span and life span just as regular routines of sustained activity are intended to do.
According to Li, however, the lack of clinical studies in humans renders it unclear as to whether fasting produces longevity.
She emphasized the much-needed strides medical science has made recently in the direction of personalized care. Such care attends to individual differences in physiology and metabolism. To the tune of $170 million, the National Institutes of Health funded the Nutrition for Precision Health study, which will place 10,000 participants in a research program designed to inform personalized nutrition recommendations.
"We are different," Li said. "There are people who do well [eating meat and potatoes] and there are people who do well with low fat, and some may do better with intermittent fasting."
In their 2021 JAMA paper, Li and Heber referenced undesired side effects, such as dehydration, headaches and difficulty concentrating, that can result from fasting.
"It is not always safe for everyone," Li said.
Older adults who succeed in losing weight through prolonged fasting could lose skeletal muscle, for example. The concern is greatest for older individuals with a diminished ability to resynthesize proteins. For them, a protracted period of not eating and not consuming amino acids—the building blocks of protein—can lead to loss of muscle mass. Muscle wasting in older people tends to come with accelerated loss of bodily function, which can further limit physical activity and then decrease bone density over the long run. This can lead to conditions such as osteoporosis that are associated with poor health outcomes, especially later in life.
Whether exercise can reliably attenuate muscle mass loss from fasting in humans, as animal research has suggested, remains an open question and almost certainly depends on the fasting approach and the individual's health profile.
Li also explained that weight loss from fasting can entail a decrease in both fat and muscle, but subsequent weight gain when no exercise is involved tends to be largely fat, which can also exacerbate health problems.
"People with medical conditions who want to fast are another concern," she added.
Patients on hypertension pills who fast without medical supervision and do not adjust the dose of their medication could face potentially life-threatening drops in blood pressure. Similar concerns surround people on medication for diabetes. If the medication isn't monitored and adjusted as needed in response to an intervention such as fasting, health can suffer.
Li said anyone who has a history of eating disorders should approach fasting with extreme caution, if at all.
"This is not the only way you get yourself healthier," Li said, emphasizing that eating whole foods, vegetables in particular, and exercising regularly remain scientifically proven ways for people to attain or maintain good health.