“Research findings suggest that there are many changes that occur in the body and brain in response to IF that can improve health and protect against chronic diseases,” says study coauthor Mark Mattson, PhD, a neuroscientist at Johns Hopkins University School of Medicine in Baltimore. “[Benefits] include improved glucose regulation, reduced blood pressure and resting heart rate, reduced inflammation, and improved muscle health,” Dr. Mattson says. In the article, researchers examined results from studies in animals and humans that focused on how IF may impact health markers such as metabolism and how IF may slow or reverse aging and disease. RELATED: 6 Types of Intermittent Fasting: Which Is Right for You?
What Happens to the Body During Intermittent Fasting
“IF is an eating pattern [that] includes extended time periods during which no or very little food and caloric beverages are consumed,” Mattson says. There are many types of IF, but Mattson sees these practiced most frequently:
Daily Time-Restricted Reeding This involves eating only during a narrow window of time each day, usually over six to eight hours.5:2 Intermittent Fasting People eat normally five days a week, then eat only a single moderate-sized meal on the other two days.
Several animal studies and some preliminary research in humans have shown that alternating between times of fasting and eating may support cellular health, probably by triggering an age-old adaptation to periods of food scarcity called metabolic switching, according to studies cited in the new article. This switch happens when cells use up their stores of readily accessible glucose, or sugars, and begin converting fat into energy in a slower metabolic process. It normally takes about 16 hours without food for metabolic switching to occur, Mattson says. During fasting, the liver converts fatty acids into ketones, a type of fuel released into the bloodstream when stores of glucose run out. Ketones also regulate the activity of many proteins and molecules that are known to influence health and aging. As part of this process, IF triggers a stress response that increases the body’s antioxidant defenses against free radicals that damage cells and tissues, boosts DNA repair, removes damaged proteins, and curbs inflammation, animal studies have found. These processes can slow aging and are suppressed in people who overeat, researchers note in the article. “Time-restricted feeding (a form of intermittent fasting) has been shown to reprogram age-related pathways in our genes and may have an ‘out with the old, in with the new’ effect on our cells and cell machinery,” says Felicia Steger, PhD, of the University of Alabama at Birmingham. Dr. Steger was not involved with the current article. “Fasting gives our cells and tissues a break from processing and storing calories, and allows time to repair and regenerate,” Steger says. RELATED: Intermittent Fasting Puts Type 2 Diabetes in Remission in 3 Men
Animal and Human Studies Offer More Clues About Intermittent Fasting’s Benefits
Decades of animal studies have linked calorie restriction and IF directly to an increased life span in mice and rats, but results in animals have been mixed, and there aren’t long-term studies in humans. Both animals and humans have experienced improved physical function with IF that might also contribute to longevity, according to the article. For example, mice with similar weight in one referenced study published in December 2014 in Cell Metabolism had more running endurance with IF than with unlimited access to food. And in another study published in October 2016 in the Journal of Translational Medicine, young men who fasted 16 hours a day lost fat while maintaining muscle mass during two months of resistance training. If may also help slow aging processes in the brain, some research in animals and people suggests. Studies in animals show that IF improves cognition, with gains in things like spatial memory, which is needed for navigation and recalling where things are located; associative memory, which is needed to understand the relationships between unrelated things; and working memory, which is needed for reasoning. IF also appears to reverse the negative effects of obesity, diabetes, and inflammation on spatial learning and memory in animals. In a clinical trial published in the Proceedings of the National Academy of Sciences, older adults improved verbal memory with IF. In another study of older adults with mild cognitive impairment, published in March 2016 in The Journal of Clinical Endocrinology & Metabolism, IF was associated with improvements in verbal memory, executive function, and global cognition after 12 months. And a separate clinical trial published online in April 2019 in CNS Spectrums found that two years of daily calorie restriction led to better working memory. In addition to the aforementioned benefits, the article suggests IF may modify risk factors associated with obesity and diabetes, says Mattson. Six short-term studies of overweight and obese adults have found that IF works as well as standard diets for weight loss specifically. Two studies found that IF is associated with a reversal of insulin resistance, the body’s failure to respond normally to the hormone insulin, which is a hallmark of type 2 diabetes. RELATED: 12 Possible Intermittent Fasting Benefits
Diet Quality Still Matters When You’re Not Fasting
While many human studies in the article have had encouraging results, much of the research cited is from studies that are too small and too brief to draw broad conclusions about long-term outcomes, the article authors caution. “We don’t have long-term studies looking at adherence, which we know is absolutely key to successful lifestyle change,” says Steger. “We also don’t have nuanced results to individualize recommendations or clinical care,” Steger adds. “For instance, we don’t know if healthy individuals gain any benefit to restricting the eating window in terms of preventing disease.” It’s also important to keep in mind that IF does not give you permission to binge on pizza, doughnuts, cheeseburgers, and hot fudge sundaes. “While intermittent fasting can lead to modest weight loss, changing when you eat is probably not as effective as changing what you eat because losing weight ultimately depends on eating fewer calories than your body needs,” Steger says. “Intermittent fasting can lead to modest weight loss alone but will be more effective when combined with traditional weight management recommendations to reduce calories and increase exercise.” Fasting may also work best when paired with healthy eating habits, like a Mediterranean diet rich in fruits, vegetables, legumes, whole grains, fish, and unsaturated fats. Meanwhile, other people may combine IF with the popular ketogenic diet in the hope of supercharging their weight loss, though many experts caution that research on this combined approach is lacking. “IF is not distinct necessarily from a Mediterranean diet or other eating plans,” says Susan Roberts, PhD, a professor of nutrition at Tufts University in Massachusetts, and founder of the iDiet weight loss program. “You can do them both together.” It’s possible that IF may offer benefits even when people don’t have good eating habits, but those benefits won’t necessarily be significant, says Dr. Roberts, who was also not involved in the article. RELATED: Intermittent Fasting May Improve Metabolic Health, Small Study Finds
Certain People Should Avoid Intermittent Fasting
Some people shouldn’t try IF, including pregnant or nursing women, children, the elderly, and individuals with any history of eating disorders, says Krista Varady, PhD, a professor of nutrition at the University of Illinois in Chicago. People with diabetes and some other chronic health problems should consult with a doctor before doing IF, Dr. Varady adds. (She also was not involved in the current article.) Getting started with IF may mean tolerating side effects like headaches, nausea, constipation, and irritability, Varaday says. Most of these problems come from failing to drink enough water. “People don’t realize there’s so much water in foods that they’re not getting when they start fasting, and they get hangry and irritable,” Varady says. “If you just do eight glasses of water a day, you’ll probably be fine.” While no serious side effects have been seen with short-term IF regimens, little is known about the long-term outcomes. “The big question is whether it is healthy for the long term and manageable for more than a tiny percent of the population,” says Roberts. “There may be side effects with implications for long-term health that are not apparent for months, or even years.” For now, there are only about 25 to 30 studies of IF in humans, Varady says. Much more research is needed that follows people beyond 12 months. Steger agrees that more long-term research is needed. “The intermittent fasting field is still young, and we don’t have long-term data on sustainability, nor do we have feasibility for different groups of people,” Steger says. “While IF may benefit many (or most) people, it may not work for everyone.”
