Fasting

Intermittent Fasting Offers Benefits Beyond Weight Loss

Intermittent fasting may trigger a process called metabolic switching, which sets off a cascade of health benefits.

Each new year brings fresh proclamations about the best ways to lose weight and get healthier. There’s a lot of hype around extreme diets that have little research to support them, but an article published in December 2019 in The New England Journal of Medicine suggests that one popular approach — intermittent fasting (IF) — may be worth considering.

"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, include improved glucose regulation, reduced blood pressure and resting heart rate, reduced inflammation, and improved muscle health,"

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.

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.

There are many types of IF, but these are practiced most frequently:

Several studies and 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. 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, fasting gives our cells and tissues a break from processing and storing calories, and allows time to repair and regenerate," Steger says.

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, donuts, 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. 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.

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."

"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."