Fasting has been a cultural and religious tradition for centuries. More-recent trends like “intermittent fasting,” a term used to describe time-restricted eating, have focused the conversation on fasting’s impact on the body.
Intermittent fasting is often associated with dieting. But the practice has been studied for many effects unrelated to weight loss, like reduction of inflammation and cellular repair.
“Fasting activates an evolutionary program in which our body diverts nutrients normally used for growth and reproduction toward mechanisms for repair and survival,” says Whitney English, a Los Angeles–based nutritionist and founder of the practice Whitney E. RD.
Fasting is generally considered safe for adults who:
- Don’t have diabetes
- Aren’t pregnant or breastfeeding
- Don’t have to take medications with food
- Don’t have a history of disordered eating
But before you begin fasting, it’s important to understand the possible benefits and drawbacks, and how they might impact your health and quality of life.
Possible Benefits of Fasting
There are many ways to fast, and many names for these methods. The majority of modern research on the subject uses the term intermittent fasting, an eating plan where you restrict your meals to a specific period of time each day. This could mean you eat one meal per day, or eat within a certain window, like six to eight hours a day.
Here’s what the research says about the possible benefits of intermittent fasting.
Lower Inflammation
Some studies have shown that fasting can reduce inflammation in your body. One study showed that intermittent fasting decreased cytokines, the signaling molecules that promote inflammation.
Cell Repair
Fasting triggers a cellular process called autophagy, in which your body breaks down and recycles damaged parts of your cells. Autophagy removes cellular “debris,” which in turn improves the function and health of cells. Autophagy also destroys pathogens in cells, such as viruses and bacteria.
Improved Insulin Sensitivity
For people without underlying conditions like diabetes, eating triggers the release of insulin. That’s the hormone that shuttles sugar, also called glucose (the product of carbohydrate digestion), into cells to be used for energy or stored for later.
When we take a break from eating, insulin levels drop. The result? Improved insulin sensitivity, which is protective against diabetes.
“Limiting calories also translates to reduced blood glucose, improved lipids, and reduced blood pressure,” says Amy Rothberg, MD, director of the Metabolism, Endocrinology, and Diabetes Weight Management Clinic at the University of Michigan in Ann Arbor. “This has been shown in a number of studies of low-calorie and very low-calorie diets, and time-restricted feeding has some of the same impact.”
While animal studies have shown that time-restricted feeding patterns can help to reverse the progression of metabolic diseases like diabetes in mice, findings from clinical trials done on people report mixed results. If you have diabetes, talk with your doctor before fasting.
Weight Loss, With a Catch
Some research shows that fasting can lead to losing pounds and fat. One review of studies on the subject found that people can lose as much weight while intermittent fasting as they can with continued calorie restriction.
But there’s a catch, English says. “People do not lose more weight from fasting than they do with traditional low-calorie diets,” she says.
So why are meal plans that incorporate fasting often treated like the better way to shed pounds?
“They work simply by limiting the number of calories consumed,” Dr. Rothberg says. “Consider the average American who usually consumes meals and snacks over 12 hours every day. Limiting consumption to eight or even fewer hours generally will limit caloric intake.”
Drawbacks to Fasting
Mood Swings Some people have reported feeling negative emotions while fasting, such as anxiety or depression. If you’re used to having frequent meals and snacks, going without food for an extended time may lead to irritability, or what some people call feeling “hangry.” Fasting could also cause headaches and fatigue.
Potential to Overeat You may feel tempted to overeat at the end of a fast, which can lead to weight gain over time.
Interactions With Meds If you take medications for blood pressure or heart disease, fasting could cause imbalances of minerals like sodium and potassium. And if you need to take your meds with food, fasting might prove hard to maintain.
Fasting is not for everyone. The practice is not recommended for children up to 18 years old, who have different nutritional needs than adults. Fasting is also not recommended for pregnant women — especially in the second trimester — who actually do need a more-constant supply of nutrients.
And recommendations from the Academy of Nutrition and Dietetics discourage fasting for anyone with a history of eating disorders or disordered eating, because it can make restrict-and-binge cycles worse.
The Takeaway
- Intermittent fasting is considered safe for many adults. But some adults should not try it, like pregnant people and people with a history of eating disorders.
- People on medications and those who have diabetes should talk with a doctor before starting a fasting regimen.
- Intermittent fasting may bring benefits such as weight loss, better insulin regulation, and possible reduction of inflammation.
- Still, fasting can also cause mood swings, overeating, and complications with medications.
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