is fasting good for you?

Does When You Eat Matter?

What you eat matters. Many studies have shown that the types of food you eat affect your health. But what about the timing? Scientists are just beginning to understand that when you eat may also make a difference.

Throughout history, people have experienced periods when food was either scarce or completely lacking, says Dr. Valter Longo, an NIH-funded longevity researcher at the University of Southern California. “So, they were forced to fast,” he says.

But current technology—like refrigeration, transportation, and electric lighting—have made food more readily available.

“This has shifted our eating patterns,” explains Dr. Vicki Catenacci, a nutrition researcher at the University of Colorado. “People now eat, on average, throughout a 14-hour period each day.”

Studies suggest that this constant food intake may lead to health problems. Researchers have started looking at whether fasting can have potential benefits for some people.

Fasting diets mainly focus on the timing of when you can eat. There are many different fasting diets, sometimes called “intermittent fasting.”

In time-restricted feeding, you eat every day but only during a limited number of hours. So, you may only eat between a six- to eight-hour window each day. For example, you might eat breakfast and lunch, but skip dinner. In alternate-day fasting, you eat every other day and no or few calories on the days in between. Another type restricts calories during the week but not on weekends.

But scientists don’t know much about what happens to your body when you fast. Most research has been done in cells and animals in the lab. That work has provided early clues as to how periods without food might affect the body.

In some animals, certain fasting diets seem to protect against diabetes, heart disease, and cognitiveRelated to the ability to think, learn, and remember. decline. Fasting has even slowed the aging process and protected against cancer in some experiments.

“In mice, we’ve seen that one of the effects of fasting is to kill damaged cells, and then turn on stem cellsImmature cells that have the potential to develop into many different cell types in the body.,” explains Longo. Damaged cells can speed up aging and lead to cancer if they’re not destroyed. When stem cells are turned on, new healthy cells can replace the damaged cells.

Now, studies are starting to look at what happens in people, too. Early results have found that some types of fasting may have positive effects on aspects of health like blood sugar control, blood pressure, and inflammationHeat, swelling, and redness caused by the body’s protective response to injury or infection. . But fasting can also cause weight loss. So researchers are studying whether the beneficial changes seen in the body are side effects of the weight loss or the fasting process itself.

For many people, the main reason to try fasting is to lose weight. Currently, most people try to lose weight by restricting how many calories they eat each day.

“That doesn’t work for everyone,” Catenacci explains. “It takes a lot of focus. It takes a lot of math, and a lot of willpower.”

One of Catenacci’s studies showed that, over a two-month period, adults who were overweight or obese were equally likely to lose about 15 pounds when they either completely fasted every other day or restricted their calories every day.

“For some people, restricting calories every day may be the best approach. For others, it might be easier to not have to count calories every day and use an intermittent fasting strategy for weight loss,” says Catenacci. “The best diet for any given person is the one that they can adhere to. I don’t think weight loss is a one size fits all approach.”

Now her research team is running a similar study to compare how much weight participants lose with fasting versus calorie restriction, but over a one-year period. They’re also testing whether adding a small meal on fasting days will make it easier to stick to as a longer-term weight loss strategy.

But are the benefits from fasting all due to weight loss or is there something more to it? “There’s a lot of debate about whether the benefits of intermittent fasting are due to the extended fasting period itself,” says Dr. Courtney Peterson, an NIH-funded nutrition researcher at the University of Alabama.

To understand this better, Peterson did a study in pre-diabetic men. It was designed so the volunteers would not lose weight. The men ate an early time-restricted feeding diet for five weeks. They could eat only between 8 am to 2 pm. They then fasted for the next 18 hours. Next, they ate the same amount of food but only during a 12-hour period per day for five weeks. None of the men lost weight.

The longer fasting period alone made a difference. “The early time-restricted diet improved their blood sugar control,” Peterson says. “And we found a blood pressure lowering effect equivalent to what you see with a blood pressure medication.”

These findings suggest that an extended fast or the timing of when you eat—even when it doesn’t affect your weight—can bring health benefits for some people.

Fasting may bring health benefits, but Longo cautions that there’s still a lot we don’t know. For some, fasting may cause problems. For example, studies have found that people who regularly fast more than 16 or 18 hours a day have a higher risk of gallstones. They’re also more likely to need surgery to remove the gallbladder.

Eating for 12 hours and then fasting for 12 hours is likely safe for most people, Longo explains. “That pattern of eating is very common among people who have record lifespans,” he says. “It seems to match both science and tradition.”

Longo and his team are also looking at fasting-mimicking diets, which they hope will be safer and easier to follow than completely fasting. They designed a five-day, monthly fasting-mimicking diet that allows some food, but is low in calories. They tested the diet for three months in a recent study. Those who stayed on the diet lost weight and showed decreases in age-related disease risk factors.

But he and other experts caution against people trying fasting diets that are not based on research. If you’re considering fasting, talk with your health care provider first. People with certain health conditions or who are taking certain medications should not try fasting at all (see the Wise Choices box).

Early results have found that some types of fasting may have positive effects on aspects of health like blood sugar control, blood pressure, and inflammation . But fasting can also cause weight loss.

People fast for a variety of reasons. It might be a religious practice, or an effort to build self discipline or benefit health.

In this article, we’ll delve into how your body responds to fasting and whether it really has health benefits.

Fasting can be intermittent or prolonged.

“Intermittent” means that you fast for less than a couple of days or you switch between eating and fasting. “Prolonged” means fasting from 2 days to several weeks.

Some of the most common methods include:

Other versions of intermittent fasting include alternate day fasting and the 5:2 diet. This involves only eating on 5 days and fasting for the other 2 days of the week.

But in both of these versions, people often eat small amounts on their “fasting” days.

Prolonged, or periodic, fasts last for several days or weeks. Before starting this, talk to a healthcare professional to make sure it’s appropriate for you.

Regardless of the type of fast, your body responds to the lack of fuel. There may be potential health benefits, but there are also some risks. Fasting may not be appropriate for everyone.

And no matter your eating schedule, the foods you choose are important.

With the ZOE at-home test, we analyze your blood sugar and blood fat responses, as well as your gut microbiome. Using this information, we’ll give you personalized nutrition advice so you find the best way to eat for your body.

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Your body has an incredible ability to adapt. Anytime you go without fuel, your body makes several changes in response. Here’s what happens to your body when you fast.

Your body uses compounds in food for energy. Typically, it breaks down carbohydrates into simple sugar molecules.

This is your body’s main source of energy. Whatever isn’t used gets stored in your liver as glycogen and as fatty acids in your fat cells.

For the first few hours of not eating, your body pulls from its glycogen stores for energy. But as these stores eventually run out, your body finds a different fuel source. This is known as the metabolic switch.

When glucose is no longer available, your body begins to break down body fat into ketones. Next, these are transported to your cells and used as energy. When you have an increased amount of ketones in your blood, you’re said to be in “ketosis.”

Depending on how much energy you use, what your last meal was, and the amount of stored glycogen in your liver, this switch can take place after going 12–36 hours without food.

As you repeatedly go without food, your cells undergo a coordinated response to the stress that fasting exposes them to.

Scientists believe that this adaptive response allows your cells to improve their resistance to stress and disease — though most of the relevant research stems from animal and cell studies.

Autophagy means “self-eating.” It's the process your cells use to clean out and recycle old, damaged, or abnormal proteins and cell components.

This process is an important part of maintaining healthy cells. Autophagy likely plays a key role in preventing diseases, including heart disease, diabetes, cancer, and infections.

Autophagy happens all the time in our cells. If it doesn’t work properly, it’s harmful to cells in a way that’s linked with health conditions.

Fasting may be able to enhance autophagy, and it could be a simple and safe way to do this.

However, most of our evidence currently comes from animal studies. The evidence in humans is limited.

In your gut, there are trillions of bacteria and other microorganisms, collectively known as your gut microbiome. The gut microbiome affects your overall health in many ways.

Evidence suggests that some forms of fasting may benefit your gut microbiome.

Studies involving men who fasted for 16 hours a day found that the participants had more beneficial bacteria in their gut microbiomes. These bacteria have been linked with a variety of health benefits, including better metabolic health, improved heart health, and a lower risk of obesity, cancer, and inflammatory bowel disease.

In one study, however, when the men returned to their normal eating patterns, the makeup of their microbiomes went back to how it was before they fasted.

During sleep, the activity in your gut slows down, which allows the cells lining your gut to be repaired.

Studies suggest that increasing this fasting window could strengthen your gut barrier, which may reduce chronic inflammation.

Research into the link between fasting and gut health is still ongoing, but these effects could help explain the reported benefits. Still, fully understanding this topic will require more research.

Most research on fasting and long-term health focuses on intermittent fasting.

This area of study is in its early stages. Overall, understanding the effects of fasting over months or years requires more long-term studies. Recent evidence suggests that some forms of fasting may benefit your health over time. Here’s what the current research says.

Some forms of intermittent fasting may help you control your blood sugar.

Multiple studies suggest that the alternate-day method of intermittent fasting may help improve insulin sensitivity and blood sugar control.

Still, fully understanding the relationship between fasting and blood sugar control in the long term requires more research.

Intermittent fasting may also support heart health over time.

Evidence from multiple studies suggests that intermittent fasting may help improve heart health by:

Limited research has associated these effects with alternate day, time-restricted, and modified methods of intermittent fasting.

Short-term fasting, such as intermittent fasting, may help some people lose weight. But growing evidence suggests that this may not be more effective than traditional calorie-restricted diets.

There’s less research about prolonged fasting. While it may also help people lose weight — evidence shows that any lost weight may not come from body fat.

In one small study, men who fasted for 10 days experienced, on average, a 7% decrease in body weight. However, 60% of the weight loss was from a decrease in water, protein, and other lean tissues.

Different types of fasting may help improve your health, but there are risks to consider.

These include:

Fasting may not be appropriate for everyone. Those who should avoid it include:

Fasting can mean going without foods and drinks that contain calories. There are different types, depending on how long you fast.

Fasting may have positive effects on your health, such as promoting heart health, better blood sugar control, and weight loss.

However, there are risks. These include feeling tired and irritable, particularly when you start, and having migraine headaches. Also, people with certain health factors should avoid fasting.

With ZOE’s personalized nutrition program, you can discover how to eat for your body and your long-term health goals.

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There's a ton of incredibly promising intermittent fasting (IF) research done on fat rats. They lose weight, their blood pressure, cholesterol, and blood sugars improve… but they're rats. Studies in humans, almost across the board, have shown that IF is safe and effective, but really no more effective than any other diet. In addition, many people find it difficult to fast.

But a growing body of research suggests that the timing of the fast is key, and can make IF a more realistic, sustainable, and effective approach for weight loss, as well as for diabetes prevention.

IF as a weight loss approach has been around in various forms for ages but was highly popularized in 2012 by BBC broadcast journalist Dr. Michael Mosley's TV documentary Eat Fast, Live Longer and book The Fast Diet, followed by journalist Kate Harrison's book The 5:2 Diet based on her own experience, and subsequently by Dr. Jason Fung's 2016 bestseller The Obesity Code. IF generated a steady positive buzz as anecdotes of its effectiveness proliferated.

In the Obesity Code, Fung successfully combines plenty of research, his clinical experience, and sensible nutrition advice, and also addresses the socioeconomic forces conspiring to make us fat. He is very clear that we should eat more fruits and veggies, fiber, healthy protein, and fats, and avoid sugar, refined grains, processed foods, and for God's sake, stop snacking.

IF makes intuitive sense. The food we eat is broken down by enzymes in our gut and eventually ends up as molecules in our bloodstream. Carbohydrates, particularly sugars and refined grains (think white flours and rice), are quickly broken down into sugar, which our cells use for energy. If our cells don't use it all, we store it in our fat cells as, well, fat. But sugar can only enter our cells with insulin, a hormone made in the pancreas. Insulin brings sugar into the fat cells and keeps it there.

Between meals, as long as we don't snack, our insulin levels will go down and our fat cells can then release their stored sugar, to be used as energy. We lose weight if we let our insulin levels go down. The entire idea of IF is to allow the insulin levels to go down far enough and for long enough that we burn off our fat.

Initial human studies that compared fasting every other day to eating less every day showed that both worked about equally for weight loss, though people struggled with the fasting days. So, it's very reasonable to choose a reduced calorie plant-based, Mediterranean-style diet. But research suggests that not all IF approaches are the same, and some IF diets are indeed effective and sustainable, especially when combined with a nutritious plant-based diet.

We have evolved to be in sync with the day/night cycle, i.e., a circadian rhythm. Our metabolism has adapted to daytime food, nighttime sleep. Nighttime eating is well associated with a higher risk of obesity, as well as diabetes.

Based on this, researchers from the University of Alabama conducted a study with a small group of obese men with prediabetes. They compared a form of intermittent fasting called "early time-restricted feeding," where all meals were fit into an early eight-hour period of the day (7 am to 3 pm), or spread out over 12 hours (between 7 am and 7 pm). Both groups maintained their weight (did not gain or lose) but after five weeks, the eight-hours group had dramatically lower insulin levels and significantly improved insulin sensitivity, as well as significantly lower blood pressure. The best part? The eight-hours group also had significantly decreased appetite. They weren't starving.

Just changing the timing of meals, by eating earlier in the day and extending the overnight fast, significantly benefited metabolism even in people who didn't lose a single pound.

But why does simply changing the timing of our meals to allow for fasting make a difference in our body? An in-depth review of the science of IF recently published in New England Journal of Medicine sheds some light. Fasting is evolutionarily embedded within our physiology, triggering several essential cellular functions. Flipping the switch from a fed to fasting state does more than help us burn calories and lose weight. The researchers combed through dozens of animal and human studies to explain how simple fasting improves metabolism, lowers blood sugar levels; lessens inflammation, which improves a range of health issues from arthritic pain to asthma; and even helps clear out toxins and damaged cells, which lowers risk for cancer and enhances brain function.

According to metabolic expert Dr. Deborah Wexler, Director of the Massachusetts General Hospital Diabetes Center and associate professor at Harvard Medical School, says "there is evidence to suggest that the circadian rhythm fasting approach, where meals are restricted to an eight to 10-hour period of the daytime, is effective." But still she recommends that people "use an eating approach that works for them and is sustainable to them."

So, here's the deal. There is some good scientific evidence suggesting that circadian rhythm fasting, when combined with a healthy diet and lifestyle, can be a particularly effective approach to weight loss, especially for people at risk for diabetes. (However, people with advanced diabetes or who are on medications for diabetes, people with a history of eating disorders like anorexia and bulimia, and pregnant or breastfeeding women should not attempt intermittent fasting unless under the close supervision of a physician who can monitor them.)

Adapted from a Harvard Health Blog post by Monique Tello, MD, MPH

Effects of intermittent fasting on health, aging, and disease. de Cabo R, Mattonson MP. New England Journal of Medicine, December 2019.

Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Internal Medicine, May 2017.

Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. American Journal of Clinical Nutrition, January 2005.

The Obesity Code, by Jason Fung, MD (Greystone Books, 2016).

Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database of Systematic Reviews and Implementation Reports, February 2018.

Metabolic Effects of Intermittent Fasting. Annual Review of Nutrition, August 2017.