If you’ve ever Googled intermittent fasting, you probably know that it is an intervention for weight loss that has captured public imagination within the last decade. You might even be considering it as a way to shed the extra pounds you might have packed on during quarantine. Intermittent fasting grabbed the attention of metabolism and aging researchers and even some clinicians long ago. What started as an alternative to continuous caloric restriction (often considered a gold standard for delaying aging) has been the subject of a vast array of animal and human studies and numerous clinical trials. Intermittent fasting has been investigated as an intervention not only for weight loss but also for metabolic health more broadly.

“Periods of voluntary abstinence from food and drink (i.e., intermittent fasting) has been practiced since earliest antiquity by peoples around the globe.” – Patterson et al., 2015

You might want a “new beach bod” in 2021. You might want to stave off metabolic dysfunction associated with aging. Or you might just want to “feel like yourself” again and be able to run after your children and grandchildren. Intermittent fasting could help you achieve these goals this year. However, not all intermittent fasting schedules are created equal for every person when it comes to weight loss or metabolic health. Your approach to nutrient restriction or intermittent fasting should take your energy needs and goals into account. We encourage you to work with a physician, clinical nutritionist or dietician to talk about your energy needs. However, we’ve curated some scientific literature to help you approach intermittent fasting with your personal health goals in mind.

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Check out our infographic on fasting for weight loss – click on the graphic above.

Getting Started with Intermittent Fasting

Intermittent fasting is becoming a popular term to describe a range of different eating schedules and approaches to off-and-on calorie restriction. But what is it, really? Intermittent fasting or IF often involves a daily or weekly cycle of fasting and feeding. Intermittent fasting is an umbrella term that within scientific literature refers to at least three different approaches to restricting calories intermittently. These popularly include alternate day fasting, which in research studies typically involves eating fewer than 500 calories every other day, the 5:2 diet, which involves two fast days of fewer than 500 calories per day, and time-restricted feeding, which involves only eating within a 6 to 12-hour window each day while fasting the other 12 to 18 hours or more.

Intermittent fasting is typically well tolerated in most individuals, with no evidence of harm according to a 2017 review.

If you are just getting started with intermittent fasting, we recommend slowly working up to your target fasting schedule over the course of several weeks or months. For example, you can start with practicing a daily prolonged overnight fasting period of 12 or more hours. Even this modest fasting schedule can require an adjustment period for the typical Western Diet practitioner. From there, you might work up to 14 to 16 hours for a typical daily time-restricted feeding practice, or up to 24 to 48 hours on a monthly or weekly basis, depending on your health goals.

“I tried to ease into fasting, first with just a 12 hour fast, moving up to a 16/8 normal schedule and eventually working up to at least 20 to 22 hours of fasting with one meal per day,” says Robert Brown, a LIFE Fasting Tracker user who has lost 20 pounds with intermittent fasting this year. “The one lesson I’ve learned from this is that I had to work my way up to it. I didn’t just decide one day to fast for 20 hours. There were hunger pangs, but by easing myself into it, I was able to manage any issues that seemed to pop up. Ultimately, I’d like to lose weight but more importantly just live a long and productive life.”

Robert, a LIFE Fasting Tracker user, has lost 20 pounds this year with intermittent fasting.

Robert, a LIFE Fasting Tracker user, has lost 20 pounds this year with intermittent fasting.

Ballerina with a fire-colored shroud.Your Body on Fire

“Excess energy intake, weight gain and subsequent adiposity are consistently linked to illness, disability and mortality.” – Harvie & Howell, 2017

Intermittent fasting has been shown to change the body and its metabolism in many ways, in both animals and in humans. One of the most important metabolic changes brought on by fasting is a “metabolic switch” from carb-burning to fat-burning. The fat-burning metabolic state is known as ketosis, and is characterized by the production of ketone bodies that serve as an important fuel source for the brain as well as other tissues. Recent findings suggest that ketones may also serve as signaling molecules, with functions that are more like that of hormones in the body. Ketones may modify the activity of proteins and change gene expression associated with the metabolism of fats, repair of DNA damage and prevention of cellular stress.

“In humans, depending upon their level of physical activity, 12 to 24 hours of fasting typically results in a 20% or greater decrease in serum glucose and depletion of the hepatic glycogen, accompanied by a switch to a metabolic mode in which nonhepatic glucose, fat-derived ketone bodies, and free fatty acids are used as energy sources.” – Longo & Mattson, 2014

Intermittent metabolic switching from carb-burning to fat-burning, which fasting can trigger, has been shown to be beneficial not only for weight loss but also for brain health. John Newman at the Buck Institute for Research on Aging has found that switching in and out of ketosis over a lifetime significantly improves the memory of aging mice. The switch back to carb-burning and glucose metabolism upon refeeding after a fast may be an important component of the brain health benefits of fasting. Refeeding after a fast triggers cellular recycling (technically known as autophagy) the growth of new nerve cell connections, for example.

“With fasting and extended exercise, liver glycogen stores are depleted and ketones are produced from adipose-cell-derived fatty acids. This metabolic switch in cellular fuel source is accompanied by cellular and molecular adaptations of neural networks in the brain that enhance their functionality and bolster their resistance to stress, injury and disease.” – Mattson et al., 2018

Not all approaches to intermittent fasting have the same metabolic impacts. We are also realizing that when it comes to nutrition and the impacts of nutrient restriction, different bodies respond differently on an individual level. That being said, here is what your body might experience on various intermittent fasting schedules…

Weight Loss

“Randomised trials demonstrate that intentional weight loss reduces type 2 diabetes, all-cause mortality and increases cognitive and physical function.” – Harvie & Howell, 2017

Studies of intermittent fasting, periodic or multi-day fasting and time-restricted feeding have all been shown to various extents to foster weight loss, improve metabolic health indicators and reduce the risk for cardiovascular disease and cancer. Intermittent fasting is often practiced as a significant or total* energy restriction protocol two days per week or every other day.

*With a total energy restriction, people only drink water, water with electrolytes and non-calorie drinks like coffee and tea.

In a 2017 study, obese individuals who restricted their eating window to 8 hours for 12 weeks lost 3% of their body weight and their blood pressure improved.  One major advantage of limiting your eating window to 6-8 hours a day is that you don’t have to keep count of your calories, which makes it more sustainable and effective at achieving and maintaining weight loss compared to caloric restriction.

Whether you want to lose 5 pounds or 100 pounds with intermittent fasting, you’ll want to tune into what you eat and when you eat. Learn to recognize actual hunger from external triggers that make you eat, eat a balanced diet with plenty of plant fats and fiber and stick to your fast-day calorie goals.

Greater Insulin Sensitivity

Insulin sensitivity describes how sensitive various tissues in your body are to the effects of insulin. Insulin sensitivity can depend on our level of physical activity, our sleep patterns, what we eat and even when we eat.

Alternate day fasting seems to be more effective at improving insulin sensitivity and reducing fasting insulin compared to caloric restriction.

Time-restricted feeding is another approach to intermittent fasting that may target processes and pathways associated with our circadian rhythms, including insulin sensitivity. Did you know that your metabolism changes throughout the day, as regulated by your internal biological clock as well as external factors such as exposure to bright light and food intake? For example, you are naturally more insulin resistant (your tissues don’t take up glucose as efficiently) in the evening as your body prepares to burn fat as you sleep. Courtney Peterson and colleagues published a study in Cell Metabolism in 2018 showing that early time-restricted feeding, or eating within a 6-hour “feeding period” every day with breakfast around 8 am and dinner before 3 pm, improved insulin sensitivity, blood pressure, oxidative stress and appetite in men with prediabetes. These health benefits occurred even in the absence of weight loss.

What does this mean? Daily eating schedules that tune nutrient intake to your biological clock are metabolically favorable. Metabolic health indicators like insulin sensitivity and low levels of inflammatory factors are in turn associated with reduced risk of heart disease, diabetes and cancer.

A Different Gut Microbiome Profile

Time-restricted feeding has also been shown to modify the gut microbiota, or the collection of microorganisms living in the gut. Like other aspects of our metabolism, our gut microbiomes ideally change routinely throughout a 24-hour period, according to a circadian rhythm that is largely dictated by when we eat. In fact, the daily cycling of the gut microbiome is thought to be a biological clock of sorts. This clock helps regulate metabolic activity in various tissues throughout our body, including our brain.

We’ve known for a long time that what we eat can change our gut microbiota. But many animal studies have revealed that when we eat can also impact our gut microbes. According to a 2020 study, mice that fasted for 16 hours every day for 30 days showed increased levels of Akkermansia and reduced levels of Alistipes bacteria, compared to mice that weren’t subjected to fasting. More Akkermansia and less Alistipes are associated with reduced intestinal inflammation and decreased fat accumulation in the liver. Therefore, many of the benefits of fasting likely come from the alteration of our gut bacteria.  

In a human study, participants who were fasting 17 hours a day for 30 days during Ramadan showed lower total cholesterol, lower fasting glucose levels, and more Akkermansia muciniphila in the gut–  a type of bacteria associated with lower body weight. Intermittent fasting may also kill off “bad” bacteria while fostering the growth of gut microbes that produce beneficial compounds like lactate and short-chain fatty acids that help fight inflammation and glucose intolerance. These gut microbe-produced compounds also protect your heartFiber also helps to feed these beneficial gut microbes.

Reduced mTOR Activity and Increased Autophagy

18 hours of fasting in humans has been shown to increase mTOR as well as cellular markers of autophagy.  Autophagy is a recycling mechanism for cellular components that our cells use to deal with stress and prevent untimely cell death. mTOR, or the mechanistic target of rapamycin, is an enzyme that is the master regulator of protein synthesis and cell growth. Lowering of mTOR activity has been associated with autophagy and extended lifespan and healthspan in animals.

Recycle your body in 2021 with intermittent fasting!

Reduced Oxidative Stress and Inflammation

Oxidative stress occurs as potentially harmful reactive oxygen species build up within the cells of your body that are metabolically active. Reactive oxygen species (ROS) are, as their name suggests, reactive. They can react with and cause harm to your proteins and even your DNA. If a cell in your body can’t counteract this damage quickly enough, it might become senescent, spew out inflammation, or even die.

Intermittent fasting forces muscle, brain and other tissues (although possibly not fat, thus the rapid fat burning seen during ketosis!) to become more efficient in their breakdown of metabolites and production of energy. This can help reduce ROS and levels of inflammation. Fasting also triggers the body to produce more antioxidants that help clean up ROS.

A study with prediabetic men who shortened their eating window to 6 hours a day for 5 weeks showed that this fasting regimen reduced plasma levels of the oxidative stress marker 8-isoprostane, in addition to improving insulin sensitivity and blood pressure. Alternate day fasting (or eating every other day) has been shown to increase the expression of superoxide dismutase and catalase, two powerful antioxidant enzymes that can clean up ROS in your cells. Another way to increase antioxidant enzymes is through exercise! Physical activity increases the levels of superoxide dismutase, which has a lot of benefits for all of the organs in your body.

Avoiding late-night meals and beginning your fast early in the evening was associated with lower risk of breast cancer in a 2015 study. Taking in calories in the evening was linked to higher levels of inflammatory molecules in the circulation:

“Each 10 percent increase in the proportion of daily calories consumed after 5 pm was associated with a significantly higher concentration of CRP—a biomarker that has been associated with increased risk of breast cancer, as well as a variety of chronic conditions.”

Starting your fast earlier in the evening is also associated with lower breast cancer recurrence.

Healty eating plateHow to Fast for Weight Loss

Weight loss studies of intermittent fasting have typically investigated the 5:2 diet or alternate day fasting interventions lasting three to six months. For most people in such studies, it takes two to three months to lose 10 pounds.

The good news is that most studies of intermittent fasting using the 5:2 diet protocol or minimal calorie fasting days have reported statistically significant weight loss, approximately equivalent to what can be achieved through continuous calorie restriction. Reported weight loss outcomes of the 5:2 diet range from 3.2% weight loss in comparison to a control group over a 12-week period to 8.0% weight loss in an 8-week trial enrolling overweight adults with asthma.

Obese adults who participated on an alternate day fasting study for 6 months lost roughly 6% of their body weight during the first 3 months. They were allowed to consume up to 600 kcal on their fasting days and ate a low-carb diet on non-fasting days. In addition to losing weight, participants were also able to reduce their total cholesterol, blood pressure, and fasting insulin.

Shorter fasts are also effective at inducing weight loss in humans. Obese patients who restricted their eating windows to 8 hours lost significant weight and improved their blood pressure.

How much weight you lose with intermittent fasting will depend on where you start, what your daily energy requirements are, what you eat and when. You’ll typically lose as much weight practicing intermittent fasting as with a continuous calorie restriction diet that limits your energy intake to 80% of your daily needs. IF schedules that more naturally restrict calories through longer or more often fasting, such as alternate day fasting, will promote more rapid weight loss than time-restricted eating. However, healthier food choices and greater calorie restriction in conjunction with any IF schedule can help you accelerate weight loss.

7 Tips for Weight Loss with Intermittent Fasting

1. Increase your fiber intake for gut health and satiety between meals.

2. Increase your lean protein intake if needed to improve satiety and muscle mass. One exploratory study found that individuals who successfully lost weight during a randomized controlled trial of 12 months of alternate day fasting report greater protein intake, increased fullness and decreased hunger than individuals who didn’t lose weight.

3. Stick to your fast-day calorie goals.

4. Avoid energy-dense meals and snacks on “feast” days.

5. Eat early in the day. A recent study found that snacking or eating dinner late at night prevents you from burning fat as you sleep. Our internal clock regulates when our bodies switch from primarily using carbs to primarily using fat to create energy, but this cycle goes out of whack when we reach for that midnight snack. According to this study, If you eat late, when you go to bed you will be preferentially burning carbs, delaying the time in which your body flips into fat- burning mode.

6. Eat in a narrow window on calorie restriction days. For example, if you follow a modified alternate day fasting protocol where you eat up to 500 calories on fasting days, eating all of those calories in one meal may give you the added metabolic benefits of time-restricted eating.

7. Eliminate added sugars. While both low-fat and low-carb diets can lead to significant weight loss, consuming added sugars is particularly strongly linked to insulin resistance and metabolic dysfunction. In addition to weight gain, added sugar intake is associated with higher blood pressure, inflammation and fatty liver disease. Added sugars cater to particularly energy-hungry but metabolically inefficient cells, including senescent cells, cancerous cells and even quickly proliferating pathogenic bacteria in the gut.

Stoking the Fire for Long-Term Health

Weight loss is a top priority health goal for many of us. While intermittent fasting or nutrient restriction is often an effective way of achieving weight loss, there are other lifestyle factors that are key to maintaining your metabolic health long-term. These include things like exercise, adequate sleep and stress management.

You can help your body accelerate and maintain weight loss and your broader health with the following lifestyle interventions that complement fasting.

1. Eat more fruits and vegetables.

2. Exercise. While exercise is often not in and of itself a sufficient intervention for achieving weight loss, it health impacts go far beyond weight. Physical activity and structured exercise can help you maintain a healthy weight and prevent metabolic diseases such as obesity and diabetes. Exercise preserves your bone strength and cognitive function as you age. It also makes your body and brain better able to handle stress with adaptation responses such as increased production of antioxidants and anti-inflammatory factors. You can effectively improve your heart health and prevent chronic diseases including obesity with 150 minutes per week of moderate-intensity aerobic activity or 75 minutes per week of vigorous aerobic activity. National recommendations for physical activity also include two days per week muscle-strengthening activity.

3. Maintain a balanced diet.

4. Avoid added sugars. and other simple carbohydrates (white bread, white rice, etc.)

5. Practice mindful or intuitive eating. Learn more at lifeapps.io/mindfulness/

Learn more about fasting for weight loss and how the LIFE Fasting Tracker app can help here.

Paige Jarreau, PhD

I am the VP of Science Communication for LifeOmic and an avid blogger. I'm interested in how scientists use social media to promote public engagement and health behaviors.

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