Your Questions about Intermittent Fasting, Answered (Part 1)

What is intermittent fasting? Who is it safe for? What are the health benefits? LifeOmic has answered some of your most pressing questions on intermittent fasting in time for our LIFE Fasting Tracker app release!

Paige Brown Jarreau
May 2, 2018 · 10 min read

Great news! LifeOmic’s first LIFE app is out in the wild on the Apple App Store! To guide you in your LIFE intermittent fasting journey, we’ve created an FAQ on intermittent fasting, which we will be publishing as a series here on Medium (Part 2 next week!) Explore the questions and answers below to learn how to safely practice fasting for metabolic health.

Have questions about IF? Let us know at, or ask your Q on Twitter, IG or FB using the hashtag #SeizeLifeFast.

The following FAQ responses are based primarily upon human research studies conducted by intermittent fasting expert Dr. Krista Varady, an associate professor of nutrition at the University of Illinois, in Chicago. Varady’s primary research interest is the metabolic health impacts of alternate day fasting and time-restricted feeding. She has published results from over 10 clinical trials of intermittent fasting interventions.

Credit: Meg,

What is Intermittent Fasting?

Intermittent fasting (IF) often involves a daily cycle of fasting and feeding. IF is an umbrella term that refers to at least three different approaches to restricting calories intermittently. These include alternate day fasting, which involves eating fewer than 500 calories every other day, the 5:2 approach to fasting, which involves two fast days of fewer than 500 calories per week, and time-restricted feeding, which involves eating within a narrow 8–10-hour window each day.

A popular time-restricted feeding approach is the 16:8. This involves fasting during a 16-hour window and eating during an 8-hour window, either every day or several days per week. During your 8-hour eating window, for example from 10 a.m. to 6 p.m., you don’t have to count calories or necessarily change the way you eat. You simply confine any calories you consume to this window.

“It’s important to remember that with intermittent fasting, you do typically get to eat at least a little bit every day,” Dr. Krista Varady says. “There’s also no limitation on types of food or number of calories you eat during your feeding windows or ‘feast’ days.”

What is the metabolic health rationale behind intermittent fasting?

Intermittent fasting is a form of hormetic or “good” stress, which the body often responds to with improvements in metabolic and cellular performance. Theoretically, fasting prompts “self-eating” within the body, helping the body clean up damaged cells and cellular components including proteins that can cause issues when they accumulate later in life. If you are overweight, fasting and losing weight can help you shrink your body in such a way that many damaged and senescent cells are eliminated naturally.

Human research studies on fasting have focused on its potential benefits for people at risk of developing obesity and cardiovascular disease, major health problems in the United States. These studies have revealed that IF can have substantial positive impacts on weight loss, metabolic and heart health.

Dr. Krista Varady has found in human clinical studies that IF is an effective weight maintenance strategy, where people are more likely to stick to fasting regimens as opposed to diets long term, helping them lose weight and maintain weight loss as opposed to yo-yo dieting.

Fasting can improve metabolic flexibility. Credit: Adam McGuffie,

Is intermittent fasting safe for me?

Most people can benefit from practicing moderate fasting (shorter than 24 hours). However, there are safety concerns for pregnant women, children, individuals with type 1 diabetes and individuals who are malnourished or not getting proper nutrition. These individuals are advised not to practice intermittent fasting and should talk to their doctor before contemplating doing so in the future.

There are also concerns for young children, although not eating after dinner and fasting overnight for 8–12 hours is probably not a concern for young adolescents. Preliminary data suggest that alternate day fasting (with 500 calories or less on fast days) may be safe and effective for obese young adolescents. However, Dr. Krista Varady recommends against IF for children under 12 years of age.

Is there a best time (of day) to fast?

Based on current research in humans, there is limited consensus around optimal meal timing for people who practice intermittent fasting. We generally become more insulin resistant as the day progresses, or less able to clear blood glucose and get it to where it needs to go in our bodies. For this reason, it’s probably best to consume most of your calories earlier in the day and to start your fasting window several hours before you go to sleep.

“Your body can’t deal with nutrients (glucose) as efficiently later in the day, so you might as well give your body a break from glucose then,” Dr. Krista Varady says. “Early in the day, your body is primed and ready to deal with an influx of nutrients.”

If you are practicing an alternate day fasting regimen in which you eat under 500 calories on your fast days, you can either eat these calories in a single meal or divide them into three small meals throughout the day. Allowing for flexible meal timing on these fast days may help you maintain your fasting practice long term.

Is there a best way to break my fast?

Scientific research studies have not yet fully evaluated the efficacy of different types of post-fast meals on metabolic health in humans.

A handful of existing studies suggest that some people may experience an acute blood sugar spike, associated with insulin resistance, following a post-fast meal high in carbohydrates. However, this acute postprandial insulin resistance may be more likely to occur in individuals not accustomed to prolonged periods of fasting (16–24 hours). If you are new to fasting, you may want to break your overnight or longer fasts with meals low in glycemic index and high in fiber and plant fats (olives, seeds, nuts, coconut, avocado, etc.) After practicing intermittent fasting for several months, your body will likely experience different metabolic reactions to an influx of nutrients post-fast. IF generally leads to improved insulin sensitivity and lower blood glucose levels over time.

What should I eat when I’m not fasting?

Fasting has metabolic health benefits independent of what you are eating during your feeding periods. However, fasting will not turn a junk food diet into a good one.

In a study of people practicing IF and eating either a regular (25% fat) or high-fat diet (45% fat), Dr. Krista Varady found no significant differences in metabolic health between these two treatment groups over time. Each group displayed similar decreases in weight and coronary heart disease risk factors including LDL cholesterol and triglyceride levels.

Varady is currently exploring whether there are any links between fasting, low-carb diets and improved metabolic health. Many people who practice IF pair their fasting schedule with a ketogenic or very low-carb diet. However, there is little scientific research from human trials to determine any benefits or issues of pairing these interventions. The problem with ketogenic dieting is that it often leads people to eat an overabundance of unhealthy animal fats as opposed to healthier plant fats.

“I’d recommend eating your typical American Heart Association diet, or a balanced diet of protein, carbs and fats,” Dr. Krista Varady says.

An anti-inflammatory diet may reinforce the metabolic health benefits of fasting. Credit: IGphotography

Is it OK to exercise while fasting?

Absolutely. In fact, combining fasting with endurance exercise can produce superior changes in body weight, body composition and lipid indicators of heart disease risk, compared to either exercise or fasting alone.

Contrary to popular belief, it is safe to exercise on alternate fast days or during fasts not exceeding 24 hours. In fact, in studies of IF, research participants often report boosts of energy on days when they fast or eat fewer than 500 calories.

“In the first 10 days of an alternate day fasting practice, you may notice decreases in energy or concentration levels,” Dr. Krista Varady says. “But after these first 10 days, most people find it easy to still exercise on their fast days.”

In one of the first studies to combine fasting and exercise interventions, Varady ran a 12-week study of intermittent fasting combined with endurance exercise (brisk walking / cycling). Obese research participants on an alternate day fasting schedule came into the lab three days per week to exercise on stationary bikes and treadmills. When given the option of training during either “feast” or fast days, participants chose to exercise on fast days just as often as on feast days.

If your primary goal is weight loss, however, you may want to wait until after exercise to eat during an alternate fast day, according to Varady’s study. Participants who consumed their one allotted fast day meal before exercise tended to get a burst of hunger an hour or so after exercising, leading them to often cheat and eat extra calories that day.

Regular exercise, especially weight-bearing or resistance training, can alleviate muscle loss during IF regimens that lead to weight loss. Credit: andresr

Will I lose muscle if I practice intermittent fasting long term?

As in all forms of dietary restriction that lead to weight loss, intermittent fasting may cause both fat and lean mass loss over time. However, there is some evidence that losing weight through IF may lead to less muscle loss as compared to fat loss than traditional dieting or caloric restriction. While traditional dieting without exercise typically leads to body mass loss that is 75% fat, 25% muscle, IF may lead to either an equivalent or smaller percentage of muscle loss, or as little as 10%. However, you will always lose some percentage of muscle when you lose weight. Regular exercise, especially weight-bearing or resistance training, can alleviate muscle loss during IF regimens that lead to weight loss.

Should I still fast if I’m close to my ideal body weight?

Intermittent fasting has potential benefits independent of weight loss, including increased insulin sensitivity, fat oxidation and reduced inflammation. You can still practice IF even if you don’t want to lose weight, although you will need to make a concentrated effort to eat enough on your “feast” days or during your daily feeding window to maintain your current weight and energy use.

“People who don’t want to lose weight can add a few more calories to their fast days,” Dr. Krista Varady says. In a study of research participants of healthy weight, she found that these people lost half a pound per week, on average, while practicing alternate day fasting, while obese individuals lost 2–3 pounds per week. “Healthy individuals need to make sure they get enough calories and carefully monitor their weight to make sure they don’t fall into the underweight category. But they can still expect to see metabolic benefits with very little weight loss.”

Will I feel hungry while fasting?

Studies of intermittent fasting have generally not shown any significant impacts on ghrelin, the gut hunger hormone. It’s normal to feel hungry while fasted, even after practicing intermittent fasting for several months. Drinking plenty of calorie-free fluids or eating up to 500 calories on a fast day can help manage your feelings of hunger while in a fasted state.

In studies of alternate day fasting, Dr. Krista Varady has tracked dietary restraint, hunger, fullness hormones including glucagon-like peptide-1 (GLP-1) and peptide YY (PYY), and adipokines or cell signaling proteins secreted by adipose tissue including leptin. In clinical studies, IF has been found to decrease leptin, a hormone that helps control energy expenditure, and increase experiences of satiety after meals.

How long does it take to get into ketosis?

It can take anywhere from 12 to 24 hours to get into a state of partial ketosis. During a fast, once your serum glucose level drops by 20% your liver will start producing ketones to supplement the energy your brain needs.

Achieving full ketosis can up to 10 days on a ketogenic diet. Upon entering ketosis, some people notice a fruity smell on their breath, a decrease in appetite or lethargy, while other people don’t experience these symptoms at all. Remaining in full ketosis over time requires consuming fewer than 30 grams of carbohydrates per day.

The metabolic benefits of intermittent fasting, however, don’t depend on achieving a state of full ketosis while fasting. In studies of healthy obese individuals, Dr. Krista Varady has observed insulin resistance levels decrease by up to 40% when these individuals stick to an IF regimen for just a few weeks.

Late night snacking? Try setting your feeding window to end early evening. Credit: domoyega

Will it be difficult for me to start an intermittent fasting practice if I’m accustomed to snacking or eating every few hours?

First-time fasters often find the first 10 days of a time-restricted feeding or alternate day fasting regimen to be the toughest. According to Dr. Krista Varady, it typically takes people an average of five fast days to become accustomed psychologically and in terms of experiencing normal energy levels during fast days. However, after the first 10 days most individuals can adhere to their fast days or fasting windows and exercise as easily while fasted as fed.

Life and Tech @ LifeOmic

LifeOmic Engineering Blog

Paige Brown Jarreau

Written by

#SciComm nerd. Intermittent Faster. Director of Social Media for @LifeOmic. I’m a science blogger, blog researcher and social media consultant. Ask me anything!

Life and Tech @ LifeOmic

LifeOmic Engineering Blog

Paige Brown Jarreau

Written by

#SciComm nerd. Intermittent Faster. Director of Social Media for @LifeOmic. I’m a science blogger, blog researcher and social media consultant. Ask me anything!

Life and Tech @ LifeOmic

LifeOmic Engineering Blog

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