Paige Jarreau, PhD
Paige is the VP of Science Communication for LifeOmic and an avid blogger. She’s interested in how scientists use social media to promote public engagement and health behaviors.

Paige Jarreau, PhD
Paige is the VP of Science Communication for LifeOmic and an avid blogger. She’s interested in how scientists use social media to promote public engagement and health behaviors.

This beginner’s guide to intermittent fasting is for those who have just started their journey with intermittent fasting. You might be a bit overwhelmed with options and questions. Should you fast everyday, or practice the popular 5:2 schedule? Can you drink coffee and bone broth? Should you fast through dinner or breakfast? How long should you fast and how often?

People in the online fasting community love to share their personal experiences with fasting, which is great! However, just as with diet and exercise, one size of intermittent fasting does not fit all. The best way to develop an intermittent fasting practice that advances your health is to self-experiment. Also, speak with your own healthcare team, primary care physician or dietitian/nutritionist if you have any questions about how fasting fits in with your own health goals, health status, medications, diet, etc.

To help you get started, we’ve created the beginner’s guide for intermittent fasting below. We have answers to the questions that many people have when they first start fasting!

What you will find in this article:

What is intermittent fasting?

Intermittent fasting methods

Fasting FAQ

Shot of a group of friends having a meal together outdoors.
How about having an early dinner this week followed by a late breakfast? Easy, peasy overnight fast!

What is intermittent fasting?

Fasting is an ancient practice that involves avoiding calories for a period of time. It is a simple way to control your weight that doesn’t require you to follow a specific diet, count calories or measure food.

Intermittent fasting methods

There are many ways to fast and to reap the underlying benefits of going for some time without calories. 

Common intermittent fasting schedules include:

Time-restricted eating

This involves consuming all of your meals within a period of 4-12 hours in a day. For example, you could choose to have your meals and calories between 8 am and 6 pm. Limiting your calorie intake to daylight hours is called “eating with the sun”. Time-restricted eating is particularly beneficial if you eat your meals before sunset. Not only does this help you eliminate bad habits like late-night snacking, but it improves your sleep and blood sugar control. Over time, time-restricted eating can lower your blood sugar levels, make you more sensitive to insulin, and reduce your blood pressure.

One-meal-a-day (OMAD)

This involves packing all of your day’s calories in a single meal that you consume within 1-2 hours. A small study showed that fasting for 24 hours 3 times a week and eating only dinner on fasting days, eliminated the need for insulin in type-2 diabetes patients. This fasting approach also resulted in improved HbA1C, lower body mass index, and reduced waist circumference.

Alternate-day Fasting (ADF)

This involves alternating between days in which you consume no calories and days in which you eat normally. Another way of looking at ADF is as three 36-hour fasts per week.  For example, eat normally on Monday until 6pm. Don’t eat until breakfast on Wednesday morning. Finish dinner by around 6pm Wednesday night. Now don’t eat again until breakfast on Friday. Eat normally the rest of the day and finish dinner by 6pm or so Friday night. Now don’t eat again until breakfast on Sunday. This way of fasting increases life span in rodents by 80%, possibly by decreasing glucose levels and regulating insulin. It also reduces the incidence of cancer in lab animals genetically predisposed to it. Alternate-day fasting is the most studied method of fasting in humans, and it’s been shown to lead to weight loss and reduce circulating levels of LDL or ‘bad’ cholesterol.

The 5:2 diet

This involves fasting for two days per week, either consecutively or non-consecutively, and eating normally the rest of the week. For example, you might decide to fast Monday and Tuesday and then eat normally the rest of the week. You can also look at it as one 60-hour fast per week. Finish dinner at 6pm on Sunday night and don’t eat again until breakfast on Wednesday morning, for example.

Prolonged fasting for 3-5 days

This involves going with no calories for 3-5 days. These types of fasts are usually done under medical supervision. All studies of prolonged fasting allow up to 500 calories during fasting days. These calories should come from low-protein, low-carb foods. This usually means vegetable juices and vegetable soup, averaging a total calorie intake of 200–250 kcal and 25–35 g of carbohydrates per day. Our CEO does quarterly 5-day modified fasts in which he eats 2 cups of mixed greens with olive oil and a handful of almonds for a total of 250 calories.

Do not attempt a multi-day fast with water only. All clinical trials of multi-day fasts involve at least minimal calorie and micronutrient intake.

Fasting FAQ

Is intermittent fasting safe for me?

Most people can benefit from practicing moderate fasting. However,  pregnant or lactating women, children, individuals with type-1 diabetes, individuals who are malnourished, and those who have struggled with eating disorders should not fast for weight loss.

Some general safety tips

  • Drink water when you are thirsty.
  • Adjust your caloric intake during your feeding windows so that you consume enough calories to meet your energy needs according to your weight goals.
  • If you feel faint, dizzy or nauseous, we recommend breaking your fast early and speaking with your primary care physician. You may have symptoms of low blood sugar or another issue.
  • Talk to your physician before upping your fasting window beyond 18 hours per day.
  • Break your fast (especially fasts of over 24 hours) with low glycemic index foods, such as lean protein and healthy fats like avocados
  • Never do fasts longer than 2 days by yourself. Always tell someone, a family member or friend (in addition to your physician) and keep them updated on how you are feeling as you progress.
  • During fasts longer than 2 days, watch for signs of electrolyte imbalance: muscle spasms, weakness, blood pressure changes, irregular heartbeat, numbness and confusion.

Why should I fast?

There are many benefits of intermittent fasting. They include weight loss, reduced inflammation, improved lipid levels or biomarkers of heart health, improved blood sugar control, improved insulin sensitivity and increased cellular recycling known scientifically as “autophagy.” Overnight fasting can also help prevent weight gain and preserve metabolic health and glycemic (blood sugar) control. Intermittent fasting may be particularly beneficial for overweight individuals in terms of improving their expected healthspan, or length of healthy life.

Fast with friends for social support.
Are you a breakfast person? If not, prolonged overnight fasting could be natural for you! Could you go an extra few hours in the morning without eating anything? Try drinking some tea or coffee and bringing some fruit to work with you to eat breakfast a bit later! If you do enjoy breakfast, try eating an early-bird dinner and not having any calories after that until bed. (Sleeping counts as fasting!)

Why does fasting work for weight loss?

Intermittent fasting naturally makes you cut your daily energy intake without the work of counting calories or measuring what you eat. Those who practice alternate-day fasting, the 5:2 diet, or time-restricted eating reduce their daily calorie intake by 10-30%. Intermittent fasting also induces a metabolic switch around 12 hours into a fast: Your body switches from burning sugar to burning fat to generate energy, which leads to weight loss.

Will I lose muscle mass with intermittent fasting?

A common misperception about fasting is that the body quickly starts to break down muscle during periods without food. This would actually be pretty dumb on the part of our bodies – to tear down valuable muscles and other proteins when another source of energy, fat, is so plentiful. In reality, we see very little muscle loss during fasts of short duration. In fact, those who lose weight with intermittent fasting lose less muscle mass compared to those who lose weight with continuous calorie restriction.

I’ve never fasted before.  How should I start?

If you’ve never water fasted before, we recommend that you start with overnight fasting. This form of fasting has added benefits of reinforcing healthy circadian rhythms if you start fasting a few hours before bedtime – even your gut microbes and brain will love it! You can practice this fasting schedule most nights of the week, if you’d like.

Even 12 hours of overnight fasting can require a few weeks of adjustment for many people. But it should get easier (with little to no hunger pains) within a month! If you normally eat a snack or enjoy a drink late in the evening, try to move that up to at least 1-2 hours before you go to bed. Then try waking up and just drinking some hot tea or coffee with no cream or sugar.

If you can begin to fast for 10, 12 or even 14 hours overnight, you’ll be well on your way to an intermittent fasting lifestyle! You can work up from there. Let us know how it goes – share your story with us on Instagram or Facebook!

How can I fast if I work the night shift?

We recommend fasting through your shift and having your meals at home. Eat at home before you go to your night shift. While you work, use bone broth or coffee while you adjust. When you get off in the morning, have your breakfast at home and go to sleep. This way you get in 12 hours of fasting and avoid eating during the night when digestion is slowest. Another alternative is going to sleep immediately after you get home. When you wake up, have your first meal and eat again before your night shift.

How long and how often should I fast for?

Great question! How long and how often you should fast for on a regular basis depends on your health goals and current health status. For example, if you are overweight or have a high BMI, you might be able to fast more often that someone who is already lean. However, you still need to ensure that your daily eating window gives you enough time to consume a healthy variety of fruits, vegetables, legumes, nuts and plant oils, whole grains and protein.

It’s best to begin your intermittent fasting journey by progressively working up from overnight fasting (12+ hours per night). And yes, sleeping counts as fasting! If you are looking for significant weight loss, you might consider working up to 18-20 hours of daily fasting (OMAD or one-meal-a-day), alternate day fasting (fasting every other day, with up to 500 calories on fasting days) or a 5:2 schedule (fasting for two days per week). Learn more about fasting for weight loss and other benefits here.

But what if I get hangry?!

Hunger is normal during a fast, especially when you first start fasting. However, if you keep a set fasting schedule daily, you’ll probably find that you begin to not even feel hungry until near the end of your typical fasting window. You might also begin to relate to your hunger differently, recognizing true hunger from cravings.

But don’t worry – even on fasting days you get to eat some calories! Clinical trials of intermittent fasting typically involve having people eat up to 500 calories on fasting days of 24 hours. There are still scientifically validated benefits of having regular 500-calorie fasting days.

If you ever feel very hungry, hangry, irritable or light-headed during a fast, try the following:

  1. Drink some water with added electrolytes! SmartWater or water with lemon and a dash of pink salt are some great choices. You may also need to supplement potassium and magnesium while fasting.
  2. Have a small snack. A small low-carb snack is fine to have during a fast, especially when you first start fasting to help you achieve your goal of 12, 16 or 24 hours. A few calories rich in healthy plant fats or fibers may help ease your stomach groans but won’t substantially decrease your ketone body production or activate insulin signaling and mTOR (the pathway that normally keeps your cellular recycling processes turned off). Based on clinical trials of intermittent fasting, you can have up to 500 calories on fasting days of roughly 24 hours. Try some celery sticks or some bell pepper slices with olive oil or hummus! You can also try drinking a bit of bone broth during a longer fast – this provides both electrolytes and nutrients without substantial calories.
  3. Just end your fast early, especially if you feel dizzy or weak. There’s always another day to fast! This is a lifestyle, so it’s best to be kind to yourself and make it sustainable for you.

What can I drink while fasting?

You can drink anything that doesn’t have calories! Ideally, also avoid beverages sweetened with artificial sweeteners, as these can impact your metabolism through your gut microbes. Some artificial sweeteners raise your insulin levels more than others (aspartame more than stevia, especially when consumed with other calories), but there’s a connection between many artificial sweeteners and negative gut microbe changes, insulin resistance and metabolic disease. While a recent meta-analysis suggests we need further research, it may be best to avoid sucralose, aspartame and saccharin in general.

While fasting, you can drink things like black coffee, unsweetened hot and cold teas, water with added electrolytes, carbonated water, water with lemon, etc. Learn more about what you can drink while fasting here.

If you are feeling like you need a little pick-me-up during a longer fast, you can enjoy some bone broth, some kombucha tea or other beverages with very small amounts of natural flavorings or healthy fats added. For example, you might enjoy some coffee with a little bit of butter or coconut oil added.

Can I have cream and sugar in my coffee while fasting?

To stay in a fasted metabolic state, it’s best not to add any sugar to your coffee. Cream is a better option. If you’ve already fasted overnight for 12 or so hours and you just can’t drink black coffee, a bit of cream, butter (a very small amount), coconut oil, MCT oil or other healthy plant fat likely isn’t going to kick you out of ketosis or a fat-burning state.

At most, these coffee fixings might lengthen the time it takes you to get into ketosis after an overnight fast. They can slightly and temporarily activate your insulin and mTOR pathways, telling your body that nutrients are around and thus delay entry into some fasting metabolic processes such as autophagy (cellular recycling). However, it’s normal in human clinical studies of fasting to allow participants up to 500 calories on fasting days; fasting benefits occur regardless of these few calories!

Sugar is another beast, as it can quickly raise your blood sugar levels. Try to avoid adding sugar into your coffee, opting for cream or other healthy fats instead. Also watch out for sugar-added creamers.

What should I eat to break my fast?

There are many great ways to break a fast – you should experiment with what helps you feel your best. Intermittent fasting is great in that it is a relatively simple intervention that appears to have health benefits regardless of diet.

However, you may notice that with a regular intermittent fasting practice you start to tune into your body and sensations of hunger and satiety more closely. You can let your own hunger sensations while fasting prompt you to explore what your body is really craving and asking for! Also tune into how your body feels as you break your fast, and experiment breaking with different types of foods. I used to break my fast with my favorite breakfast foods of bagels or muffins, but found that I felt nauseous and tired afterwards. I started breaking my fasts with a crisp apple and found that I felt fantastic, with no symptoms reflective of a blood sugar spike. Needless to say, I keep a dozen apples of different colors and varieties in my house at all times now!

A good rule of thumb is to break a fast slowly and intentionally by enjoying a snack or balanced meal with plenty of vegetables and fruit, healthy plant fats (olive oil, coconut oil, avocado, etc.), some protein and some whole grains. Trying going low-carb in the first meal after a long (18+ hours) fast to avoid overly spiking your blood sugar and insulin levels.

Healthy buddha bowl lunch with grilled chicken, quinoa, spinach, avocado, brussels sprouts, broccoli, red beans with sesame seeds.
Healthy buddha bowl lunch with grilled chicken, quinoa, spinach, avocado, brussels sprouts, broccoli, red beans with sesame seeds.

What is ketosis, and how will I know that I’m there?

Ketosis is a metabolic state in which your body starts to break down and burn fat. Ketosis is characterized by the production of ketone bodies in your liver, which include acetoacetate, beta-hydroxybutyrate and the breakdown product of acetoacetate, acetone.

If you have water fasted for over 12 hours, you are likely in at least nutritional or light ketosis! Your liver glycogen has been depleted. Your body is producing ketone bodies as an alternative fuel source for your brain through the breakdown of fat and fatty acids. (Ketone bodies have health benefits for your body and brain, included reduced oxidative stress and inflammation.) You can also reach ketosis with an alternate day fasting or 5:2 protocol with up to 500 calories per fasting day, but it’s best if these calories come from low-carb or keto-friendly foods.

The best way to measure ketosis is through a blood ketone meter or a breath ketone meter. You can measure your ketone levels to see if anything you are drinking during your fast is kicking you out of ketosis. Urine ketone strips are not very accurate. They don’t detect all forms of ketone bodies, such as the important beta-hydroxybutyrate or BHB. So the best bet is to grab a blood ketone meter like Keto-Mojo, or a breath ketone meter like BIOSENSE.

I’m trying to fast but I feel like it’s reducing my energy levels. Is this normal?

It is very normal to feel tired or to have low energy levels when you first start fasting. This can also happen when you first start a new fasting schedule. But if you stick with a set intermittent fasting schedule or fasting duration for 2-3 weeks, you should adapt and have similar energy levels during your fasting windows or days as during your eating windows or days.

However, if you continue to feel tired or to have low energy levels while fasting, you may be overdoing it. You may fasting for too long too often, or not getting enough calories in during your eating windows. If you are already at a healthy BMI, reduce your fast durations or spread your fasts further apart.

To boost your fasting energy levels, you should also:

  1. Stay hydrated! Drink water with added electrolytes during longer fasts. 
  2. Eat enough calories to reach your daily energy requirements when you aren’t fasting. The Mayo Clinic and American Cancer Society have online calorie calculators you can use to determine your daily needs.
  3. You can enjoy some black coffee and tea in the mornings during your fasting windows.
  4. Make sure you get enough sleep – at least 7 hours per night. Don’t push your eating window late into the night if possible.
  5. Keep moving and exercising even while you are fasting! It’s generally safe to exercise while fasting. Physical activity will help with both energy levels and preservation of muscle mass during weight loss.

Can I exercise while fasting, and when is the best timing?

Yes! It’s generally safe to exercise while fasted. However, talk to your physician if you have diabetes or have experienced hypoglycemia during or after fasted exercise.

People practicing alternate day fasting choose to exercise on fasted days as often as on their feeding days. However, when approaching the end of a long fast you may want to stick to light to moderate physical activity.

There aren’t necessarily best practices for exercise timing with fasting. However, there’s evidence that fasted exercise can send you into deeper ketosis (and autophagy, or cell recycling) more quickly. Light exercise right before or after you eat can also help your body soak up glucose more quickly. High intensity exercise can even raise your blood glucose levels temporarily as your muscles release glycogen and your liver produces new glucose, which could leave you feeling uplifted during a prolonged fast. There are many ways to combine fasting with exercise. Learn more here.

How quickly will I lose weight with Intermittent fasting?

The amount and rate of weight loss people achieve with intermittent fasting depends on various factors, including their eating habits and current weight. With alternate day water fasting and the 5:2  overweight and obese individuals typically lose up to 8% of their body weight in 12 weeks

You may experience slower weight loss with daily time-restricted eating (up to 9% weight loss in 12 months) , but if you increase your protein, fruit, vegetable, fiber, and plant fat intake and reduce your carbohydrate intake, even just at some meals, you’ll probably see more rapid weight loss. Some people also find that early time-restricted eating (fasting after sundown) stops their late-night “empty calorie” intake, which can help a lot with both weight loss and metabolic health.

The important thing to remember is that fasting has benefits beyond weight loss, including better sleep, better blood glucose control, lowered insulin levels and greater cellular recycling. Learn more about the weight loss and other benefits of fasting here.

What if fasting slows my metabolism? Will I gain more fat if I stop fasting?

Intermittent fasting does not slow your metabolism, counter to popular belief. In other words, intermittent fasting does not significantly impact your resting metabolic rate (RMR) – it does not decrease the amount of energy your body burns at rest. In fact, fasting may increase the amount of fat you burn over time. See this related study on alternate day fasting and metabolic rate.

Resting energy expenditure has also been found to be better preserved with intermittent fasting than with other forms of weight loss – learn more here. But if you are concerned, another way to increase your resting energy expenditure is to engage in moderate to vigorous physical activity! Examples of this include walking briskly for 1-2 miles or riding a bike in hilly terrain (info via Harvard Health). Also eat plenty of fiber-containing plant foods including fruits, veggies and nuts during your re-feeding windows!

It’s also important to practice intermittent fasting not as a fad diet that you might “rebound” from in terms of weight loss and regain, but as a long-term healthy lifestyle. For example, you could practice 10-16 hours of overnight fasting for the rest of your life with metabolic health benefits even in the absence of weight loss, while occasionally practicing longer or more frequent fasts if you are trying to lose weight.

Find Support!

Doing a fast? Share your experiences with us on Instagram and Facebook  with the hashtags #SeizeLIFEFast #LIFEfastingapp.

Download the LIFE Fasting Tracker app here.

This post was updated by Luisa Torres, PhD.

Paige Jarreau, PhD

I am a health literacy advisor at LifeOmic and a science communicator at Kelly Services and the National Institutes of Health.

LifeOmic® is the software company that leverages the cloud, machine learning and mobile devices to improve healthspans – from prevention and wellness to disease management and treatment.



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