Intermittent fasting, or going 12 to 36 hours with minimal to no calories on an intermittent basis, is a health trend that has robust underpinnings in science of our cellular metabolism. Fasting even has an evolutionary rationale. It’s why we consider it to be one of the primary pillars of health, along with exercise, plant-heavy nutrition, sleep and mindfulness or stress reduction. Intermittent fasting is the subject of over 100,000 peer-reviewed papers listed on! It has been studied as an intervention in both animal research and a range of human studies and randomized controlled clinical trials. Its safety as a lifestyle intervention has been investigated in a number of human trials, in terms of impacts on health and even on eating disorder symptoms.

If you are concerned about the safety of intermittent fasting, whether you practice it yourself or watch this health trend from the outside, perhaps in curious disbelief about how someone could go for 16 hours without food, there’s good news. Human studies of intermittent fasting have overwhelmingly reported positive outcomes and health benefits for this lifestyle intervention. These benefits have been found particularly among individuals who are overweight and/or suffering from mild metabolic dysfunction (e.g. prediabetes) and inflammation.

Most people can benefit from practicing moderate intermittent fasting (under 24 hours at a time), with some exceptions. But too much of a good thing almost inevitably becomes a bad thing, especially when it comes to our health.

Learn to fast safely – jump to our section below on how to practice overnight fasting, 16:8, OMAD, 5:2, alternate day fasting and prolonged fasting safely.

Know safe intermittent fasting practices, which include having at least a few hundred calories per day during 24-hour and longer fasts.
Know safe intermittent fasting practices, which include having at least a few hundred calories per day during 24-hour and longer fasts.

As scientists at LifeOmic, we understand that precision is everything. As with any kind of health lifestyle intervention, whether related to diet, exercise or something else, it is important to recognize that not all of these interventions are created equal. They nearly all have a dose-dependent response.

Exercise is the leading lifestyle intervention known to extend healthspan and delay or prevent diseases of aging. Most Americans don’t meet the national guidelines for physical activity; they rarely reach the goal of 30 minutes, most days of the week, of moderate to vigorous physical activity to maintain their health and prevent chronic disease. And yet there does exist a U-shaped relationship between exercise and heart-related death. Extreme exercise or overtraining can cause stress, heart health issues and even sudden death, with symptoms of overtraining including fatigue, insomnia, metabolic imbalances and depression. In terms of overtraining, we are talking about vigorous activity practiced at 5 to 10 times the recommended weekly amount.

“Exercise is unparalleled for its ability to improve [cardiovascular] health, quality of life, and overall longevity. If the current mantra “exercise is medicine” is embraced, [physical activity] might be best analogized as a drug, with indications and contraindications, as well as issues related to underdosing and overdosing. As with any powerful therapy, establishing the safe and effective dose range is fundamentally important – an insufficiently low dose may not bestow full benefits, whereas an overdose may produce dangerous adverse effects that outweigh its benefits.” – O’Keefe et al., 2014

The same relationship is likely true with intermittent fasting. At moderate amounts, intermittent fasting has been repeatedly found to promote sustainable weight loss, reduce inflammation, improve lipid levels and even improve blood glucose regulation in some cases. But ideal fasting durations and schedules depend on individual factors such as weight and energy requirements, nutritional and disease status.

Intermittent fasting serves as a metabolic switch from sugar burning to fat burning that lowers insulin levels, reduces mTOR signaling – which drives cell growth and proliferation – and activates autophagy (molecular recycling). Autophagy, which our body ramps down when it senses that we have plenty of dietary glucose and protein around, is a cellular clean-up process that is critical for the proper functioning of our tissues (such as the brain) and clearing of dysfunctional proteins that can cause neurodegenerative disease, for example. This is part of what explains why intermittent fasting is much better for our tissue function, especially as we age, than constant grazing and snacking. Eating constantly, at all hours of the day, chronically elevates our insulin levels and prevents autophagy.

But of course, just as with overtraining, too much fasting can become a bad thing. “Over-fasting”, which may or may not involve a disordered relationship to food, may be marked by health issues similar to those of undernutrition and chronic stress. Undernutrition and malnutrition (marked by nutrient deficiencies) can lead to fatigue, a weakened immune system, slow wound healing, loss of muscle mass, gut issues, stress, depression and other health issues.

Human studies and clinical trials of the safety and impacts of intermittent fasting have focused on intermittent fasting interventions that involve going without calories for 12 to 18 hours per day (as in this study of an early-day 6-hour eating window) and alternate day or 5:2 fasting interventions that involve eating fewer than 500 calories within a 24 to 36-hour window every other day or two days per week. These moderate intermittent fasting interventions have generally been found to be very safe (at least on the scale of 6 months to a year). Most participants in these studies have been overweight or at a normal weight. Moderate intermittent fasting generally produces relatively slow, steady and sustainable weight loss. It also can produce improvements in mental stress, physiological stress, inflammation, insulin sensitivity and blood sugar control. Women who fast for 12 or more hours overnight also report better sleep patterns. They also have a reduced breast cancer risk.

Intermittent researcher Dr. Krista Varady has also investigated the impacts of various intermittent fasting interventions on disordered eating patterns and body image perceptions. In a study published in 2015, she found that alternate day fasting did not cause increased eating disorder symptoms among 59 obese subjects. In a study published this year, she and colleagues found that a 12-week time restricted feeding intervention (a 16:8 fasting schedule) had no negative impacts in obese subjects in terms of physical and mental health. Individuals in the study instructed to eat within an 8-hour window every day for 12 weeks lost significant body weight (average of 2.6% body weight reduction). But they had no changes in their blood cell counts (reflective of immune function) and no increase in nausea, constipation, negative body image perceptions, binge eating or purgative behaviors, avoidance of “forbidden foods”, uncontrolled eating, irritability or fatigue, depression or other disordered eating patterns.

Young woman jumping box and talc powder departs from under feet.
Tune into how fasting makes you feel. After an initial adjustment period, you shouldn’t experience fatigue or loss of energy or rapid and unsustainable weight loss due to fasting. Hydrate while you fast and eat enough before and after your fasts to maintain your energy requirements. You should be able to fast without weight loss by adjusting your calorie intake during your eating windows if necessary.

Dangerous Fasting?

Not all types of fasting are safe or supported by scientific research. Even “safe” forms of intermittent fasting tested in human trials are not safe for all people. To fast safely, you should 1) consult your primary care physician, and 2) avoid prolonged water fasting or fasting with only water (no calories) beyond 36 hours if you don’t have a medical reason for doing so (for example, substantial weight loss, curbing cancer therapy side effects, etc.) and haven’t consulted your physician.

There are types of fasting that have been invented and popularized by people outside of the scientific and medical communities (or by quacks) that are dangerous. These include dry fasting, or fasting without water, which has zero scientific weight behind it. No human studies or clinical trials have investigated the impacts of dry fasting, and it does not promote autophagy – a cellular clean-up process – any more than does water fasting, despite popular claims. Where this practice has been studied, in the context of people who dry fast for religious reasons or in hunger strikes, it has been found to have very harmful health impacts and can result in sudden death.  

“In dry fasting, the person refuses all solid or fluid intake. Death occurs in 4 to 10 days, depending on factors such as ambient temperature and humidity and the striker’s level of stress and physical activity. Severe electrolytic imbalance can rapidly cause death due to cardiac arrhythmia or damage to the central nervous system. A hypovolemic state [severe blood or fluid loss makes the heart unable to pump enough blood to the body] causes multiorgan dysfunction and acute renal insufficiency.” – Restellini & Restellini, Prisons and Health, WHO

If you do practice a form of fasting not mentioned here, make sure that this form of fasting has been shown to be safe in clinical trials. Use or Google Scholar to search for published research studies reporting the results of clinical trials. If you aren’t sure, ask your physician or a clinical registered dietitian for advice.

Safety Concerns of Intermittent Fasting

The primary safety concerns associated with overdoing intermittent fasting include undernutrition and malnutrition. In women, some forms of intermittent fasting could also impact menstrual cycles and reproductive health, although again the dangers reside primarily in under- and malnutrition. Substantial weight loss, and even excessive exercise, can cause irregular or absent menstrual periods.

People who are underweight, who have a nutritional deficiency or who are at risk of such (for example, cancer patients suffering rapid weight loss and people with certain eating disorders) should not practice intermittent fasting. More specifically, they should not regularly practice anything over a typical overnight fast (~12 hours). Other people who should not be fasting include pregnant women and people with type 1 diabetes.

There isn’t any evidence to suggest that intermittent fasting can promote or spur the development of disordered eating. However, it is important to maintain a healthy relationship with your body and food while practicing fasting, not “overtraining”, and staying vigilant of the symptoms of undernutrition and malnutrition. The societal acceptance of fasting as a popular health trend may hide the signs and symptoms of eating disorders for certain individuals and their friends and family members. An eating disorder is “a type of mental illness involving abnormal eating patterns that can negatively impact an individual’s health and feelings about their body.” It requires diagnosis and treatment by a medical professional.

Learn more about the warning signs and symptoms of anorexia nervosa.

It is important to point out that one person’s healthy intermittent fasting schedule could be another person’s eating disorder, regardless of their weight or other health metrics. A good rule of thumb for practicing intermittent fasting safely is to take note of your relationship to your fasts, your food and your body. Do you enjoy re-feeding as much as fasting? Do you have positive perceptions of and emotions about your body? Do you let yourself enjoy any food in moderation without judgement, or you see certain foods as the enemy? Do you eat to feel good and full of energy? Is it enjoyable to eat? Do you know how to identify true feelings of hunger and differentiate them from food cravings and emotional triggers? As long as you have these positive relationships with your body, food and fasting, you have the tools to fast safely.

The re-feeding phase of fasting is one of if not the most important components of intermittent fasting. The cellular re-building and growth that happens after a fast is partly responsible for the tissue function and cognitive function improvements seen with fasting interventions. New proteins are formed to replace old recycled one. More effective cells and tissues, such as muscle cells with shiny new mitochondria formed as a result of temporary energy deprivation, produce fewer reactive oxygen species and inflammation. So it’s just as important to eat a healthy, balanced diet and enough calories to meet your body’s energy demands as it is to fast to improve your metabolic health.

Fast with a friend and follow the other safety guidelines below to avoid creating health issues with your approach to fasting.
Fast with a friend and follow the other safety guidelines below to avoid creating health issues with your approach to fasting.

Fasting Safely, by Intermittent Fasting Schedule

Time-restricted eating (12-18 hours)

Almost anyone can practice daily time-restricted eating or overnight fasting safely, even normal weight individuals and athletes [see here and here]. Some of what we know about the safety and metabolic health benefits of time-restricted eating comes from observational studies of Ramadan fasting in healthy individuals. Time-restricted eating involves fasting for 12 to 18 hours per day, while eating the other 12 to 6 hours. A time-restricted eating approach to fasting can be practiced with or without calorie restriction. In other words, you might naturally reduce your daily calorie intake as well as improve your blood sugar control by not eating salty and sweet snacks close to bedtime, but you can practice fasting this way without losing weight.

I’ve personally been practicing an 16:8 fasting schedule on a near daily basis for over a year now; my BMI is 20.5 (normal range) with lots of muscle. I have not lost any weight in a year of time-restricted eating, on average. I’ve also occasionally practiced a 24-hour fast and even one 5-day modified fast.

To practice time-restricted eating safely:

One-meal-a-day, or 20:4 and longer

Time-restricted eating can also be practiced with a tighter eating window, on the range of 2-4 hours. This is also referred to by some as one-meal-a-day or OMAD, where you’ll be eating all of your daily calories within one (often longish) meal.

With a shorter daily eating window, it becomes even more important to ensure that you are eating quality calories and enough of them during this window to account for your energy demands, particularly if you are at a normal weight.

To practice OMAD safely:

  • Talk to your physician before upping your fasting window beyond 18 hours per day.
  • As with other time-restricted eating schedules, adjust your caloric intake during your feeding windows so that you consume enough calories to meet your energy/exercise demands and weight maintenance or weight loss goals. Calculate your daily calorie needs here.
  • Follow our best practices for avoiding blood sugar spikes when you break your fast; favor low glycemic index meals when you break your fast, especially longer fasts.
  • Make sure that in your short daily eating window you can still consume the daily recommended amounts of vegetables and fruits for a healthy diet, along with other nutrients and micronutrients such as fiber, protein, vitamins etc. You’ll likely need to plan out your meals and nutrient intake more carefully than someone who has all day to achieve these nutritional goals. There’s also good evidence that you can’t just take supplements or vitamin pills to achieve these nutritional goals with the same health benefits.
  • Monitor your blood ketone and blood glucose levels while fasting and before, during and after your OMAD meal, to ensure that you are staying within a healthy blood glucose range. You might want to save higher glycemic index foods and carbohydrates for later in your fasting window.
  • Consider taking breaks from your OMAD schedule one or more days per week, especially if you are at a normal weight.
  • 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.

5:2 Diet and Alternate-day fasting

The 5:2 and alternate-day approaches to intermittent fasting are the most commonly investigated fasting interventions for weight loss and related health impacts. They’ve most often been investigated in clinical research studies to determine their safety and impact in overweight individuals and individuals with metabolic syndrome or inflammatory disorders. They involve alternating days (24-36 hours) of limited calories (usually around 500 calories, or 25% of one’s daily energy requirement), either every other day or two days per week. In a 5:2 diet, the fasting days can be consecutive or nonconsecutive, with some evidence that consecutive fasting days help more robustly elevate serum levels of stress-busting ketones and reduce insulin resistance.

To practice 5:2 or alternate-day fasting safely:

  • Talk to your physician before starting this fasting schedule.
  • Favor the 5:2 diet or even just one fasting day (<500 calories) per week if you are already at a normal weight. There is evidence that the 5:2 and alternate-day approaches, while they have health benefits for overweight individuals, are not well tolerated in normal weight individuals. These individuals may experience distracting levels of hunger and irritability while fasting, and transient insulin resistance at the end of a fast due to high levels of free fatty acids in their bloodstream (making it more important to break their fasts with low glycemic index foods). However, other studies have found health benefits with no tolerance issues for normal weight individuals who practice alternate-day fasting.
  • Pay attention to what your body needs and eat normally on “feast” days. Don’t “binge” or consciously try to eat extra calories. If you are at a normal weight, you will naturally consume some extra calories on these “feasting” days, which is fine.
  • As for the other types of fasting listed above, maintain a balanced diet with plenty of vegetables and fruits on your non-fast days.

Periodic / Prolonged fasting

There is limited research on the impacts or safety of long-term periodic fasting for three to five days at a time. Dr. Valter Longo, Director of the USC Longevity Institute, has conducted some clinical research investigating the impact and safety of fasting-mimicking diets, including his ProLon regimen. This regimen involves five “fasting” days of consuming a limited number of daily calories (~1,000) derived primarily from plant fats and vegetables, with low protein and carbohydrate content.

Prolonged fasting that has scientific backing is similar to alternate day fasting protocols, but with up to five days of consecutive fasting, days of consuming less than 500-1000 calories. The goal of this type of fasting is cellular rejuvenation, particularly the activation of autophagy and the lowering of inflammation.

If the complexity of fasting-mimicking diets such as ProLon tells us anything it is that prolonged fasts are not interventions to take lightly. Practiced sparingly (no more than once per month), 3-day to 5-day fasts may help with weight loss and health issues such as chronic inflammation and immune or metabolic dysfunction. However, these fasts should not be practiced without consultation with a physician, and should never be practiced consecutively.

To practice multi-day fasts safely:

  • Consult your physician and a registered dietitian before starting a multi-day fast. You may need to modify medication timings and dosages during the fast, etc. Talk to a dietitian about hydration and possible supplemental electrolyte requirements during a multi-day fast.
  • Reconsider attempting a multi-day fast if you don’t have a meaningful health or medical reason for doing so. Multi-day fasting and fasting-mimicking diets have been investigated as interventions for people with significant chronic inflammatory conditions, cancer, age-related declines, brain injury, etc. The impact of such interventions on health in especially young, healthy and normal weight adults is unclear.
  • Be at a healthy weight before you start a multi-day fast.
  • Never fast alone. Always tell someone, a family member or friend (in addition to your physician) that you are doing a multi-day fast and keep them updated on how you are feeling as you progress through your fast.
  • 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.
  • Watch for signs of electrolyte imbalance: muscle spasms, weakness, blood pressure changes, irregular heartbeat, numbness and confusion.
  • 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.
  • Take care to replenish your body based on national nutritional goals and recommendations before and after a prolonged fast.


Have other safety questions about intermittent fasting? Let us know. With our LIFE Fasting Tracker app and the adoption of intermittent fasting as one of our 5 Pillars of healthspan, we always put safety first. We have zero tolerance for user-generated Circles and social content in our app that promotes disordered eating patterns or other forms of self-harm.