Alternate-Day Fasting: Benefits, Drawbacks & Safer Approaches To Weight Loss
Alternate-day fasting (ADF) is a restrictive type of intermittent fasting that is often used for weight loss. However, its long-term safety is questionable. Here's what you need to know about the benefits and risks of ADF, plus some alternatives to the fasting regimen.
What is alternate-day fasting?
Alternate-day fasting, as the name suggests, involves fasting every other day. So, if you were to follow this fasting schedule, you would eat normally one day and fast the next day.
The alternate-day fasting protocol has gained popularity largely due to the research of Krista Varady, Ph.D., a professor of nutrition at the University of Illinois, Chicago. In her book The Every Other Day Diet, Varady describes how you can lose weight by eating what you want—half of the time.
The idea behind alternate-day fasting is that instead of having to limit your calories every day, as many diet regimens require, you only have to restrict your calorie intake on alternate days.
When you fast, your body starts to burn stored fat for energy, a process known as lipolysis1. Over time, it goes into a metabolic state known as ketosis2, where it uses fat as its main source of energy, instead of sugar.
When done correctly, ADF can also give you the benefits of fasting beyond weight loss. Emerging research shows that alternate-day fasting can be a useful tool for improving metabolic markers, boosting heart health, lowering chronic inflammation, and promoting cell repair, says Madiha Saeed, M.D., an integrative family physician and author of The Holistic Rx.
However, most people will find fasting every alternate day challenging. Furthermore, this diet may not be safe for everyone and should only be undertaken under medical supervision, says Bindiya Gandhi, M.D., an American Board-certified physician who practices family medicine and integrative medicine.
What to eat on fasting days.
People who follow strict alternate-day fasting3 routines don't eat anything at all on their fasting days, leading to an approximately 36-hour period without any calorie intake.
However, others prefer to practice modified alternate-day fasting4, where they restrict their calorie intake to around 500 calories per fasting day (or approximately 25% of their normal caloric intake).
On fasting days, someone who is following a strict fast would limit themselves to calorie-free beverages such as water, green tea, or black coffee.
On the other hand, someone who is practicing a modified version of the alternate-day fast can opt for nutritious but low-calorie meals or snacks during fasting days, such as:
If you decide to practice alternate-day fasting, Gandhi also recommends taking a multivitamin and mineral supplement every day, even on fasting days, to help meet your nutritional needs.
Science-backed benefits of alternate-day fasting.
While the research on alternate-day fasting is still preliminary, emerging results show that it may be able to improve cardiometabolic health, encourage cell repair, reduce inflammation, and improve longevity. Here's a look at the science behind each of these benefits:
It may improve cardiometabolic health.
Research shows that most forms of intermittent fasting, including alternate-day fasting, 5:2 fasting, and time-restricted eating can help improve cardiometabolic health5.
In fact, a 2020 review that analyzed seven randomized controlled trials of alternate-day fasting with a total of 269 participants concluded that it can benefit cardiometabolic health6 by reducing participants' weight, body mass index (BMI), cholesterol levels, triglyceride levels, and blood pressure.
Improved cardiometabolic health can help reduce the risk of developing heart disease, heart attack, and stroke7.
It may enable cell repair.
A 2019 review published in Nutrients notes that alternate-day fasting enables autophagy8, which is an internal housekeeping process that your body uses to repair or dispose of damaged cells and molecules. For instance, the body might patch up or recycle damaged or miscopied proteins or DNA molecules, Steve Hendricks, a health journalist, previously told mindbodygreen.
By increasing autophagy9, fasting essentially helps clean and reboot your cells, Chris Palmer, M.D., a psychiatrist at Harvard Medical School, previously told mindbodygreen.
Your body's ability to repair itself declines with age10, but by promoting autophagy, alternate-day fasting may help prevent age-related health conditions.
It may reduce inflammation.
Inflammation is your body's natural defense against injury or illness. However, inflammation can be harmful if it persists for too long. Chronic inflammation has the opposite effect on your system, and it increases your risk of developing chronic diseases.
Inflammation can be measured by checking your levels of C-reactive protein (CRP), via a hs-CRP blood test. A 2023 randomized control trial with 80 participants found that alternate-day fasting helped reduce hs-CRP levels11 in patients with metabolic syndrome.
A 2022 review notes that the various types of intermittent fasting may be able to prevent or improve inflammatory and autoimmune conditions12 such as diabetes, arthritis, psoriasis, spondylitis, multiple sclerosis, and asthma.
It may promote longevity.
Alternate-day fasting can help improve molecular and physiological markers of aging, according to Saeed.
For instance, a 2019 randomized controlled trial with healthy adult participants found that within four weeks of practicing alternate-day fasting, participants had higher levels of beta-hydroxybutyrate13 in their blood. This compound delays aging and promotes longevity, Saeed explains.
The study also found that alternate-day fasting helped reduce methionine, an amino acid associated with aging14, and sICAM-1, an age-associated inflammatory marker.
It's worth noting that improved cardiometabolic markers, increased autophagy, and reduced inflammation also contribute to improved longevity.
Is alternate-day fasting effective for weight loss?
Alternate-day fasting can be an effective way to achieve weight loss, says Saeed. "Research has found that it can help you lose between 4.5 to 6 kilograms15 (10 to 13 pounds) of body weight in three months."
A 2023 review notes that in addition to reducing body weight, it can also be an effective way to reduce11 body mass index (BMI), fat mass, waist circumference (WC), and waist-hip ratio (WHR).
However, Saeed explains that weight loss depends on various factors. For instance, she says making sure you exercise while you're following this fasting regimen can help you preserve muscle mass while losing fat. On the other hand, she cautions that eating excess calories on feeding days will derail weight loss goals16.
Therefore, it's important to eat a balanced and nutritious diet on feeding days and maintain an active lifestyle while you follow this fasting regimen.
If you've tried alternate-day fasting for weight loss and found it too restrictive, it may be helpful to try other forms of fasting that are easier to sustain, such as 16:8 fasting. The healthiest weight management strategy is the one you can stick with in the long term.
Downsides and side effects.
While alternate-day fasting can be helpful to some people, it may not be safe or appropriate for everyone.
Gandhi recommends avoiding this fasting regimen if you are under 18 years old, breastfeeding, pregnant or planning on becoming pregnant, underweight, or have a history of eating disorders.
If you do plan on trying alternate-day fasting, Gandhi recommends working with a health care provider such as a doctor or dietitian who can determine whether it's safe for you and tailor it to your needs.
It's important to practice fasting under medical supervision. Otherwise, you can easily become fatigued or dehydrated, or experience vitamin, mineral, and electrolyte deficiencies, says Gandhi.
Other side effects of fasting17 include fatigue, hunger, low mood, irritability, difficulty concentrating, food-related thoughts, and fear of losing control and overeating on non-fasting days. "If you ever experience blood sugar instability, dehydration, or fainting, you need to stop and seek help immediately," says Gandhi.
Fasting can also lead to orthorexia18, an obsession with healthy eating, and contribute to the development of other eating disorders, Gandhi adds.
How long can you do it safely?
While the ideal length of time to follow this fasting protocol varies from person to person, it's not advisable to follow such a restrictive fasting plan long term, Gandhi cautions.
Furthermore, most people will find it difficult to comply with such a challenging fasting regimen on a long-term basis. Research studies often see a high dropout rate, putting the sustainability of the diet into question.
An analysis of an alternate-day fasting study conducted over 12 months found that people had difficulty sticking to their calorie goals19 on both fasting and feeding days; they were eating more calories than they should on fasting days and fewer calories than they should on feeding days, so by the end, their diet more closely resembled that of a traditional calorie restriction diet.
Other types of fasting.
If you want to try intermittent fasting but think that alternate-day fasting may be too challenging for you, these are some less restrictive types of fasting to explore:
- 5:2 fasting: Instead of fasting every alternate day, 5:2 fasting involves eating normally for five days a week and fasting twice a week. On fasting days, you consume 25% of your normal calorie intake. You can choose which two days of the week you want to fast, depending on your schedule, and they don't have to be consecutive. This is essentially a modified, less restrictive form of alternate-day fasting. Research shows that it benefits cardiometabolic health5, improves blood sugar levels20, and promotes weight loss21.
- 18:6 fasting: 18:6 fasting is a form of time-restricted eating that involves fasting for 18 hours a day and eating within a six-hour window. You can adjust the timing of the window based on your preferences. This form of fasting can help promote weight loss and boost your metabolic rate22.
- 12-hour fast: 12-hour fasting is another form of time-restricted eating that involves fasting for 12 hours and feeding for 12 hours. This is one of the easiest forms of intermittent fasting to practice since you're likely to be sleeping for six to eight hours of the fasting period. It simply means, if you last ate dinner at 9 p.m., you wait until 9 a.m. to eat your breakfast the next day. Research shows that your body starts mobilizing stored fatty acids22 after a 12-hour fasting period.
What does alternate-day fasting do to your body?
Alternate-day fasting causes your body to go into a state of ketosis, where it burns stored fat as energy. In addition to helping you lose weight, this metabolic state also offers other health benefits, such as reduced inflammation and better cardiometabolic health.
Is alternate-day fasting more effective than intermittent fasting?
Alternate-day fasting is a type of intermittent fasting. Research shows that it is more effective for weight loss than other forms of fasting such as 5:2 fasting, time-restricted eating, and traditional calorie restriction diets. This is likely because it is more restrictive than these diets.
Can you do alternate-day fasting long term?
It is not advisable to do alternate-day fasting on a long-term basis. You may also have difficulty adhering to this diet in the long term.
Alternate-day fasting is a type of intermittent fasting that requires you to fast (or severely restrict your calories) every other day. Whether you chose to do a strict or modified version, this fasting regimen can be difficult to follow and unsafe for some people. It's best to consult a health care provider to determine whether it's appropriate for you and how you can practice it safely. You also may want to explore other types of intermittent fasting that are more sustainable in the long term.
Sanjana Gupta has been a health writer and editor since 2014. She has written extensively for platforms like Insider, Livestrong.com, and Verywell Mind. Her work spans various health-related topics, including nutrition, fitness, mental health, medical conditions, and wellness.
Sanjana has a master's degree in digital journalism from New York University. She also holds a master's degree in management from the University of Mumbai.
She balances her love for chocolate with a penchant for fun workouts like aerial yoga and kickboxing.