This Type Of Fasting Isn't Safe Long Term: Here's What To Do Instead

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:

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.

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.

Your body's ability to repair itself declines with age10, but by promoting autophagy, alternate-day fasting may help prevent age-related health conditions.

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.

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? 

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:

The takeaway

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.