Alternate-Day Fasting: Benefits, Drawbacks & Safer Approaches To Weight Loss

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.

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.

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.

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.

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. 

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.

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.

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.

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. 

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.