Modified Alternate-Day Fasting (MADF): How It Works and What the Evidence Says
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If daily calorie counting has never worked for you, alternate-day fasting may sound like a refreshing change: eat very little one day, eat normally the next. It's one of the most studied forms of intermittent fasting, and the research is more nuanced than the headlines suggest. In this guide I'll explain how modified alternate-day fasting (MADF) works, what the latest evidence says about weight loss, who should avoid it, and how to make it work if you decide to try it.
What is modified alternate-day fasting? MADF alternates "fast days" of about 500 to 600 calories, roughly a quarter of your usual needs, with "feast days" of normal eating. It reliably produces weight loss in adults with excess weight, and a 2025 analysis of 99 trials found slightly more weight loss than daily calorie restriction, about 1.3 kg on average. In the longest trial, though, results were similar to daily calorie restriction and more people dropped out. It isn't suitable for everyone, including anyone with a history of disordered eating.
Before you start
Fasting isn't safe for everyone. Please don't try MADF if you're pregnant, breastfeeding or trying to conceive, underweight, under 18, or have a current or past eating disorder. If you take medication for diabetes, blood pressure, cholesterol or weight loss, or have a long-term health condition, talk to your doctor first. You'll find the full list below.
What is MADF?
MADF is a form of intermittent energy restriction. Instead of eating a little less every day, you alternate between two kinds of days:1
- Fast days: about 500 to 600 calories. Unlike a full water fast, the "modified" version lets you eat a small amount, which makes it easier to stick to.
- Feast days: normal, unrestricted eating.
There's a lot of flexibility within that. Studies have found that it doesn't matter whether you eat your fast-day calories as one meal or spread them across the day,2 and both higher-fat and lower-fat fast days produced weight loss.3 On fast days, you can also have calorie-free drinks like water, sparkling water, black coffee and unsweetened tea.
What a MADF week looks like
Because there are seven days in a week, your fast days shift from week to week. Here's one way to set it up over two weeks:
Does MADF work for weight loss?
Yes, for adults with overweight or obesity, the evidence is consistent. An umbrella review of meta-analyses found that MADF had some of the strongest evidence among intermittent fasting approaches for reducing body weight and BMI.1 In 2025, a network meta-analysis of 99 randomized trials in more than 6,500 adults found that alternate-day fasting was the only form of intermittent fasting to beat daily calorie restriction, by about 1.3 kg on average, with moderate certainty of evidence.4
That said, a few caveats keep the picture balanced:
- The difference is small. The extra 1.3 kg didn't reach the 2 kg threshold usually considered clinically meaningful for people with obesity, and the authors concluded that intermittent fasting and daily calorie restriction have broadly similar benefits.4,5
- Over a year, results were similar to daily calorie restriction. In the longest randomized trial so far, both groups lost about 6% of their body weight, and 38% of the alternate-day fasting group dropped out, compared with 29% of the daily restriction group.6
- Exercise roughly doubles the result. In a 12-week trial, combining alternate-day fasting with endurance exercise produced about twice the weight loss of either approach alone.7
Why it works
There's no magic involved. MADF works mainly because it creates a weekly calorie deficit, as long as you don't fully make up for fast days on feast days. A 2025 network meta-analysis concluded that weight loss mostly depends on the total amount of energy restriction, regardless of when you eat.8 In practice, people tend to eat less on feast days than you might expect: in the year-long trial, participants actually ate less than prescribed on feast days.6
Several studies in adults with excess weight have found that most of the weight lost on MADF comes from fat rather than muscle.7,9 This doesn't seem to apply to lean people, though: in a trial of lean adults, alternating fasting days led to less fat loss and more loss of lean tissue than daily calorie restriction.10 That's one more reason MADF is best suited to people with weight to lose.
Is MADF easy to stick to?
Adherence matters more than almost anything else in a weight loss plan, and MADF has real strengths here:
- It's simple. There are only two kinds of days to remember, and no tracking at all on feast days.
- It's flexible. You choose what to eat on fast days, as long as you stay within the calorie limit.3
- There's no constant restriction. Knowing tomorrow is a normal day can make today easier, and in one trial in normal-weight and overweight adults, hunger on fast days eased within a few weeks.9
The year-long dropout figures above are a reminder that it isn't easy for everyone, though. If you find yourself thinking about food constantly, or overeating on feast days, a gentler daily approach may suit you better. The best plan is the one you can sustain.
Side effects and who should avoid MADF
MADF is generally well tolerated in healthy adults. Some people notice headaches, tiredness, light-headedness, irritability, constipation or hunger in the first few weeks, which usually ease as the body adapts.9,11 Drinking plenty of water and getting enough electrolytes helps.
Please don't try MADF if you:
- Are pregnant, breastfeeding or trying to conceive
- Are under 18, or older and frail
- Have a current or past eating disorder
- Are underweight
- Have type 1 diabetes
- Take medication for diabetes, blood pressure, cholesterol or weight loss, unless your doctor agrees and adjusts it, as fasting can change how these work
Talk to your doctor first if you have liver, kidney, heart, lung or digestive disease, type 2 diabetes, cancer, epilepsy, depression, thyroid disease, Gilbert syndrome (fasting can raise bilirubin),12 or a history of bariatric surgery. Most MADF studies excluded people with these conditions, so there isn't enough evidence to know whether it's safe for them.6
Practical tips
- Plan your fast days. Decide what you'll eat in advance, so you're not making choices when you're hungry.
- Prioritize protein and fiber. Both help you feel full on few calories, and protein helps protect muscle. Think eggs, fish, Greek yogurt, legumes and plenty of vegetables.
- Stay hydrated and keep up your electrolytes. Fasting, exercise and hot weather all increase losses. Unless you've been told to limit salt, salting your food normally on fast days can help with headaches and light-headedness.
- Keep feast days nourishing. Unrestricted doesn't have to mean unhealthy. Diet quality on feast days affects both your results and how you feel.
- Move. Adding regular exercise may double your results.7
- Get support. Doing it with a friend, a community or a practitioner makes it easier to keep going.
Track how it's working
Fasting can change more than your weight, including cholesterol, triglycerides and blood sugar markers. If you're having bloodwork before and after, my Blood Test Tracker lets you chart those changes over time in Excel or Google Sheets. For day-to-day notes on energy, hunger and symptoms, see my daily health trackers.
Frequently asked questions
How many calories can I eat on a fast day?
Most MADF studies use about 500 to 600 calories on fast days, roughly a quarter of usual energy needs. You can eat them as one meal or spread them across the day, and calorie-free drinks like water, black coffee and unsweetened tea are fine.
Is alternate-day fasting better than calorie counting?
Slightly, in shorter trials: a 2025 analysis of 99 trials found about 1.3 kg more weight loss than daily calorie restriction. In a year-long trial, though, weight loss was similar and more people dropped out of alternate-day fasting. The best approach is the one you can stick with.
Will I lose muscle on alternate-day fasting?
In adults with excess weight, studies have found that most of the weight lost comes from fat. Lean people may lose more lean tissue, though, so MADF is best suited to people with weight to lose. Eating enough protein and staying active help protect muscle.
Can I exercise on fast days?
Yes. In one trial, combining alternate-day fasting with endurance exercise roughly doubled weight loss. Start gently, stay hydrated, and save your hardest sessions for feast days if you feel light-headed when fasting.
How long can I do MADF for?
Studies have run from a few weeks to a year, so there's limited evidence beyond that. Many people use MADF for a set period, then move to a less intensive pattern for maintenance. It's worth reviewing with a practitioner if you plan to continue long term.
Who shouldn't try alternate-day fasting?
Anyone pregnant, breastfeeding or trying to conceive, under 18, underweight, or with a current or past eating disorder, as well as people with type 1 diabetes. If you take medication or have a long-term health condition, speak to your doctor first.
The bottom line
Modified alternate-day fasting is a well-studied, flexible way to create a calorie deficit, and it works well for many adults with excess weight. It isn't magic, though: over the long term it performs about as well as daily calorie restriction, and it doesn't suit everyone. If the idea of two simple kinds of days appeals to you and none of the cautions apply, it may be worth a try, ideally with support from a qualified practitioner.
References (12)
- 1Umbrella reviewPatikorn C, et al. Intermittent fasting and obesity-related health outcomes: an umbrella review of meta-analyses of randomized clinical trials. JAMA Netw Open. 2021;4(12):e2139558. Source ↗
- 2Clinical trialHoddy KK, et al. Meal timing during alternate day fasting: impact on body weight and cardiovascular disease risk in obese adults. Obesity. 2014;22(12):2524-2531. Source ↗
- 3RCTKlempel MC, et al. Alternate day fasting (ADF) with a high-fat diet produces similar weight loss and cardio-protection as ADF with a low-fat diet. Metabolism. 2013;62(1):137-143. Source ↗
- 4Network meta-analysisSemnani-Azad Z, et al. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. BMJ. 2025;389:e082007. Source ↗
- 5EditorialIntermittent fasting as a nutritional tool. BMJ. 2025;389:r1156. Source ↗
- 6RCTTrepanowski JF, et al. Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults: a randomized clinical trial. JAMA Intern Med. 2017;177(7):930-938. Source ↗
- 7RCTBhutani S, et al. Alternate day fasting and endurance exercise combine to produce pronounced weight loss in obese humans. Obesity. 2013;21(7):1370-1379. Source ↗
- 8Network meta-analysisNetwork meta-analysis of continuous energy restriction and intermittent fasting regimens for weight loss and metabolic improvement. 2025. PubMed ID 40367516. Source ↗
- 9RCTVarady KA, et al. Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutr J. 2013;12:146. Source ↗
- 10RCTTempleman I, et al. A randomized controlled trial to isolate the effects of fasting and energy restriction on weight loss and metabolic health in lean adults. Sci Transl Med. 2021;13(598):eabd8034. Source ↗
- 11ReviewIntermittent fasting: the choice for a healthier lifestyle. Cureus. Source ↗
- 12Case reportGilbert's syndrome and Ramadan: exacerbation of unconjugated hyperbilirubinemia. Source ↗
This article is for educational purposes only and isn't a substitute for personalized medical advice. Please talk to your doctor or a qualified practitioner before making significant changes to your diet, especially if you have a health condition or take medication. See my full medical disclaimer.
