Alternate-day fasting (ADF) has been gaining popularity as a helpful way to not only lose weight, but also experience a range of other health benefits. But are you really expected to go full days without eating anything? Not exactly.
Though its name suggests completely going without food every other day, studies on ADF typically involve eating about 25 percent of your usual intake one day, followed by a “feast day” of eating as much food as you like. The daily see-saw approach is still restrictive, yet for some, there’s appeal to a diet that doesn’t tell you what you can and can’t eat, doesn’t require macro tracking, and doesn’t say “kitchen’s closed” at any particular hour of the day. The only thing you need to track with alternate-day fasting is your calories: You simply cut them down to 25 percent of your normal intake every other day, then eat how you like on opposite days.
It’s certainly not right for everyone, but if you’re wondering whether or not ADF lives up to its reputation for weight loss and other health perks, we’ve gathered the research and asked the experts.
- ABOUT OUR EXPERTS: Melissa Mitri, M.S., R.D., is a weight-loss dietitian, nutrition writer, and owner of Melissa Mitri Nutrition. Amy Davis, M.S., R.D., is a registered dietitian.
Alternate-Day Fasting and Metabolism
One of the most touted benefits of ADF is its impact on metabolism. According to weight-loss dietitian and nutrition writer Melissa Mitri, M.S., R.D., owner of Melissa Mitri Nutrition, ADF’s influence on metabolism is not unlike that of a keto diet. “Alternate-day fasting can support metabolic health by giving the body time to enter ketosis,” she says. “This is a process where your body starts to use fat as its primary energy source, rather than carbohydrates, and typically starts after at least 18 hours of fasting.” Research shows that ketosis may help preserve muscle mass, which in turn can support a more active metabolism.
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The metabolic boost ADF may give is no small feat for body composition. A 2024 systematic review and meta-analysis found that people who followed this eating pattern for six months had reduced body mass index (BMI), body weight, and total cholesterol. That’s a pretty impressive combo for metabolic health—and one of the more promising alternate-day fasting benefits supported by research.
The potential benefits of ADF for metabolism don’t stop there. “It may also be beneficial for those with diabetes or pre-diabetes, as some studies suggest ADF can reduce fasting insulin levels more than daily calorie restriction, though results are still mixed and may depend on the individual,” Mitri says. Some early studies have explored ADF’s effect on heart disease risk markers, such as cholesterol and blood pressure, Mitri points out. “However, results are mixed on this, and more recent research suggests ADF does not have any positive impact on heart disease,” she notes.
Not every research study has shown dramatic results for metabolism, either. A 2017 study, for example, found that people who tried ADF didn’t experience more weight loss, improved insulin sensitivity, or other metabolic measures than those who restricted calories every day, or even those who had no dietary intervention at all.
Alternate-Day Fasting and Weight Loss
No shocker here: Burning fat and preserving muscle could lead to weight loss. Though studies have had varying results, some show that the back-and-forth of alternate-day fasting is more than the sum of its caloric parts. “Research shows people following ADF can lose 3-7% of their body weight in 8-12 weeks, along with reductions in body fat, waist circumference, and visceral fat,” says Amy Davis, M.S., R.D. These are some of the most sought-after alternate-day fasting benefits, especially for those with weight-related health concerns.
If you’ve ever struggled with diets that pigeonhole your eating into narrow food categories or time windows, Davis says alternate-day fasting can be a smart choice. “Those who might benefit most include people who struggle with moderation, those who don’t like tracking food or macros, people who like large meals and find portion control difficult, and individuals with prediabetes or metabolic syndrome,” she says.
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Alternate-Day Fasting and Longevity
Who wouldn’t want to live longer? Some research shows that ADF could promote longevity, though Davis cautions that many studies have been conducted on animals, and more research is needed.
Three processes that could help explain why alternate-day fasting may support longevity are autophagy, metabolic switching, and enhanced cellular function. “Autophagy is the process of cellular repair that can enhance mitochondrial function and reduce oxidative damage,” Davis explains. Some research has found that fasting triggers these helpful activities known for extending lifespan.
Metabolic switching, on the other hand, is the cycling between feasting and fasting that’s the hallmark of ADF. “Fasting periods deplete glycogen and trigger fat oxidation, which boosts metabolic flexibility,” Davis says. Some researchers believe a flexible metabolism may play a role in supporting healthy aging.
Finally, supporting cellular health and metabolic efficiency is often considered beneficial for longevity. A fasting-based eating plan has been studied for its potential to influence markers associated with aging and metabolic function, though it’s not currently recommended as a treatment for any medical conditions.
That said, it’s important not to offset the potential benefits of fasting with the food you eat on your “off” days. “While ‘feast day’ may sound like a free-for-all, the quality of the foods consumed on those days matters, especially for longevity,” Davis says. “Certain foods have been linked to longevity, including fiber-rich plant-based options like vegetables, beans, whole grains, and nuts.” On the other hand, a diet high in ultra-processed foods has been associated with negative health outcomes, including markers tied to a shorter lifespan.
Who Shouldn’t Try Alternate-Day Fasting
Like any diet, ADF isn’t right for everyone. “Certain individuals should avoid alternate-day fasting,” says Davis. Those with a history of eating disorders should steer clear, she says, since the restrictive nature of fasting can be a trigger. “Pregnant or breastfeeding women, children, and teens should also avoid ADF, since they need consistent nutrition to support growth and development.” Lastly, those taking insulin or other diabetes meds should avoid fasting to prevent hypoglycemia.
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If you’re OK flexing your willpower muscles, alternate-day fasting could be your sweet spot for a trimmer figure, higher metabolism, longer life, and more. Just be sure to check in with your healthcare provider before starting a new diet, especially one that involves fasting.
This article was reviewed by Brian Tanzer, nutritionist and Director of Scientific and Regulatory Affairs at The Vitamin Shoppe.