What Is Alternate-Day Fasting—And Is It A Safe Weight-Loss Method?

Another day, another intermittent fasting diet that's gaining serious buzz in the weight-loss-wanting community. This one? Alternate-day fasting (ADF), which is, at its most basic, what it sounds like.

Alternate-day fasting involves eating very minimally every other day, and eating whatever you want in between. Sounds pretty simple, right? Well, it's actually a bit complex...and more extreme than it sounds.

That's why I brought in the experts: Jessica Levinson, MS, RD, CDN, a culinary nutrition expert in New York City and author of 52-Week Meal Planner, and Alyssa Dweck, MD, a women's health expert. They've got the answers to all the Qs, because when it comes to any type of fasting, real news matters. Let's dig in...

So....what exactly is alternate-day fasting?

What Is The Best Intermittent Fasting Schedule?

You already know that it's a type of intermittent fasting (IF), which means you won't be eating during a designated period of time. But as mentioned, alternate-day fasting stands out from hours-based intermittent-fasting diets (like the ever-popular 16:8 diet), as it revolves around full days at a time. Typically, on fasting days (say, Monday, Wednesday, Friday), most people limit themselves to 500 or 600 calories for the entire day, Dweck says.

Keep in mind, according to the Dietary Guidelines for Americans, women should be consuming about 1,800 to 2,400 calories a day. So limiting yourself to just 500 calories a dayâ€"even if it's just a few days of the weekâ€"is quite extreme, and can bring with it some serious side effects, both good and bad.

Ooh, let's start with the good. What are the health benefits of alternate-day fasting?

First, like most IF diets, alternate-day fasting can be easier for some people to maintain than other types of restrictive diets. "I like the fact that it's not eliminating a specific food group," says Levinson. This also means that you're less likely to miss out on essential nutrients that removing, say, grains from your diet could involve.

Then there's the weight-loss benefit, which I know you're interested in (obviously...you're here). Recent research looked at strict alternate-day fasting over a four-week period and found an improvement in (healthy, middle-aged) subjects' overall health, as well as a 37 percent average reduction in calories. The study also showed improved cardiovascular health, reduced fat mass, and improvements in fat-to-lean-mass ratio.

The asterisk, though? "My hesitation is that we don't have long-term studies on it," Levinson says. That's why she recommends moderation and portion control as a better, more reliable alternative. Dr. Dweck, on the other hand, is a huge fan of the Mediterranean diet, for all of its weight-loss, heart-healthy and brain-boosting bennies.

Want to learn more about the benefits of the Mediterranean diet?

Got it. But how does alternate-day fasting work for weight loss?

In short, it’s all about fat breakdown, according to Dweck. "The sugars and starches in your body are depleted first when you're expending calories [a.k.a. doing any kind of movement or work], and that doesn’t take very long. So after you use lose all of your stored carbohydrates, you start to burn fat," she explains.

But there are a few caveats. "If someone does overeat on a non-fasting day, it's very possible they don't lose weight at all," Levinson says. You can probably go over the recommended 1,800 to 2,000 calories and be okay, since you're consuming way less several days of the week, but you don't want to go overboard. At the end of the day (or week, rather), calories in versus calories out still matters.

It's not just the calories that count but what type they are, too, btw. "Planning is the most important thing, so when it comes time to eat, you’re not just grabbing the first thing you see," Dweck says. (Anyone who's ever fasted knows how easy that can be when you're ravenous and hangry AF.) So if you're thinking of adopting alternate-day fasting, you might want to start meal prepping at night, to avoid those unnecessary indulgences.

Understood! But...is alternate-day fasting safe?

Here's where it gets a bit fuzzy: The safety of ADF depends on the person. Overall, it's not terribly risky, but there are some people who shouldn't try it.

For starters, that's pregnant women or women who are breastfeeding."The nutrients needed during pregnancy [and breastfeeding] are so much higher," Levinson says. If this is you, any fasting diet isn't a good idea.

This type of fasting is also dangerous for diabetics, because they need to keep their insulin levels steady. People with a history of eating disorders should steer clear of ADF as well, since any kind of fasting diet can quickly devolve into something even more extreme.

On that note, if you're someone who works out multiple hours a day, alternate-day fasting could be a bit risky for you, too. "You need a certain amount of energy to work out," Levinson says. Without enough cals in, you might not have the energy you need to exert yourself (not to mention, enough nutrients to nourish your muscles afterward).

'I Lost 85 Lbs. When I Got My PCOS Under Control'

Now, if you're not one of those people, alternate-day fasting could be very helpful to you. Dr. Dweck deals with lots of women who currently struggle with PCOS, a hormone imbalance that occurs due to minimal ovulation. Women with PCOS often experience weight gain as a side effect and have a hard time losing weight; Dr. Dweck thinks alternate-day fasting could help them.

She also recommends ADF to her perimenopausal and menopausal patients. Due to age, loss of muscle mass, and hormonal changes, they often struggle with weight loss, too.

I'm ready! Anything else I need to know?

Yep! First, know that for many people, their first attempt at alternate-day fasting can make them feel fatigued, nauseous, dizzy, and/or foggy. You might have to stick with it for several days to see if those reactions turn around (for most, they do). If they don't, stop fasting, or try doing one fasting day every few days to work your way up to the alternating pattern.

Should You Combine Intermittent Fasting And Keto?

There are also certain nutrients that you could be missing out on by adopting this diet, Levinson says. For example, if you start eating way less dairy because you're cutting calories during fasting days, then you may not be getting the calcium you need. The same goes for protein. If you're skipping a few chicken dinners a week, you may not be getting the vitamin B12 you need.

That's why Dweck says, "when you do eat, you still have to mindful of the kind of diet you're ingesting." Be sure you're non-fasting days rely on wholesome, nutritious foods (fruit and veggies, lean proteins, legumes, etc....not packaged junk).

And the number one thing both experts agree on? Hydrate. With water first and foremost, of course. But also feel free to have coffee, which Dweck says can help curb your appetite (thanks, caffeine!).

So...should I try alternate-day fasting or not?

"With all the intermittent-fasting approaches, it's really what works for each individual person," Levinson says. "For some people alternate-day fasting is really helpful and beneficial to them." For others, though, it's really hard.

"I go back to moderation, knowing how to balance your plate, and portion control."

Either way, Levinson considers herself more of a mainstream-diet advocate. "I go back to moderation, knowing how to balance your plate, and portion control," she says. "I really believe that all foods for the most part can fit into a diet when consumed in moderation." Going on a diet that eliminates food for a specific period of time isn't as manageable for the long-term, she emphasizes.

Dweck, on the other hand, is a bit more optimistic about this fasting plan. While she sees does acknowledge it has its risk, she's seen a good number of patients have weight-loss success with it.

Regardless of their professional opinion, both experts emphasize the major bottom line: Talk to your doctor first! They'll know your medical conditions, health issues, and weight-loss historyâ€"all the info you need to get real, personalized, and honest advice about whether or not alternate-day fasting is right for you.

Alexis Jones Assistant Editor Alexis Jones is an assistant editor at Women's Health.