What Is Alternate-Day Fasting, and Is It Even Safe? Here's What Nutritionists Have to Say

Let's get real for a sec: The thought of any kind of fasting diet sounds like the exact opposite of fun (and is likely making your stomach growl at the mere thought of it). And yet, it seems to be the weight-loss trend that just won't go away (think: OMAD, and the 5:2 diet).

Now, there seems to be another type of intermittent fasting—alternate-day fasting—that's been making headlines recently. And while the name definitely suggests some longer stretches of time without eating, it's not totally clear what the diet's all about. Here's what experts have to say about alternate-day fasting—and (spoiler!) it's definitely nothing good.

What is alternate-day fasting?

Alternate-day fasting (aka, ADF), is basically a type of intermittent fasting (IF)—a practice in which you alternate between periods of regular eating and fasting (not eating, or severely restricting calorie content). Depending on the IF program, you can fast for a few hours, or a full day or longer.

ADF, in particular, is considered one of the most extreme forms of intermittent fasting. In a new study published in the journal Cell Metabolism, ADF is defined as "strict 36-h[our] periods without caloric intake ('fast days') followed by 12-h[our] intervals with ad libitum food consumption ('feast days')."

Essentially, that boils down to not eating anything for 36-hours (basically a day and a half), and then eating whatever you want for the remaining 12 hours in the 48-hour cycle. "It's eating one day and not eating on another day," Keri Gans, a New York-based RDN, tells Health.

Are there any benefits to alternate-day fasting?

So back to that new research in the journal Cell Metabolism: According to the results of a clinical trial which involved 90 participants, ADF can reduce caloric intake—as much as 37% on average. This means that the amount of calories consumed by participants overall went down, likely because they spent every other day fasting.

Because of this, Gans says that ADF could potentially help someone lose weight, in a very specific circumstance. “If someone was eating anything they wanted seven days a week and now they’re [doing that] 50%” of the time, it could help them lose weight, Gans explains.

However, she’s quick to point out that there are other ways to lose weight—and that she definitely doesn't recommend it for weight loss. Instead, Gans recommends that someone learn more about portion control, instead of focusing on deprivation, which is what the ADF diet emphasizes.

As far as other health benefits go, the study also showed that those who followed the plan for six months had lower levels of LDL cholesterol (often called the "bad" cholesterol, since it can lead to build up in your arteries, per the US National Library of Medicine) triglycerides (a type of fat that can increase your risk of heart disease, again, per the NLM) compared to those who ate normally.

Is alternate-day fasting safe?

Let's be clear: Even the study authors suggest that even healthy people should only try ADF through consultations with physicians. That means it's definitely not something to be taken lightly.

“First, [you’re] missing out on important nutrients your body needs—this is even for healthy individuals,” says Gans. The diet can result in a “total lack of energy,” Gans says, which is easy to understand given that your body needs food for energy. Gans likens ADF (or any fasting diet, really) to "trying to run a car without gasoline." Dehydration is also a concern with ADF. According to Gans, we get a good amount of our daily water intake from food—therefore, refraining from food all day can lead to dehydration. Intermittent fasting overall can also lead to muscle loss, poor sleep, and rebound overeating.

Gans also explains that ADF (and more extreme forms of IF) can be especially harmful if you take medicine to manage an ongoing health problem. “If you’re taking medications, you should never not eat throughout the day,” Gans says, explaining that if you take medications on an empty stomach you can induce a headache or upset your gastrointestinal tract.

Safety aside, ADF can also interfere with your overall wellness—that means your mental and social health, too. Gans says it’s important to understand the consequences of adhering to an ADF diet, one of which is not being able to partake in social celebrations that involve food a whopping 50% of the time, not to mention the fact that a strict diet can force you to focus on your eating habits in an unhealthy way, which can be especially dangerous for those with a history of disordered eating.

So, should you try alternate-day fasting?

Honestly, it doesn't come highly recommended. "There's so many other ways you can lose weight without starving," Abby Langer, RD, a Toronto-based nutritionist tells Health.

Langer adds that, health risks aside, ADF is far too intense, which makes it even less sustainable—and therefore possibly harder to stick to for long-term, sustained weight loss, which should focus on simple adjustments you're willing to live with for a long time, even after you hit your weight-loss goals.

Still, if you're dying to try it out, it's best to do so with a doctor's guidance. But just be aware: ADF is likely not the long-term weight-loss solution you're looking for, and there are other, healthier ways to lose weight (like eating a balanced diet filled with whole foods and exercising a bit) that are actually nutritionist-approved.

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