Jessica Biel’s 20-pound Epsom salt bath ahead of the Met Gala raised eyebrows. (Getty Images) (Gilbert Flores via Getty Images)

Ahead of attending the Met Gala on Monday night, Jessica Biel revealed a unique part of her red carpet prep: an Epsom salt bath. In a video taken on Sunday night and posted to TikTok, the actress — who wowed in a pink Tamara Ralph couture gown at the star-studded New York City event — filmed herself preparing to soak in a bathtub filled with a whopping 20 pounds of salt.

The video shows Biel counting out five bags of Dr. Teal’s Pure Epsom Salt Soaking Solution before emptying them all into her bathtub. “With water as hot as you can take it, [have a] 30-minute soak [the] night before,” she shared. “Drink tons of water then, and off to bed early. See you tomorrow, Met Ball.”

What is the salt supposed to do — and is that much really necessary? Here’s what to know about Epsom salt baths, and why there’s some controversy surrounding them.

What does an Epsom salt bath do?

Epsom salt is also known as magnesium sulfate or bath salt and contains magnesium, which is associated with a number of health benefits, including improved sleep and decreased levels of inflammation. Bathing in Epsom salt is said to reduce muscle pain and inflammation, as well as promote relaxation and soften skin. Some doctors have also recommended it as an easy, natural remedy to address daily aches and pains.

“[An] Epsom salt bath is well known to help soothe achy muscles and help you relax,” Dr. Laura Purdy, a family medicine physician and medical director of Swell Medical, tells Yahoo Life. “Another believed benefit of Epsom salt baths is magnesium absorption in the skin. Magnesium is an important mineral that supports overall health — specifically brain and heart health, sleep quality and muscle relaxation, and when looking specifically [at] your diet, it can help lower blood sugar levels.”

The scientific evidence to support these largely anecdotal claims, however, is lacking. There are no definitive studies showing that magnesium can be absorbed through the skin (via a soak) in amounts sufficient to provide the benefits seen after ingesting it through food or oral supplements.

Some claim that soaking in Epsom salt can help with weight loss. However, Purdy says that any perceived change in the body’s appearance or weight after a soak in Epsom salt would have less to do with the magnesium components and more to do with the dehydrating effect of the salt and warm water combined.

“Epsom salt baths are known to reduce swelling and remove toxins from the body. These factors may impact weight management,” she says. “There is no known evidence that Epsom salt baths are effective for losing weight, but taking a hot bath can cause you to sweat and ultimately burn calories. So there could be a correlation of weight loss due to sweating, which would reduce water weight.”

Biel, for instance, said she made the bathwater as hot as she could handle, which Purdy says would lead to sweating and dehydration. It’s worth noting that the 20 pounds of Epsom salt that the actress used is significantly higher than the 2 cups recommended by the Mayo Clinic and Dr. Teal’s.

“For a standard-sized bathtub, the desired therapeutic effects are achieved by combining approximately 2 cups of Epsom salt with warm bathwater,” dermatologist Dr. T.N. Rekha Singh tells Yahoo Life. “Overuse or excessively concentrated solutions, however, can lead to dehydration, salt imbalance or magnesium overdose.”

When used in a bath for soaking purposes, Purdy says, Epsom salt is generally considered safe. However, “there may be some skin reactions if you have sensitive skin, like a rash or itchy skin,” she says.

“Individuals with kidney problems or severe skin conditions should consult a health care provider before using Epsom salt baths,” adds Singh.

Why there’s some controversy

Biel didn’t share why her Met Gala prep involved an Epsom bath soak, but the implication — as suggested by People’s Instagram post — that it allowed her to “slip into” her gown has been called “so unhealthy and toxic” by some social media users. Other commenters shared their own experiences taking Epsom salt baths, from taking them to slim down before a wrestling competition to regarding them as a “known hack” for models and bodybuilders.

Emily Van Eck, a registered dietitian who specializes in body image and eating disorders, tells Yahoo Life that it’s important that people don’t take Biel’s routine as guidance.

“This can definitely cause harm,” she says. “The more you try behaviors like this that perpetuate trying to be as thin as possible, the more at risk you are for developing an eating disorder.”

“Any fast fads for dropping pounds that start to circle the internet can impact someone’s mental health if they are personally experiencing body image issues,” says Purdy. “Oftentimes these fads are not sustainable and are considered quick fixes based on how someone wants to look short-term and not considering their overall long-term health.”