Before attending the Met Gala, actress Jessica Biel revealed a unique part of her red carpet preparation: a bath with Epsom salt.

In a TikTok video, the actress, who stunned in a pink couture dress by Tamara Ralph at the star-studded event in New York, showed how she prepared to take a bath filled with a whopping 9 kilograms of Epsom salt.

“Fill the water as hot as you can handle, [take] a 30-minute bath [the night before],” she shared. “Drink tons of water and go to bed early,” she advised.

What does an Epsom salt bath do?

Epsom salt, also known as magnesium sulfate or bath salt, contains magnesium, which offers several health benefits, including improved sleep and reduced inflammation. It’s said that soaking in Epsom salt can alleviate muscle pain and inflammation, as well as promote relaxation and soften the skin. Some doctors also recommend it as a simple and natural remedy for everyday aches.

“A bath with Epsom salt is well known for soothing sore muscles and helping you relax,” Dr. Laura Perdikis tells Yahoo Life. “Another purported benefit of Epsom salt baths is the absorption of magnesium through the skin. Magnesium is an important mineral that supports overall health, particularly brain and heart health, sleep quality, muscle relaxation, and may help lower blood sugar levels.”

However, there’s insufficient scientific evidence to fully support these largely anecdotal claims. There are no definitive studies showing that magnesium can be absorbed through the skin in amounts sufficient to provide the benefits observed after oral intake or supplementation.

Some claim that an Epsom salt bath can aid in weight loss. However, Perdikis says that any noticeable change in appearance or body weight after bathing in Epsom salt would be more related to the dehydrating effect of the combination of salt and warm water.

“It’s known that Epsom salt baths reduce swelling and flush toxins from the body. These factors can impact weight control,” she says. “There’s no evidence that Epsom salt baths are effective for weight loss, but taking a hot bath can make you sweat and ultimately burn calories.”

For example, Biel said she made the water in the bath as hot as she could, which, according to Perdikis, would lead to sweating and dehydration. It’s worth noting that the 9 kilograms of Epsom salt used by the actress significantly exceed the 2 cups recommended by doctors and experts.

Some social media users have labeled Biel’s method as “unhealthy and toxic.” Other commentators shared their own experiences of taking Epsom salt baths, from using them for weight cutting before wrestling competitions to considering them a “well-known trick” for models and bodybuilders.

Emily Van Eck, a registered dietitian, noted that it’s important for people not to perceive Biel’s method as a guide to action. “This can definitely be harmful,” she says. “The more you engage in behaviors that perpetuate attempts to be as thin as possible, the more at risk you are for developing disordered eating behaviors.”