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

In a video posted to TikTok, the actress, who stunned in a pink Tamara Ralph couture dress at the star-studded event in New York, showed herself preparing to take a bath filled with a whopping 20 pounds (around 9kg) of Epsom salt.

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

What are the benefits of an Epsom salt bath?

Epsom salt, also known as magnesium sulfate or bath salt, contains magnesium, which provides a number of health benefits, including improved sleep and reduced inflammation. Bathing in Epsom salts is said to reduce muscle pain and inflammation and promote relaxation and softening of the skin. Some doctors also recommend it as a simple and natural remedy for daily pain.

"It's well known that an Epsom salt bath can help soothe sore muscles and help you relax," Dr. Laura Purdy tells Yahoo Life. “Another purported benefit of Epsom salt baths is the absorption of magnesium into the skin. Magnesium is an important mineral that supports overall health, especially brain and heart health, sleep quality and muscle relaxation, and can also help lower blood sugar levels.”

But there is insufficient scientific evidence to support these largely anecdotal claims. There are no conclusive studies showing that magnesium can be absorbed through the skin in sufficient quantities to provide the benefits observed after taking it through food or oral supplements.

Some claim that an Epsom salt bath can help you lose weight. Purdy, however, says that any noticeable change in appearance or body weight after bathing in Epsom salts will be due less to the magnesium components and more to do with the dehydrating effect of the salt and warm water combination.

“Epsom salt baths are known to reduce swelling and remove toxins from the body. These factors can impact weight management,” 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."

Biel, for example, said she turned the bath water as hot as she could, which Purdy said would have led to sweating and dehydration. It's worth noting that the 20 pounds of Epsom salts the actress used is considerably more than the two cups recommended by doctors and experts.

Some social media users called Biel's method "unhealthy and toxic." Other commenters have shared their own experiences with Epsom salt baths, from taking them to lose weight before wrestling competitions to seeing them as a "famous gimmick" for models and bodybuilders.

Emily Van Eck, a registered dietitian, said it's important that people don't take Biel's formula as a guide to action. “It can definitely cause harm,” she says. “The more you engage in behaviors that perpetuate trying to be as thin as possible, the greater your risk of developing an eating disorder.”

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