Cold plunges are all the rage. But what does the science say?

Hundreds of Canadians rang in the new year by jumping into near-freezing bodies of water on Monday.

The New Year's Day cold plunge, or Polar Bear Dip, is an annual tradition more than a century old — a social event of perhaps limited appeal that unites shivering strangers in joyful discomfort. 

But the cold plunge has entered the mainstream in recent years, along with lofty claims about its supposed health benefits, from reducing inflammation and boosting immunity to alleviating depression and anxiety. 

Influencers are taking ice baths all over TikTok, and the practice has been touted by celebrities including Drake, Justin Bieber and Joe Rogan. Cold plunge tubs are being sold for hundreds or thousands of dollars to people who want to make it part of their daily routine.

But is there any science behind these supposed benefits? Or is this just a lot of hype? 

"People come out and they usually feel just fantastic," said John Holash, assistant professor with the faculty of kinesiology's Human Performance Lab at the University of Calgary.

And yet, "when you jump into that really, really cold water, it's a real insult to your body, and your body perceives it as a threat.'

Holash says there's little science to conclusively support some of the purported health benefits of cold plunges, in part because it's difficult to find consistent responses across large groups of people, given many factors that change how our bodies respond.

Most people who do cold plunges tend to be in fairly good shape, he says, which creates a "healthy user bias."

However, there is some evidence of short-term mental health benefits. Studies of cold water exposure have found huge spikes in participants'  levels of the noraderenaline and dopamine. Both are hormones and neurotransmitters that are associated with, respectively, alertness and pleasure, among other positive conditions. 

A cold plunge "increases your blood circulation, it pumps up your heart rate [and] you have a change in a lot of circulating hormones within the body," Holash said.

While scrolling social media might give you "a small hit" of dopamine that lasts five to ten minutes, "these changes that we see in hormone profiles [from a cold plunge] tend to last for hours. So they tend to be really really robust and long-term changes."

Holash says though the practice is trendy now, it goes back thousands of years. The ancient Greeks used cold water therapy for depression.

"Some of the results might be anecdotal, but when you get anecdotal evidence that kind of spans 3,000 years you might be onto something, even though it might be hard to prove in a lab."