(TIME.com) -- Vinny Guadagnino, the fist-pumping star of the MTV reality show "Jersey Shore," made a splash when he took a dip in the freezing cold waters of the Atlantic Ocean as part of the Coney Island Polar Bear Club's 110th New Year's Day Swim.
Guadagnino, who has done the plunge twice, told OK! magazine the experience was exhilarating.
"You get a rush when you're there because there are so many people doing it," he said. "It's definitely like a shot of adrenaline... It doesn't affect you that much -- you just have to go in, and do it, and jump back out... Afterward, as soon as I get out, I have a towel waiting for me, and I dry off quickly. That's it -- and then, I just chill."
Okay, so Guadagnino was swimming for Cold-EEZE and was taking the plunge to raise money for charity, but there are plenty of others who take chilly dips for their health.
The Coney Island Polar Bear Club, which claims to be the oldest "winter bathing" organization in the United States, was founded in 1903 by publisher Bernarr MacFadden, who believed swimming in the ocean in the wintertime was "a boon to one's stamina, virility and immunity."
Doctors, however, aren't so convinced that these plunges are good for you, noting that there is no solid evidence of any physical health benefits associated with swimming in freezing cold water. In fact, they say, the sudden drop in temperature can be dangerous for people with underlying health issues.
"The biggest problem I see with these clubs is that people participate in them without having made sure from a health perspective that it's clear sailing," says Dr. David Frid, a cardiologist at the Cleveland Clinic.
People with a family history of stroke, aneurysm, blood pressure problems, hypertension, or sudden cardiac death should be extra cautious and should probably be evaluated by a doctor before jumping in near-freezing water.
When people first immerse themselves in such frigid water, their bodies go into "cold shock," and they start gasping for air, which puts a strain on the heart.
"Blood vessels on the outer part of your body constrict to try to retain heat, and that constriction (shifts) your blood demand more to your inner organs, trying to keep them warm," Frid says.
In healthy people, the discomfort lasts for about 30 seconds and then dissipates after two to three minutes.
But in people who are at a high risk for heart disease, the blood vessels in the heart can constrict, leading to chest pains like angina or a heart attack. As they desperately try to breathe and pull in more oxygen, they may inhale too much salt water into their lungs and drown. Even people who swim well in warm water are at risk for drowning in cold water.
"Your muscles get cold and are instantly paralyzed by the hyperventilation, and you can become very weak, " says Dr. Thomas Traill, a cardiologist and professor of medicine at Johns Hopkins University School of Medicine. "You hear of people drowning in a minute or two, even when they're just a few feet from safety."
Still, regular polar bear swimmers insist they feel better after their dips, describing the invigorating rush they experience when they plunge into icy water. That's caused by the adrenaline and stress hormones that the body releases in response to the shock of the cold temperature, but experts say that these rushes are not as healthy as people think.
"People talk about being full of energy, and they perceive that this has done them good," says Mike Tipton, a professor of Human & Applied Physiology at University of Portsmouth, "but that's because the body is responding to a stress with a 'fight-or-flight' response and preparing to get you out of that environment, and that may well give you a feeling of quasi-euphoria."
In healthy people, an adrenaline surge can trigger irregular heart rhythms and in those with heart conditions, the surge could trigger a heart attack.
The key to avoiding such events, as with any athletic endeavor, is training. The more you immerse yourself in cold water, the more your body will adapt to the extreme temperature change, according to Tipton, who started off 2013 by donning Bermuda shorts and participating in a polar bear plunge in his hometown of Gosport, England.
Tipton, who conducts research on the human body's physiological and psychological response to adverse environments, found that it takes as few as five immersions in icy water for the body to develop a tolerance that can reduce the risk of cold-shock response by half.
Tipton recommends five three- to five-minute immersions -- either one a day or stretched out over two to three days. Though that protection wears off as the year goes on, Tipton says people who take an icy dip as long as 14 months later seem to maintain some of their tolerance.
In one unusual but revealing study, his researchers repeatedly immersed one side of people's bodies until it showed evidence of habituation, while keeping the other side warm -- and then turned them over and immersed the side that had never been in the cold water and found that there was still a significant reduction in their cold shock response.
And what's the best way to take the plunge? By taking a slow walk into the water to acclimate the body to the cold. Diving in from a pier or a jetty causes too sudden a change in temperature and is exactly the kind of stressor that could trigger a heart attack.
If you're planning on taking a polar bear plunge, Tipton says you can prime your body for the event several days before by gradually lowering the temperature of your shower.
And once you're in a plunge, it's important to ease out of the water just as you eased in. If you've been in cold water for up to 30 minutes, Tipton recommends soaking for 10 to 15 minutes in a hot bath -- but no more than that.
"If you stay in a warm bath for too long, you'll stand up and feel faint because now all of the blood is going to the skin to cool you, and that doesn't leave sufficient blood to circulate around the heart and the brain and maintain blood pressure," he says.
And while some may feel like they need a drink or two to work up the courage to take the plunge ... alcohol isn't a good idea before taking a cold dip.
"Alcohol makes you lose heat much more quickly and makes it harder to regain heat afterwards, so don't drink until you come out of the water and rewarm," says Traill.
Polar bear plunges aren't really new; people in Russia and Scandinavia have been taking cold dips in frigid waters for health reasons for centuries. There, patrons switch off from sessions in a hot sauna and dips in nearby lakes or rivers (or a roll in snow). The contrast between the two extreme temperatures can be exhilarating and produce the same euphoria you feel from exercise.
"When you're exposed to very hot temperatures, one of your body's reactions is to dilate the blood vessels to try to keep the heat from building up inside of you," says Frid, "so then when you go to the cold, it causes some constriction of the blood vessels," in an effort to regulate your blood flow.
The feeling of relief can feel cleansing, but it may not necessarily be improving your health.
There simply isn't enough evidence to justify the benefits of swimming in freezing cold water over some laps in a warmer swimming pool.
"It could be just that exercise is good for you, maybe just dunking yourself in water is good for you, or the social aspect of doing the swim in one big group is good for you. And that does not have anything to do with the cold, per se," says Tipton.
Still, any type of physical activity is better than none, as long as you prepare yourself beforehand.
"I'm all for the attitude of 'let's do something that makes us feel good,'" says Traill. "It's better than plopping yourself on the couch and watching sports on TV."
This story was originally published on TIME.com