ByEmily Sohn
Published January 24, 2024
As an athletic teenager in Ireland, Joseph Costello often followed hard training sessions with ice baths. By the time he was a graduate student, he had found a more extreme method to chill the sore muscles and joints that come with exercise-induced inflammation: standing in a chamber cooled to -110° Celsius (-166° Fahrenheit) for a few minutes at a time.
The experience was equal parts excruciating and invigorating, says Costello, who started researching this kind of cold exposure, called cryotherapy, for his PhD, which he finished in 2012. “It was literally out of this world,” says Costello, an exercise and environmental physiologist at the University of Portsmouth in the U.K., who used to play Gaelic football and other sports. “The coldest temperature ever recorded on Earth was about 20 degrees Celsius warmer than this.”
Over a decade later, ice plunges and cold showers are trendier than ever as strategies for battling inflammation, and it’s not the only approach people are taking to hack their own health. Also popular: heat exposure, alternating heat and cold, and injections that require assistance from a healthcare provider.
(This is what a cold plunge does to your body.)
But do any of these inflammation-hacking techniques help accelerate healing? Evidence is both promising and murky, with some studies showing benefits, some showing no effects, and others showing that inflammation-control tactics can backfire in some cases. As scientists investigate, they are unraveling an enduring mystery: When is inflammation good and when does it cause harm?
“Inflammation gets a bad rap—everybody’s trying to decrease inflammation in their body,” says Dean Wang, an orthopedic surgeon and chief of the sports medicine division at the University of California, Irvine. “But not all inflammation is the same, and controlled inflammation is necessary for healing. I don’t think people understand that enough.”
Inflammation to the rescue
Painful joints are a near-universal experience. Osteoarthritis affects an estimated 80 percent of people over 55. Many more develop injuries, which become increasingly common—and slower to heal—with age. Inflammation is a major driver of musculoskeletal pain that comes with both injuries and arthritis, studies suggest. But controlling the discomfort is a tricky problem, in part because inflammation is an essential process that our bodies have developed to repair damage.
Here’s how it works. When someone strains or tears a ligament or tendon, the insult triggers the release of inflammatory molecules and cytokines, starting a sequence of events known as acute inflammation. Immediately, blood vessels dilate to allow more fluid to flow into the injured area. Swelling and clotting ensue while yet more inflammatory cells arrive to clear out damage and mobilize still other cells to rebuild damaged tendons, ligaments, and other tissues.
This acute inflammatory process is so integral to healing, Wang says, that during many shoulder and knee surgeries, he uses a sharp tool to scrape the tissue he’s working on to “muck it up a bit” and cause bleeding. That accelerates the inflammatory pathways that do the work of repair, even in cases of chronic pain and inflammation that continue longer than the acute phase.
Shots for inflammation control
The idea of tinkering with inflammation underlies an increasingly popular medical approach, called platelet-rich plasma or PRP, aimed at treating more chronic types of pain from tendonitis, arthritis, and other problems. Injections of PRP are made from a person’s own blood, which is first spun rapidly to filter out red blood cells and isolate platelets, which are essential for clotting and full of anti-inflammatory growth factors.
Mixed back together, PRP shots can contain various concentrations of white blood cells and other ingredients that dial inflammation up or down as needed, Wang says. For his arthritis patients, he uses an anti-inflammatory PRP. For people with chronic tendon injuries, like tennis elbow or hamstring strains, he makes pro-inflammatory formulations.
Efforts to evaluate the effectiveness of PRP remain a work in progress, in part because it is being used in many different ways. In a 2021 review, Wang and colleagues reviewed 132 studies on the use of PRP for 28 conditions in eight specialties, including musculoskeletal issues, cosmetic uses, and neurology. Overall, 61 percent of the studies supported the use of PRP, though only a third detailed the PRP formulation used. Also, studies varied in how they measured outcomes, making them hard to compare with each other.
“The practice of it is far outpacing the science,” Wang says. “But our science is good enough to show that there’s some promise with these treatments. For mild to moderate degeneration or knee arthritis, these things are worth trying if you’ve tried some other things and they don’t work.”
Injections of the future may include adult stem cells (which regenerate damaged tissue) and the proteins they produce (called secretomes). These shots are also widely available, Wang says, but research is still ongoing, and it’s not yet clear if positive outcomes result from biological processes or placebo effects.
Inflammation on ice
For people who want to take inflammation into their own hands without needles, cold exposure has become a leading contender, with ice plunges selling for thousands of dollars and attracting devotees who rave about their ability to ward off pain, anxiety, depression, and more. People who embrace the chill often invoke inflammation as the link between cold exposure and health benefits.
Scientists are still working on making that direct connection. Studies show that cold helps with pain, probably by lowering skin temperature, which reduces the speed that nerves can deliver pain messages, Costello says. The intensity of extreme cold exposure may also work as a distraction from what hurts. But studies that have tried to assess the effects of cold exposure on both acute and chronic inflammation have produced conflicting results.
After an injury, infusions of cold saline reduce inflammation in animals, studies show. And in a 2013 study that included 20 men who ran downhill for 40 minutes at a -10 percent grade, there was a slight reduction in inflammatory molecules among the half of participants who sat in a 40°F ice bath for 20 minutes afterwards. But those results weren’t statistically significant, and the ice bath made no difference in muscle soreness. For a different study, researchers took muscle biopsies from nine young men who did a strength workout followed by a 10-minute cold plunge in 50°F water. They found no difference in the inflammatory response compared with a post-exercise light spin on an exercise bike.
Cold therapy may also counteract the muscle gains that come with exercise, studies show, possibly by interfering with the inflammatory process. In some cases, cold interventions might even increase inflammation, Costello says.
“There’s more evidence coming out on an annual basis,” he says. “But there are still only a handful of research studies that are supporting the effectiveness of cold therapy for reducing the inflammatory response.”
Playing with fire
Heat might be another way to address inflammation and improve health. Regular sauna users have a lower risk of cardiovascular disease, according to research on men in Finland. And a type of physician-supervised procedure that raises body temperature has shown promise for treating depression and cancer.
In a 2021 review of research on repeated sauna and hot-tub use, University of Oregon environmental physiologist Chris Minson and colleagues found evidence of multiple ways that heat can suppress pro-inflammatory pathways and enhance anti-inflammatory ones in animals and people.
In heart failure patients, two weeks of bathing in hot springs led to a drop in multiple inflammatory molecules. And 30 one-hour sessions of hot tubbing over 10 weeks reduced levels of a main measure of inflammation, called C-reactive protein, in the blood of women with polycystic ovary syndrome, Minson’s group reported in 2019.
People vary quite a bit in their inflammatory levels, which can also change in a single person from day to day and even throughout the day, experts say, making it tricky to give evidence-based advice that applies to everyone. Costello advises against icing after every workout because it could interfere with healing and athletic gains.
On the other hand, cold might help with pain that makes it hard to move or interferes with daily activities, or when chronic low-level inflammation sets in and inhibits the normal repair process, Minson says. He recommends separating cold exposure from exercise by, for example, plunging first thing in the morning and exercising later in the afternoon.
Exercise itself has powerful anti-inflammatory effects, Minson adds. Taking anti-inflammatory medications or other measures that impede the inflammatory process can interfere with those benefits, at least in healthy young people. “Inflammation is super important for the healing process,” Minson says. “By knocking it back, it’s not necessarily a healthy thing.”
Back to nature
If hot tubs and cold plunges feel good, the experience may have benefits beyond inflammation that, in fact, ultimately affect it. People who swear by cold plunges often report a sense of euphoria, like they’ve woken up, feel more alive and less stressed, Minson says. One soak in 55°F seawater for up to 20 minutes led to a large drop in negative thinking and a major boost in mood for 42 undergraduate students who soaked for a 2021 study.
That sense of well-being could have important physiological benefits. Studies of cold plunging, which tend to be small, have shown reductions in the stress hormone cortisol. When chronically elevated, cortisol can lead to persistent low-level inflammation.
Cold plunges in natural settings might be especially beneficial, adds Minson, who is also chief science officer of a free app called NatureDose that aims to get people outside more. Spending time in nature, according to many studies, improves physical and mental health.
The routines and community that habitual cold-water swimmers and sauna bathers often develop have yet more benefits for the immune system, says Costello. To reap benefits safely, he advises people to check with a medical professional before exposing themselves to extreme temperatures.
Minson recently put a sauna in his backyard next to a small workout space. Most days, he spends about half an hour in the hot room, punctuated by a cold shower. His research has convinced him that the practice is good for him. More than that, he is motivated by how good it makes him feel. “When I take care of my body by doing these kinds of things, I sleep better,” he says. “I’m happier. I feel healthier.”
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