Feeling chilly? It's good for you!

© Peter Arkle

When you first put on the ice vest, you will feel cold. Not intolerably cold, but cold enough to make you think, Or, at least, as numbness spreads across your shoulders and down your back, There must be better ways to lose weight. And there are. But as an adjunct to those better ways, the vest carries some unlikely promise.

The sturdy Han Solo - style garment is loaded with ice packs, and it's inspired by a theory gathering momentum among scientists: namely, that environmental thermodynamics can be harnessed in pursuit of weight loss. The basic idea is that because your body uses energy to maintain a normal body temperature, exposure to cold expends calories. The vest's inventor, Wayne B. Hayes, an associate professor at the University of California at Irvine, claims that wearing it for an hour burns up to 250 calories, though his data are very rough. A little more than a year ago, he began selling the vest, which he calls the Cold Shoulder, out of his Pasadena apartment. Name notwithstanding, people won't ignore you when you wear it.

Ken K. Liu, a principal at a hedge fund in Los Angeles, has been wearing the vest under his suit jacket on and off for about a year. He told me that some people's first reaction to the unwieldy getup is "What the hell are you doing?" As soon as Liu explains the concept, though, many of them say it sounds like a good idea. Others still think it's "stupid" - as did my colleagues, when I wore one - but Liu has not been deterred. Each morning while his coffee is brewing, he takes his vest out of the freezer and dons it without shame. Liu was never "fat," by his estimation, but he says he did carry a few extra pounds that he had trouble dropping, despite exercise and attention to diet. The Cold Shoulder closed that gap.

Hayes's ice vest was inspired by the work of Ray Cronise, a former materials scientist at NASA who now devotes himself to researching the benefits of cold exposure. During the swimmer Michael Phelps's 2008 Olympic gold-medal streak, Cronise heard the widely circulated claim that Phelps was eating 12,000 calories a day. Having been fastidiously trying to lose weight, he was incredulous. Phelps's intake was more than five times what the average American eats daily, and many thousands of calories more than what most elite athletes in training need. Running a marathon burns only about 2,500 calories. Phelps would have to be aggressively swimming during every waking hour to keep from gaining weight. But then Cronise - who knows enough about heat transfer to have been employed keeping astronauts alive in the sub-zero depths of space - figured it out: Phelps must be burning extra calories simply by being immersed in cool water.

Fascinated, Cronise began a regimen of cold showers and shirtless walks in winter, and he lost 26.7 pounds in six weeks. He began measuring his metabolism during and after cold exposure, and found that his body was burning a tremendous amount of energy. Rather than storing energy as fat, his body was using it to sustain his core temperature. Cronise's preliminary experiments led him to put together what is now a pretty high-tech lab in his Huntsville, Alabama, home, where he conducts miniature scientific studies, mostly on himself. All of this attracted publicity, naturally. Timothy Ferriss hyped Cronise's unorthodox weight-loss success in the 2010 best seller . That same year, Cronise gave a popular TEDMED Talk. ran a feature story describing his home laboratory, titled "The Shiver System." Through it all, Cronise endured not just the obvious physical discomfort of his endeavors, but the discomfort of personal and public criticism. Some detractors raised concerns about regularly exposing one's skin to cold (Cronise shared these worries); others accused him of diverting people away from solid principles of weight management and toward dubious shortcuts.

Cronise believes that his weight-loss story was misunderstood and may have distracted people from the important issue of nutrition. "You can't freeze yourself thin," he told me. "When I first started, I had kind of a naive approach that I was going to suck calories out of people." But his interest in altering metabolism through exposure to mild cold - which he defines as 55 to 65 degrees Fahrenheit - has only grown. Such temperatures are far enough below the socially accepted range that people plunked into a 50-something degree office would complain to no end. Unless, maybe, they believed it was good for them.

The notion that thermal environments influence human metabolism dates back to studies conducted in the late 18th century by the French chemist Antoine Lavoisier, but only in the past century has it really become relevant to daily life. Cronise believes that our thinking about the modern plagues of obesity and metabolic disease (like diabetes) has not addressed the fact that most people are rarely cold today. Many of us live almost constantly, year-round, in 70-something-degree environments. And when we are caught somewhere colder than that, most of us quickly put on a sweater or turn up the thermostat.

In that sense, we don't really experience seasonal variations in temperature the way our ancestors did. Even people in tropical regions used to get cold on rainy nights, Cronise pointed out, in a quick rejoinder to my observation that not all parts of the world have four seasons. Most other species display clearly ingrained biological responses to the seasons; why would humans be any different? He casually mentioned some informal experiments he has done with squirrels in his backyard. It was October when we spoke, and he claimed that he couldn't get a single squirrel to eat a peanut. "They bury every one I give them," he said. In the spring, though, the squirrels ate his peanuts readily. "In their world, they don't eat for entertainment," he added. Few animals do.

Cronise's latest ideas are laid out in a 2014 article he co-authored with Andrew Bremer, who was then at Vanderbilt University (he is now at the National Institutes of Health), and the Harvard geneticist David Sinclair, who is well known for his recent work on resveratrol (the "anti-aging" antioxidant found in red wine) and sirtuins - enzymes that help control metabolism. Sirtuins are active during times of stress, including when a person is hungry, and are thought to be related to the known life-prolonging effects of very-low-calorie diets.

Cronise, Bremer, and Sinclair propose what they call the "Metabolic Winter" hypothesis: that obesity is only in small part due to lack of exercise, and mostly due to a combination of chronic over-nutrition and chronic warmth. Seven million years of human evolution were dominated by two challenges: food scarcity and cold. "In the last 0.9 inches of our evolutionary mile," they write, pointing to the fundamental lifestyle changes brought about by refrigeration and modern transportation, "we solved them both." Other species don't exhibit nearly as much obesity and chronic disease as we warm, overfed humans and our pets do. "Maybe our problem," they continue, "is that winter never comes."

Their article joins a growing body of research on the metabolic effects of cold exposure, some of which I've reported on previously. Earlier last year, in the journal , researchers from the National Institutes of Health likened these effects to those of exercise, arguing that a better understanding of endocrine responses to cold could be useful in preventing obesity. The lead researcher in that study, Francesco Celi, published more research in June, finding that when people cool their bedrooms from 75 degrees to 66 degrees, they gain brown fat, the metabolically active fat that burns calories to generate heat. (Having brown fat is considered a good thing; white fat, by contrast, stores calories.) Another 2014 study found that, even after controlling for diet, lifestyle, and other factors, people who live in warmer parts of Spain are more likely to be obese than people who live in the cooler parts.

Meanwhile, Wouter van Marken Lichtenbelt, a professor at Maastricht University, in the Netherlands, has headed up a spate of recent research on the weight-loss effects of "non-shivering thermogenesis," the technical name for the calorie-burning, heat-generating metabolic phenomenon that occurs in the mild cold that Cronise champions. "Mild cold exposure increases body energy expenditure without shivering and without compromising our precious comfort," Lichtenbelt and colleagues wrote in an April paper.

Cronise is currently testing whether, with a low-calorie diet and a cool environment, he can maintain a healthy weight and low body-fat ratio without going to the gym. He does not turn on the heat in his Alabama home until the coldest days of winter, which at times means letting the indoor temperature dip into the 50s. And he has - most amazing, to me - trained himself to sleep without blankets. When he talks about the practice, he uses blanket as a verb, as in: People used to blanket because bedrooms had no heat. Now we heat bedrooms and we blanket.

Even on the hottest nights, I feel like I need the weight of a blanket, or at least a sheet, to sleep. But like eating sweets or turning up the heat, he sees sheeting and blanketing as acquired habits that can be changed. He was able to wean himself from blankets gradually, by learning to sleep with them first folded down partway, and then folded further, and then, eventually, all the way down to his feet. Cold really isn't that miserable, he insists, once you've gone through withdrawal and adapted to it.

Cronise said that when people tell him they need a blanket to sleep, he asks them, "Do you walk around in a blanket all day?" (Given the choice, some of us would.) But Cronise is more affable and reasonable-sounding than his anti-blanket rhetoric might suggest. The mild cold exposure he advocates might be as simple as forgoing a jacket when you're waffling over whether you need one, not layering cardigans over flannels despite the insistence of the fall catalogs, or turning off the space heater under your desk. And if you don't want to annihilate the environment by running the air conditioner to get a taste of sweet, calorie-burning, metabolism-enhancing cold in the summer, there are devices like the ice vest, which really isn't as terrible as it sounds.

"The first time you put it on, it's a bit shocking, to be honest," Wayne Hayes, the vest's inventor, warned me. "You feel like, Holy shit, this is cold." But after wearing it a few times, he said, most people barely notice they have it on. That was my experience. (Hayes's wife has become so used to the vest that she wears it under her clothes instead of over them.) Hayes recommends wearing the vest twice a day until the ice melts - which can take an hour or longer - though he has himself worn it as many as three or four times in a single day.

"If you buy more than one," he said, drifting into salesman mode, and only half kidding, "you can cycle them throughout the day and wear them every waking hour."