Heat Could Be Making Human Bodies Less Efficient

Usually it's air conditioning that gets shit these days for making human beings a generally softer (less adaptable, less resilient) and more ecologically harmful species. You could even say there's been a minor hysteria over the global spread of high-energy climate control as more and more countries industrialize and populations concentrate in hotter locales. In an argument against air conditioning, Stan Cox, author of Losing Our Cool: Uncomfortable Truths About Our Air Conditioned World (and Finding New Ways to Get Through the Summer), made the claim that A/C is actually driving obesity, cranking up appetites and slowing down metabolisms. "There is evidence that living almost constantly in an air conditioned environment can disrupt the endocrine system," Cox told Motherboard in 2011.

Cox was at least right about the disruption (and the climate harm, but that's another subject), but he might have had the rest of the physiology backwards, according to new results from the US National Institute of Health's ICEMAN (Impact of Chronic Cold Exposure in Humans) study. This study, which examined the "plasticity" of brown fat and relative blood sugar levels among humans living in cool/cold and warm/hot conditions, found that staying cool has some potent metabolic benefits: cold makes your body burn energy faster. The changes found in the study are even marked enough to suggest that temperature could well end up as some component of future diabetic treatments.

A bit of background to start. Not all of the fat clinging to the insides of a given mammal is the same. Some of it is brown fat, which can be looked at as "good" fat. Unlike white fat, brown fat burns energy to generate heat. It's typically found in newborns and hibernating animals, and would seem to have more in common with muscle tissue (in some ways) than its white fat sibling: brown fat and muscles arise from the same stem cells, and brown fat cells boast a larger number of mitochondria (power plant) structures and more capillary access. Brown fat actually burns off white fat.

Newborns have a high percentage of brown fat, but as they grow up, the proportion tilts toward white. Adults nonetheless keep some of the brown kind around, with adults living in cold climates tending to keep more than those in warm climates. Earlier research has shown that individuals with higher proportions of brown fat tend to be overall leaner and with lower blood sugar levels, and lab experiments have already had some success in encouraging the conversion of white fat stores to brown fat. The ICEMAN study took this further, extending the concept out of the lab to actual human organisms, exposing them to carefully climate-controlled environments for extended periods of time, all while tracking fat makeups via PET/CT scans and muscle and fat tissue biopsies.

The study ran four months and tracked five adult male subjects (note: a small number for big claims). Each month of the study was characterized by a different temperature scheme in an NIH habitat of sorts (subjects slept there, but lived normally during the day). The first month, with the rooms set at 24º C (about 75º F), was "thermo-neutral." The second dropped the temperature down to 19º C (about 67º F), while the third went back up to a neutral 24º C. The last and final month cranked the thermostat to 27º C (80º F). Real-life seasons, the kind that happen outside, were adjusted for.

Those temperature variations might seem small, but that 20 or so percent drop in temperature was enough to yield 30 to 40 percent increases in brown fat concentrations among the subjects. The changes were then lost when the temperature returned to the baseline, and yet more brown fat disappeared in the final, hot month.

A related change was in the expression of the fat-regulating hormone leptin and the glucose-regulating protein adiponectin, both of which increased in the colder climate. Together, the pair has been implicated in counteracting insulin resistance, a critical feature of diabetes in which the body loses its ability to regulate glucose. "The improvement in insulin sensitivity accompanying brown fat gain may open new avenues in the treatment of impaired glucose metabolism in the future," said Dr. Paul Lee in a statement from the Garvan Institute of Medical Research, his academic home-base.

"On the other hand, the reduction in mild cold exposure from widespread central heating in contemporary society may impair brown fat function and may be a hidden contributor to obesity and metabolic disorders," Lee added.

The small pool of subjects in the current study isn't ideal, but its results align well enough with an earlier study this year looking at brown tissue concentrations in mice within similar temperature variations. That study, published in the open-access journal PLOS ONE, found that decreasing temperatures by 4º C for just eight hours a day boosted the mouse metabolisms two-fold, while encouraging the conversion of white fat to brown and improving glucose regulation. This earlier study, in particular, looked at cold as a sort of exercise analog: if it's possible for short periods of exercise to make extended, similar alterations in metabolism and fat storage, maybe limited periods of cold might also have extended effects.

The question remaining, for both studies, is if cold-induced changes extend to other exercise benefits, like sustained weight loss. We could make a reasonable prediction, but metabolism is also a funny thing. In the PLOS ONE study, the researchers found that the mice just ate more to compensate for the additional caloric demands of more brown fat tissue. And our air conditioning thinker Stan Cox was partially right about cold weather and eating: people, at least already overweight people, tend to eat more during the winter months. But whether or not that actually has to do with appetite is debatable: other researchers argue that winter just involves more high calorie food opportunities. The difference is key.

Dr. Lee, of the current NIH study, offers the slightest peak into a deeper, complicating issue: it's just getting hotter, everywhere. "Studies have been performed in the UK and US measuring bedroom, dining room and lounge room temperatures in people's homes over the last few decades," he said, "and the temperature has climbed from about 19 to 22, a range sufficient to quieten down brown fat. So in addition to unhealthy diet and physical inactivity, it is tempting to speculate that the subtle shift in temperature exposure could be a contributing factor to the rise in obesity."