Obesity epidemic may threaten mitten industry

We’re all familiar with the phrase, “Cold hands, warm heart.” A new federal study suggests a less complimentary aphorism might apply to a large and rapidly growing proportion of the population: Warm hands, big waist.

HOT HANDS, COOL BELLY These thermograms quantify the heat radiating from torso (left) and hand of an obese volunteer (upper) and normal-weight test subject (lower). Temps range from a low of 28 °C, depicted as indigo, to a high of 36°, brick red. AJCN

People who are plump — especially the obese — tend to generate more heat than do their lean pals, and for a number of reasons, note David Savastano and his colleagues at the National Institutes of Health, in Bethesda, Md. But where does heat generated by that metabolic furnace escape from big bodies — ones whose torsos are girdled by a thick layer of heat-insulating fat?

Clearly, the excess is getting out or big people would run chronic fevers, notes Jack Yanovski with the Unit on Growth and Obesity at the Eunice Kennedy Shriver National Institute of Child Health and Human Development. In fact, his team’s new study found, the core body temperature of 23 obese individuals was virtually the same as in 13 normal-weight volunteers.

So the researchers decided to thermally scan a region of the body that is relatively (but not completely) free of body fat: our hands. And in an American Journal of Clinical Nutrition paper published online, ahead of print, they now report that the hands of obese individuals — especially tiny blood vessels near the surface of their fingers — radiated substantially more heat than did these appendages in normal-weight people.

For instance, temperatures around the fingernails averaged 33.9 degrees Celsius (or 93 degrees Fahrenheit) in the obese volunteers versus just 28.6 °C (~83.5 °F) in the normal weight participants. By contrast, the surface temperature of the torso in big people, as measured by thermal imaging throughout a band encompassing the belly button, averaged 31.8 °C. That was one degree LESS than in the normal weight individuals.

While the researchers didn’t scan the feet, “We would expect to see the same thing as in the hands,” Yanovski says — that they too are major release valves for venting excess heat.

People also release heat via sweating. But over prolonged periods, sweating risks triggering dehydration and salt loss. So it’s not a very healthy approach to chronic cooling, he notes. Shedding heat from the blood via the skin is safer — but not all that comfortable when the body’s furnace really gets stoked and running, such as during exercise.

Indeed, Yanovski says, a fat-impeded release of heat in obese individuals “has consequences, we think, for exercise tolerance. Folks that are overweight tend to have more trouble with sustained exercise — and have increased sweating, which is probably a response to their difficulty in removing heat.”

The NIH team began focusing on where big individuals vent their heat “to find ways of helping them more effectively remove heat during sustained exercise,” Yanovski explains. “We now have follow-up experiments to study heat removal from areas like the hands.”

Such as wearing chilled gloves? In theory, he says, that’s the idea. “But it looks like you’ll have to do more than just make the hands cold,” he adds, “because even in obese people, if you place a cold thing on their hands, there are little controls on the vessels — little muscles — that will squeeze down and prevent blood from flowing to those areas.” 

Bottom line, he explains: “Humans weren’t really designed to be extraordinarily heavy.” Indeed, throughout most of human history, survival depended on avoiding starvation. With the obesity epidemic swelling waistlines around the globe, a new imperative is emerging: fighting human biology so that big people can burn calories without feeling like they’re burning up.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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