We’re all chilling out, new research shows
One of the most widely accepted standard measurements of the human body, a normal temperature of 98.6 degrees Fahrenheit, has declined gradually for more than 150 years in the United States by about 1.6% since the pre-industrial era, a new study published in the journal eLife finds. The cooling off owes largely to improvements in health and medicine and in part to increasingly cushy lifestyles, the study’s researchers think.
Many health practitioners are still using the old, inaccurate number of 98.6 F as the presumed norm, which was set by a German physician in 1851.
“Our temperature’s not what people think it is,” says Dr. Julie Parsonnet, a professor of medicine and health research at Stanford University School of Medicine and the senior author of the study. “What everybody grew up learning, which is that our normal temperature is 98.6, is wrong.”
Parsonnet and her colleagues say many health practitioners are still using the old, inaccurate number of 98.6 F as the presumed norm, which was set by a German physician in 1851 based on 25,000 patients in one city.
The researchers of the new study analyzed three sets of medical records that included body temperature measurements: from 1862 to 1930 on 23,710 veterans of the Civil War; from 1971 to 1975 on 15,301 people from the National Health and Nutrition Examination Survey; and from 2007 to 2017 on 150,280 adults who visited Stanford Health Care.
The mean body temperature of men born in the 2000s is 1.06 F lower than men born in the early 1800s, the researchers concluded. Among women born in the 2000s, body temps have fallen 0.58 F compared with women born in the late 1800s.
The new study is not the first to find the accepted norm is no longer accurate. In 2002, a review of studies conducted between 1935 and 1999 found the mean body temperature to be lower than the standard. For women, rectal temperature readings (generally thought to be the most accurate) ranged from 98.2 F to 98.8 F, and in men, it ranged from 98.1 F to 99.5 F. A 2017 study of 35,488 British men and women with a mean age of 52.9 years put the mean body temp at 97.9 F.
Why the decline?
The changes aren’t due to improved thermometers, the researchers say. Within the datasets, the temperature decline was found to occur across years when instruments were the same or similar.
It’s not known for sure why we’re all chilling out, but the researchers have some ideas.
“Mostly, I think this is due to our triumph over infectious diseases that affected humans since we descended from apes,” Parsonnet says. “We continue to see drops in inflammation even in the last few decades.” Less inflammation of body tissues means a lower metabolic rate since the immune system can relax.
We humans are endotherms, animals that survive by self-regulating body temperature regardless of the environment. The process starts with the body converting chemicals in food to provide energy. Metabolism, as it’s called, produces heat. Metabolic rate and body temperature vary based on a host of factors, including gender, diet, fitness, and time of day as well as extreme environmental conditions or with illness and fever.
To a lesser extent, air conditioning and central heating may have forced some human evolution, the researchers say. The body doesn’t have to expend as much energy in its effort to maintain a constant body temperature, allowing it to run at a lower metabolic rate. “It may be that our cushy lives, always at temperatures between 64 and 72, play a role,” Parsonnet says. It’s also “not impossible” that our increasingly sedentary lives have some effect, she says, given that generations past were busy farming, chopping wood, and otherwise struggling to survive day to day, and now the masses plop into office chairs for hours on end, followed by hours on the couch, with motorized transportation to and fro.
“Physiologically, we’re just different from what we were in the past,” Parsonnet says.
It’s not known whether the decline will continue, but Parsonnet suspects it may taper off as the decadeslong increase in life expectancy levels off. She also can’t say if the lower body temperature is having any follow-up effects. “The first step in figuring it out is knowing that it happened,” she says. “That’s where we are now. Since we have this information, scientists and physicians can work on figuring out why and what it means.”