Popular Mechanics (South Africa)
Why our bodies have gotten colder with each passing decade
SINCE THE GERMAN PHYSICIAN CARL Reinhold August Wunderlich published his research on human body temperature in 1868, 37°C (or 98.6°F) has been the gold standard. Now scientists say that number may be inaccurate. Thanks to improved health outcomes – meaning people are generally healthier and getting better overall medical treatment – the average human body temperature has fallen gradually over time.
‘Much as we have changed the Earth’s ecosystem, we are changing our own ecosystems,’ says Julie Parsonnet, MD, a professor of epidemiology at Stanford University. ‘We have changed who we are over the modern era.’ She and her team analysed more than 670 000 reported temperatures spanning
157 years of measurement and 197 birth years, and found that our temperature has dropped by 0.028°C (0.05°F) per decade since the mid 1800s. The study was sparked by a review of modern temperature studies where patients uniformly fell below the 37°C mark – a signal that something was rotten in the state of human body-temperature research.
Parsonnet and her team studied three large sets of data from between 1862 and 2017: temperatures taken during periodic check-ups with US Civil War veterans between 1862 and 1930; US National Health and Nutrition Examination results from the early 1970s; and data from the Stanford Translational Research Integrated Database Environment project taken from 2007 to 2017.
Controlling for changes in how temperatures were taken and advances in thermometry across the large pool of data, they compared body temperature with birth years and found that the average body temperature in men and women has gone down by 0.028°C per birth decade.
In their paper published in the journal eLife, Parsonnet and her colleagues suggest that a change in inflammation levels over time is the most likely explanation for the decrease in temperature. Inflammation can be caused by bacterial, fungal, or viral infections, surface injuries such as scrapes and punctures, or inflammatory conditions such as cystitis, bronchitis, and dermatitis. As part of the body’s immune response, inflammation produces cytokines and other proteins that increase metabolism and generate heat.
The development of germ theory (that microorganisms can cause disease) and advances in hygiene have changed how often many of these conditions occur. Today, the length of bacterial infections is shortened by antibiotics. And the symptoms of viruses can often be alleviated with anti-inflammatory drugs such as ibuprofen and naproxen, which can bring down your body temperature. Civil War veterans plagued with ailments in the 1800s weren’t as lucky.
Our environment has changed, too. We now have access to healthier foods and are more hygienic. We have learned to heat and cool indoor spaces more efficiently, which may also contribute to a lower metabolic rate. These environmental changes beget physiological changes. ‘We’re taller, fatter, and colder,’ Parsonnet says. We also live a lot longer than the average Civil War veteran.
The world has certainly become a safer, healthier place for humans, but the researchers say it’s hard to single out any one environmental change that’s lowered our temperatures.
Parsonnet says that changing the human body creates resulting mysteries: what are the outcomes, and how do they change as a consequence of our actions and environment? ‘They could be good – greater life expectancy, for example. They could be bad – causing obesity and perhaps limiting our ability to deal with new pathogens,’ Parsonnet says. ‘Changing fundamentally who we are may have surprising consequences.’
Parsonnet and her colleagues did not identify a new average body temperature guideline in their study. A recent large review, a 2017 study published in the British Medical Journal, for example, found the average body temperature of 35 488 British patients to be around 36.6°C. However, because each person’s own temperature constantly fluctuates and can be influenced by factors such as gender, age, and the time of day, many researchers have questioned whether an ideal temperature standard should even exist.
" MUCH AS WE HAVE CHANGED THE EARTH’S ECOSYSTEM, WE ARE CHANGING OUR OWN ECOSYSTEMS,’ SAYS JULIE PARSONNET, MD, A PROFESSOR OF EPIDEMIOLOGY AT STANFORD UNIVERSITY. ‘WE HAVE CHANGED WHO WE ARE OVER THE MODERN ERA.’