In the nearly two centuries since German physician Carl Wunderlich established 98.6°F as the standard “normal” body temperature, it has been used by parents and doctors alike as the measure by which fevers — and often the severity of illness — have been assessed.
Over time, however, and in more recent years, lower body temperatures have
been widely reported in healthy adults. A 2017 study among 35,000 adults in
the United Kingdom found average body temperature to be lower (97.9°F), and a
2019 study showed that the normal body temperature in Americans (those in Palo
Alto, California, anyway) is about 97.5°F.
A multinational team of physicians, anthropologists and local researchers led
by Michael Gurven, UC Santa Barbara professor of anthropology and chair of the
campus’s Integrative Anthropological Sciences Unit, and Thomas Kraft, a
postdoctoral researcher in the same department, have found a similar decrease
among the Tsimane, an indigenous population of forager-horticulturists in the
Bolivian Amazon. In the 16 years since Gurven, co-director of the Tsimane
Health and Life History Project, and fellow researchers have been studying the
population, they have observed a rapid decline in average body temperature —
0.09°F per year, such that today Tsimane body temperatures are roughly 97.7°F.
“In less than two decades we’re seeing about the same level of decline as that
observed in the U.S. over approximately two centuries,” said Gurven. Their
analysis is based on a large sample of 18,000 observations of almost 5,500
adults, and adjust for multiple other factors that might affect body
temperature, such as ambient temperature and body mass.
The anthropologists’ research appears in the journal Sciences Advances.
“The provocative study showing declines in normal body temperature in the U.S.
since the time of the Civil War was conducted in a single population and
couldn’t explain why the decline happened,” said Gurven. “But it was clear
that something about human physiology could have changed. One leading
hypothesis is that we’ve experienced fewer infections over time due to
improved hygiene, clean water, vaccinations and medical treatment. In our
study, we were able to test that idea directly. We have information on
clinical diagnoses and biomarkers of infection and inflammation at the time
each patient was seen.
While some infections were associated with higher body temperature, adjusting
for these did not account for the steep decline in body temperature over time,
Gurven noted. “And we used the same type of thermometer for most of the study,
so it’s not due to changes in instrumentation,” he said.
Added Kraft, “No matter how we did the analysis, the decline was still there.
Even when we restricted analysis to the <10% of adults who were diagnosed
by physicians as completely healthy, we still observed the same decline in
body temperature over time.”
A key question, then, is why body temperatures have declined over time both
for Americans and Tsimane. Extensive data available from the team’s long-term
research in Bolivia addresses some possibilities. “Declines might be due to
the rise of modern health care and lower rates of lingering mild infections
now as compared to the past,” Gurven explained. “But while health has
generally improved over the past two decades, infections are still widespread
in rural Bolivia. Our results suggest that reduced infection alone can’t
explain the observed body temperature declines.”
It could be that people are in better condition, so their bodies might be
working less to fight infection, he continued. Or greater access to
antibiotics and other treatments means the duration of infection is shorter
now than in the past. Consistent with that argument, Gurven said, “We found
that having a respiratory infection in the early period of the study led to
having a higher body temperature than having the same respiratory infection
more recently.”
It’s also possible that greater use of anti-inflammatory drugs like ibuprofen
may reduce inflammation, though the researchers found that the temporal
decline in body temperature remained even after their analyses accounted for
biomarkers of inflammation.
“Another possibility is that our bodies don’t have to work as hard to regulate
internal temperature because of air conditioning in the summer and heating in
the winter,” Kraft said. “While Tsimane body temperatures do change with time
of year and weather patterns, the Tsimane still do not use any advanced
technology for helping to regulate their body temperature. They do, however,
have more access to clothes and blankets.”
The researchers were initially surprised to find no single “magic bullet” that
could explain the decline in body temperature. “It’s likely a combination of
factors — all pointing to improved conditions,” Gurven said.
According to Gurven, the finding of lower-than-expected body temperatures in
the U.S., and the decline over time, had a lot of people scratching their
heads. Was it a fluke? In this study, Gurven and his team confirm that body
temperatures below 98.6°F are found in places outside the U.S. and the U.K.
“The area of Bolivia where the Tsimane live is rural and tropical with minimal
public health infrastructure,” he noted. “Our study also gives the first
indication that body temperatures have declined even in this tropical
environment, where infections still account for much morbidity and mortality.”
As a vital sign, temperature is an indicator of what’s occurring
physiologically in the body, much like a metabolic thermostat. “One thing
we’ve known for a while is that there is no universal ‘normal’ body
temperature for everyone at all times, so I doubt our findings will affect how
clinicians use body temperature readings in practice” said Gurven. Despite the
fixation on 98.6°F, most clinicians recognize that ‘normal’ temperatures have
a range. Throughout the day, body temperature can vary by as much as 1°F, from
its lowest in the early morning, to its highest in the late afternoon. It also
varies across the menstrual cycle and following physical activity and tends to
decrease as we age.
But by linking improvements in the broader epidemiological and socioeconomic
landscape to changes in body temperature, the study suggests that information
on body temperature might provide clues to a population’s overall health, as
do other common indicators such as life expectancy. “Body temperature is
simple to measure, and so could easily be added to routine large-scale surveys
that monitor population health,” Gurven said.
Reference:
Michael Gurven, Thomas S. Kraft, Sarah Alami, Juan Copajira Adrian, Edhitt
Cortez Linares, Daniel Cummings, Daniel Eid Rodriguez, Paul L. Hooper,
Adrian V. Jaeggi, Raul Quispe Gutierrez, Ivan Maldonado Suarez, Edmond
Seabright, Hillard Kaplan, Jonathan Stieglitz, Benjamin Trumble. Rapidly
declining body temperature in a tropical human population. Science Advances,
2020; 6 (44): eabc6599 DOI:
10.1126/sciadv.abc6599
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