Researchers examine
the decline in average body temperature among healthy adults over the past two
decades
Source:
University of
California - Santa Barbara
Summary:
In the
nearly two centuries since 98.6 degrees Fahrenheit (37.0 degrees Celsius) was
established as the standard 'normal' body temperature, it has been used as the
measure by which fevers have been assessed. Over time, however, lower body
temperatures have been widely reported in healthy adults -- for example, in
recent studies in the UK and the US. Researchers have now found a similar
decrease among the Tsimane, an indigenous population of forager-horticulturists
in the Bolivian Amazon.
n 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.
Story
Source:
Materials provided
by University of
California - Santa Barbara. Original written by Andrea Estrada. Note:
Content may be edited for style and length.
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