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Some physiologists analyzed case files in European and American history and found out that the body temperature of modern people has become lower. At that time, the suspicion might be related to the modern lifestyle.
For many years, people in the United States and the United Kingdom have had a body temperature lower than the usual 37°C - the generally accepted average.
Now, new research on Tsimane, a relatively remote indigenous tribe in Bolivia, shows that this trend is not accidental, nor is it a phenomenon that can only be explained by modern lifestyles. On the contrary, this situation exists even in rural and tropical regions where medical conditions are extremely poor and infections are widespread.
When anthropologists first worked with the Tsimanes in 2002, they found out that the average body temperature of a local adult was 37°C - exactly the average body temperature in Europe two centuries ago.
Only 16 years later, the measured temperature then dropped to 36.5°C (36.53 for women, 36.57 for men), a rapid decline of 0.03°C every year.
"In less than two decades, the level of decline we have seen is similar to the level of decline observed in the United States for about two centuries." explains Michael Gurven, an anthropologist from the University of California, Santa Barbara.
This result has relatively high credibility. The analysis is based on a large sample of 5,500 adults and approximately 18,000 long-term observations, with the consideration of many other factors that may affect a person’s body temperature.
No matter how the researchers segment the results—even if they only analyze fully healthy adults—the body temperature drop still exists.
Although some may be from a low-income, the results of the study are basically consistent with a recent study in the United States, which found that "the current average body temperature of the population in high-income countries is 1.6% lower than the pre-industrial era [36.4°C]".
But this study in Americans only looked at a single population, thus it cannot explain why this decline occurred.
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Anthropologists working with the Tsimane people can dig into more details. Researchers can obtain the clinical diagnosis and biomarkers of infection and inflammation for each patient, which means they can test for different factors.
One of the previous mainstream hypotheses was that improved hygiene, clean water, and better medicines reduced the number of infections people experience, thereby lowering their temperature.
Although the inflammation of the Tsimane family is indeed related to higher body temperature, the improvement of hygiene alone is not enough to explain the sharp drop in body temperature.
"For Europe and the United States, it can be explained by the rise of modern medical care and the low rate of lingering mild infections compared with the past." Gurven said, "but, although health conditions have generally improved in the past two decades, infections in rural areas of Bolivia are still widespread."
In the end, the researchers failed to find any single explanation. They said it was probably a combination of multiple factors.
It may be that human living conditions are better today, and our bodies don't have to work so hard to resist infection. It may also be related to easier access to antibiotics, vaccinations or other medical methods.
For more developed communities, it may even be related to modern luxury goods, such as air conditioning or heating, which makes it easier for our bodies to maintain internal temperature.
"Although the body temperature of Tsimane people does change with time and weather patterns, they have not yet used any advanced technology to help regulate body temperature," admitted Thomas Kraft of UCSB Research Anthropology. "However, compared to before the industrial age, there are indeed more opportunities to get clothes and blankets."
We need to conduct more research on various groups of people around the world to figure out what happens to our body temperature and why.
The research was published in "Science Progress".
