Doctors know that there isn't a single normal body temperature for everyone at all times, despite fixation at 98.6 degrees Fahrenheit (37 degrees Celsius).
Throughout the day, your body temperature can vary by about 1 Fahrenheit, with lows in the early morning and high in the late afternoon. It changes when you are sick, rises during and after exercise, varies across the menstrual cycle and between individuals, and tends to decrease with age.
You can say that body temperature is an indicator of what's going on inside your body, like a metabolic thermostat.
And an interesting study, from earlier this year in USA, found that a normal body temperature is about 97.5 Fahrenheit in Americans - at least in Palo Alto, California, where researchers have taken hundreds of thousands of temperature readings. This indicated that in the United States, the normal body temperature had decreased over the past 150 years.
A standard of 98.6 F measurement for "normal body temperature" was first established by German physician Karl Wunderlich in 1867 after studying 25,000 people in Leipzig. However, lower body temperatures have been widely reported in healthy adults. A 2017 study of 35,000 adults in the United Kingdom observed an average body temperature of 97.9 degrees Fahrenheit.
What might be causing these subtle and important changes? Are these provocative signals of changes in human physiology only occurring in urban and industrial areas like the United States and the United Kingdom?
One of the main hypotheses is that thanks to improved hygiene, sanitation and medical treatment, people today suffer fewer infections that could lead to higher body temperatures. The researchers were able to directly test this idea in a unique environment: among horticulturists in Tsimane, in the Bolivian Amazon.
The Tsimani lives in a remote area with little access to modern amenities, and we know from first-hand experience that infections are common - from the common cold to intestinal worms to tuberculosis. And after the research team worked with Tsimane to study a variety of topics related to health and aging for two decades, they had a rich opportunity to notice whether body temperatures drop similarly in this tropical environment, where infection is common.
As part of the Tsimane Health and Life History project, a mobile team of Bolivian doctors and researchers travel from village to village to monitor health while treating patients. They record clinical diagnoses and laboratory measures of infection at every patient visit.
When they first started working in Bolivia in 2002, the body temperatures of Tsimane individuals were similar to those found in Germany and the United States two centuries ago: averaging 98.6 degrees Fahrenheit, but over a relatively short period of 16 years, they noticed a rapid decline in the average Body temperature in this community. The drop is steep: 0.09 degrees Fahrenheit a year, today temperatures are around 97.7 Fahrenheit.
In other words, in less than two decades, we are witnessing the same level of degradation observed in the United States over nearly two centuries. This can be said with confidence, since the analysis is based on a large sample (approximately 18,000 observations for nearly 5,500 adults), with statistically controlling many other factors that may affect body temperature, such as ambient temperature and body mass.
Most importantly, while the incidence of certain diseases, such as respiratory infections or skin infections, was associated with a high body temperature during a medical visit, adaptation to this infection did not explain the sharp drop in body temperature over time.
So why have body temperatures decreased over time, for both Americans and the Tsimane? Fortunately, there are data available from long-term research in Bolivia to address some of the possibilities.
For example, the decrease may be due to higher modern healthcare and lower rates of lingering mild infections now compared to the past. But while the state of health has generally improved in Bolivia over the past two decades, the infection is still widespread among the Tsimane. The results indicate that the lower incidence of infection alone cannot explain the observed decrease in body temperature.
And people may be in better shape, so their bodies don't need to work as hard to fight the infection. Or, increased access to antibiotics and other treatments means the duration of infection is less now than it was in the past. It is also possible that increasing the use of certain medications such as ibuprofen or aspirin may reduce inflammation, and this is reflected in lower temperatures. However, while laboratory measurements of inflammation at the system level were associated with elevated body temperature during patient visits, calculating this in the analysis did not affect the researchers' estimate of how much the body temperature decreased annually.
Another possible explanation for the historical drops in body temperature is that bodies now do not need to put in much effort to regulate their internal body temperature, due to air conditioners in the summer and heaters in the winter. While the body temperatures of the Tsimane change with the time of year and weather patterns, the Tsimane does not use any advanced technology to regulate their body temperature. However, they are getting more clothes and blankets than they previously were.
Understanding the cause of low body temperatures remains an open question for scientists to explore. Whatever the cause, experts assert that body temperatures are less than 98.6 F outside places like the US and UK - even in rural and tropical areas with minimal public health infrastructure, where infection is still the main cause of the kill.
They hope the results will inspire more studies of how the improved conditions can lower body temperature. Because it is quick and easy to measure, body temperature may someday prove to be a simple but useful indicator, such as life expectancy, which provides new insight into a population's health.
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