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For many years we have viewed 98.6 degrees Fahrenheit  (37.0°C) as the normal body temperature of a healthy adult. It turns out that is no longer true. The average body temperature has been falling for the last few decades. Studies find the temperature decrease in different parts of the world, in both rural and urban areas. Looks like it's somewhere between 97.5°F (California), 97.7°F (among the Tsimane in the Bolivian Amazon), and 97.9°F (United Kingdom).

An international team of researchers studied the body temperature of the Tsimane of the Bolivian Amazon using 16 years of data and compared the findings to the US and UK studies. The Tsimane live a subsistence lifestyle without access to running water or sanitation, have high exposure to diverse pathogens (e.g. parasites), and many infections. During the last 2 decades they have had increased access to health care (including vaccinations), and to markets, and there has been an improvement in health and lifestyle. So it was surprising that their average body temperature also declined in the last 2 decades.

The researchers think the temperature decline among the different populations is due to a combination of factors, which include improved lifestyle, better medical care and treatments, increased use of antibiotics which alter the gut microbial communities (microbiome), fewer parasitic infections, changes in physical activity, and increasing body weight.

From Science Daily: Average body temperature among healthy adults declined over the past two decades

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.  ...continue reading "Normal Body Temperature Is Lower Than We Thought"

For years we've been told (and learned in school) that 98.6 degrees Fahrenheit is the normal body temperature of humans. But... it turns out it's not. It's actually a bit lower - closer to 97.7 degrees F. It's the lowest around 6:00 in the morning - 97.5 degrees, and it is highest around 4 to 6 pm - 98.5 degrees F. Women tend to be about 0.2 degrees warmer than men. These were the findings of a crowdsourced smartphone study (with 329 people participating) by researchers at Boston Children's Hospital.

It turns out that other research has also said this for 2 decades. Huh? And yet the outdated 98.6 degree temperature persists in our culture. From Scientific American:

Normal Body Temperature Is Surprisingly Less Than 98.6

Normal body temperature is 98.6 degrees Fahrenheit, right? Not so. There is no baseline for humans, and even if there was, it would be closer to 97.7 °F. Temperature also varies across the day, peaking in late afternoon and bottoming out in early morning. It is slightly higher for women than for men as well. For two decades research has debunked the benchmark, set way back in 1868, yet it persists. One important ramification, says Jonathan S. Hausmann, a rheumatologist at Boston Children's Hospital, who led the latest study, is to redefine fever. Most doctors use 100.4 °F or higher, but if “normal” is lower, then the fever threshold should be, too. It also should vary with the daily pattern and be tailored to each individual, Hausmann says: “A child at 99.0 °F at 4 A.M. may be highly abnormal but at 4 P.M. could be within normal limits.” ...continue reading "Our Fluctuating Body Temperature Is Lower Than 98.6 Degrees"

So perhaps mother knew best - that you'd catch a cold easily if you didn't bundle up when going outside in the winter. Study was done on mice cells, so now need to study this in humans. From Science Daily:

Cold virus replicates better at cooler temperatures

The common cold virus can reproduce itself more efficiently in the cooler temperatures found inside the nose than at core body temperature, according to a new Yale-led study. This finding may confirm the popular yet contested notion that people are more likely to catch a cold in cool-weather conditions.

Researchers have long known that the most frequent cause of the common cold, the rhinovirus, replicates more readily in the slightly cooler environment of the nasal cavity than in the warmer lungs. 

To investigate the relationship between temperature and immune response, Iwasaki and an interdisciplinary team of Yale researchers spearheaded by Ellen Foxman, a postdoctoral fellow in Iwasaki's lab, examined the cells taken from the airways of mice. They compared the immune response to rhinovirus when cells were incubated at 37 degrees Celsius, or core body temperature, and at the cooler 33 degrees Celsius. "We found that the innate immune response to the rhinovirus is impaired at the lower body temperature compared to the core body temperature," Iwasaki said.

The study also strongly suggested that the varying temperatures influenced the immune response rather than the virus itself. Researchers observed viral replication in airway cells from mice with genetic deficiencies in the immune system sensors that detect virus and in the antiviral response. They found that with these immune deficiencies, the virus was able to replicate at the higher temperature. 

Although the research was conducted on mouse cells, it offers clues that may benefit people, including the roughly 20% of us who harbor rhinovirus in our noses at any given time. "In general, the lower the temperature, it seems the lower the innate immune response to viruses," noted Iwasaki.