Average Body Temperature In The US Has Dropped Since 19th Century, Claims Study

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While sifting through historical medical records from the past few centuries, scientists noticed a curious trend: the average human body temperature in the US seems to have dropped since the 19th century.

Reporting in the journal eLife, a  team of researchers from Stanford University School of Medicine has attempted to sniff out a possible explanation for this observation.

Most people, including many physicians, will say that the average body temperature is 37°C (98.6°F), a number that stems from a medical book by German physician Carl Reinhold August Wunderlich in 1868. However, a recent study of over 35,000 British patients with almost 250,000 temperature measurements, found mean oral temperature to be 36.6°C (97.88°F).

So, what’s going on? Does this change reflect a true pattern or is it a matter of blunders, better thermometers, or different methods of obtaining temperature?

Perhaps surprisingly, the researchers speculate that the answer could be something to do with inflammation.

For this study, the researchers used 677,423 temperature measurements from datasets in the US that spanned from the 19th century to now, the earliest of which was a set of military service documents and medical records from Union Army veterans of the Civil War. The researchers attempted to iron out any biases from changes in thermometer technology or methods of measurement by checking for body temperature trends within each dataset. For example, with the veterans' dataset, which covered numerous generations and presumably used the same thermometer, they noticed a trend for each decade that was consistent with other observations and findings. 

The findings confirmed some previously known facts about changes in body temperature. For example, their data showed people’s temperatures tend to be higher towards the end of the day. It also noted that younger people and women have slightly higher temperatures on average. 

Most crucially, their data found that men born in the 2000s had an average temperature 0.59°C (1.06°F) lower than men in the early 19th century. The average body temperature of women has also dropped 0.32°C (0.58°F) since the 1890s at a similar rate of decline.

The researchers conclude that it might be founded in actual physiological change caused by changes in our environment over the past 200 years. More specifically, chronic inflammation related to poor health in the 19th century could be the culprit. Equally, they speculate lower body temperatures and reduced inflammation could be linked to a reduction in metabolic rate, which is the result of changes in the amount of energy we use.

"Inflammation produces all sorts of proteins and cytokines that rev up your metabolism and raise your temperature," Julie Parsonnet, MD, professor of medicine and of health research and policy, said in a statement.

With advances in biomedical care, better hygiene, and greater availability of food, levels of poor health and chronic inflammation started to decrease throughout the 20th century, potentially explaining this apparent drop in body temperatures. 

However, there were some factors known to influence body temperature that could not be accounted for due to missing data. For example, many of the records do not include information such as ambient temperature and time of day. Other factors, such as changes in demographics, could also help to explain the trend.

Nevertheless, the researchers argue that it’s perhaps not so surprising that our bodies have changed over the past centuries. 

"Physiologically, we're just different from what we were in the past," Parsonnet said. "The environment that we're living in has changed, including the temperature in our homes, our contact with microorganisms and the food that we have access to.

"All these things mean that although we think of human beings as if we're monomorphic and have been the same for all of human evolution, we're not the same. We're actually changing physiologically."

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