In a recent article, a number of scientists at Stanford (Protsiv et al.) raised the puzzling question of why the average human body temperature has decreased since the Industrial Revolution.

Our current standard goes back to the German physician and medical professor Carl Reinhold August Wunderlich who conducted millions of axillary temperate measurements from 25,000 patients. He established the standard human body temperature of 98.6 °F or 37°C.

Source: Photo by Matteo Fusco on Unsplash

Yet, current measurements suggest that the mean human body temperature has is actually 36.6°C. Some behind those new measurements have accused Wunderlich's data as being flawed or at least biased (perhaps another instance of the ). The controversy raises an important scientific question: have we actually discovered the 'true mean' for human body temperature or has it decreased in the last 200 years. Protsiv et al. think that there is more to this change than mere differences in the quality of measurement methods or change in the measurement method (from armpit to mouth). In addition, they suggest that common diseases at the time such as syphilis, tuberculosis, and periodontitis may have impacted the population mean leading to an average life expectancy of 38 years.

But even if the mean human body temperature has changed - why is this interesting? The 'normal' mean, after all, may not be anything but the population average in the 19th century.

Protsiv et al. argue that the decrease may hold secrets regarding the improvement of human health and longevity.

Analyzing 677,423 human body temperature measurements within three separate cohort populations allowed them to investigate 157 years of measurement and 197 birth years in total. Groups were sorted by , , age, height, weight, and . They found that "man born in the early 19th century had temperatures 0.59°C higher than men today, with a monotonic decrease of -0.03°C per birth decade". A similar effect is observed in women since the 1980s with a total decrease of -0.32°C and a rate of decline of "-0.029°C per birth decade".

They hold that these differences are unlikely due to systematic measurement because they observed a similar trend not only across but also within cohorts. The change from axillary to oral measurements, moreover, should if anything push the contemporary averages up since axillary measurements are known to be one degree Celsius lower on average. Yet, it is the contemporary oral measurements that are the lower ones. Therefore they argue that actual physiological changes are more likely.

Protsiv et al. suggest that the change is due to a decrease in resting metabolic rate. Heat they argue is a byproduct by basic metabolic processes. Many animals have a constant range of body temperatures despite vastly differing environments. Yet, humans have actually become bigger in the last centuries (both weight and height). This should have corresponded in an increase in temperature. The average temperature, however, has decreased. Therefore, they argue there must have been a decrease in our resting metabolic rate. Decrease in inflammation seems to be the best explanation as it correlates strongly with temperature. As they argue:

Economic development, improved standards of living and sanitation, decreased chronic infections from war injuries, improved dental hygiene, the waning of tuberculosis and malaria infections, and the dawn of the antibiotic age together are likely to have decreased chronic inflammation since the 19th century. - Protsiv et al.

Source: Photo by Andrea Piacquadio, used with permission

Studies in Pakistan, where inflammatory diseases such as tuberculosis are still common and access to anti-inflammatory drugs such as Aspirin harder do indeed confirm this trend: their temperature measurements do closer to those of Wunderlich in the 19th century.

In the end of their paper Protsiv et al. unfortunately, speculate a bit too much and suggest that evolution may have been an additional cause. As humans now live in temperate climates (88% of all American households are equipped with air conditioning and heating is almost universal) there is less need for a higher metabolic rate in order to ensure a stable body temperature with minimal energy expenditure. Yet, there simply has not been enough evolutionary time to allow for such a change on a human population level. This evolutionary story, however, may hold promise to explain differences in metabolic rates between human populations living in sub-Saharan Africa, the Himalayas, the Polar regions, and temperate Europe.

In conclusion, it seems as if Wunderlich's hard work has paid off and can be appreciated after all. His measurements weren't inaccurate - human body temperature actually changes due to improvements in our standards of living and access to healthcare. The new human mean temperature is 36.6°C.

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