Normal human body temperature is lower than 37°C

Carl Reinhold August Wunderlich (a German physician, pioneer psychiatrist, and medical professor) is best remembered for his measurement of mean healthy human body temperature of 37 °C (98.6 °F).

Nearly 150 years ago, he analyzed a million temperatures from 25,000 patients and concluded that normal human body temperature is 37 °C. This standard has helped generations of parents judge the gravity of a child's illness. But at least two dozen modern studies have concluded the number is too high. The findings have prompted speculation that the pioneering analysis published in 1869 by Wunderlich was flawed.

In a new study, researchers from Stanford University argue that Wunderlich’s number was correct at the time but is no longer accurate because the human body has changed. Today, they say, the average normal human body temperature is closer to 36.4 °C (97.5 °F).

Body temperature is a crude proxy for metabolic rate, and if it has fallen, it could offer a clue about other physiological changes that have occurred over time. People are taller, fatter and live longer and we don’t understand why these things have happened.

To test their hypothesis that today’s normal body temperature is lower than in the past, researchers analyzed 677,423 temperatures collected from 189,338 individuals over a span of 157 years. Overall, the temperatures of the World War II veterans were higher than measurements taken in the 1970s, and, in turn, those measurements were higher than those collected in the 2000s.

A complicating factor for the comparisons in that the Wunderlich and Stanford data used different methods and instruments. Human temperature can be measured in the mouth, armpit, ear or rectum. Ear and rectal temperatures tend to be half a degree higher than an oral temperature. Axillary temperature, taken in the armpit, tends to be one degree lower.

Wunderlich preferred the axillary method but used a thermometer that was calibrated higher than normal*. The methods used in the Stanford study vary. World War records could have included a mixture of axillary and oral temperatures taken with mercury thermometers (no one is sure about the methods and the precision of the instruments used). The 1970s measurements used readings from oral mercury thermometers exclusively and the data from the 2000s used digital oral instruments.

Age, time of day, physical activity and other factors, which the researchers couldn’t always account for, also affect body temperature.

* - In a critical study of Wunderlich’s work, Dr. Philip Mackowiak concluded that 36.8°C (98.2°F) rather than 37.0°C (98.6°F) was the mean oral temperature. He recommended abandoning the Wunderlich’s standard.

Compiled by Praveen Kumar S (rajuspk@gmail.com)

References -

1. The Wall Street Journal

2. The Journal of the American Medical Association

3. Wikipedia