Body Temperature at 37 Celsius (98.6 Fahrenheit) is No Longer the Norm – Humans Are Getting Cooler

January 10, 2020 – It’s not that humans are going to go from being endotherms to ectotherms, but a recent study is showing that 37 Celsius is no longer our normal body temperature. To make it even more interesting our body temperatures are decreasing by 0.03 Celsius (0.054 Fahrenheit) per decade.

States Julie Parsonnet, School of Medicine, Stanford University, “Our temperature’s not what people think it is. What everybody grew up learning, which is that our normal temperature is 98.6 [Fahrenheit], is wrong.” The study sampled 200,000 using data from a number of U.S. sources.

A recent British study of 25,000 concluded that our average body temperature today is 36.6 Celsius (97.9 Fahrenheit). The American research noted that the temperature of males born in the 21st century averaged 0.58 Celsius (1.06 Fahrenheit) lower than men born at the beginning of the 19th century. The difference for women was less at 0.32 Celsius (0.58 Fahrenheit).

Who first calculated human body temperature?

A German physician, Carl Reinhold August Wunderlich, was the first to document normal body temperature ranging from 36.2 to 37.5 Celsius (97.2 to 99.5 Fahrenheit). The median was calculated to be 37 Celsius (98.6 Fahrenheit) and that’s what we learned in school. But in fact, some of us run hotter and some run colder.

So why are human body temperatures decreasing or are they?

When you consider normal body temperature has a variance of 1.3 Celsius (2.3 Fahrenheit) it would suggest that the data collected fits easily within that range.

The Stanford study is an interesting piece of work. The analysis was done on 677,423 human body temperature measurements covering populations spanning 157 years. Could the differences in the 19th century be a reflection of less precise thermometers and methods in obtaining body temperatures, or are these findings real?

The proof in the pudding is the measuring of modern populations because control of the calibration can be readily verified as accurate although biases could exist based on the type of thermometer used and from where the measurement was taken (oral versus other means).

Having taken potential variables into consideration for the more recently collected data, the researchers at Stanford concluded “the observed drop in temperature reflects physiologic differences rather than measurement bias.”

One of the reasons posited is Americans over the past two centuries have increased in size and weight. They are bigger and that might affect body temperature. The other possibility is that metabolic rates have declined since the beginning of the 20th century based on other experimental data.

What other factors could explain a decline in average body temperature?

Besides a resting metabolic rate, decreasing levels of inflammation in the general population because of improved living standards, better heating and air conditioning in homes, fewer chronic infections, less dental disease, and antibiotics could all play a factor. It is particularly interesting to link how much time Americans spend in heated homes in the winter, and air-conditioned homes in the summer in the latter part of the 20th and now the 21st century, a phenomenon described as thermoneutral, and its impact on metabolic response and subsequently body temperature.

The Stanford study summary states, “normal body temperature is assumed by many, including a great preponderance of physicians, to be 37°C …. Our investigation indicates that humans in high-income countries have changed physiologically over the last 200 birth years with a mean body temperature 1.6% lower than in the pre-industrial era. The role that this physiologic evolution plays in human anthropometrics and longevity is unknown.”