Human Body Temperature Varies: Historically, by Age, and Individually

a thermometer for measuring body temperature

Debbie says:

“Normal” is a word that always makes me skeptical: “not normal” so often means “within the range of human variation” or “not like people I know” or “not like adult white men.” I’ve known for a long time that different individuals have different “normal” body temperatures, and there’s nothing magic about 98.6 (or 37 if you use the much more comprehensible Celsius scale).

I didn’t know that the 98.6 number is from the 1850s, and is based on a study of 25,000 people in Leipzig, Germany.

Brian Resnick, writing at Vox last month, reviewed new findings which say that 98.6 is no longer “normal.” Instead,

In a new paper in the journal eLife, from a group of scientists at Stanford University, researchers analyzed three different databases of human body temperature readings, starting with a cohort of Civil War veterans, then to temperatures taken in the 1970s, and ending with data collected between 2007 and 2017.

Overall, the researchers found that men born in the early 1800s had average body temperatures 1.06°F higher than men today. Women born in the 19th century were, on average, 0.57°F warmer than women today.

Data analysis rules out the possibility that it’s just better thermometers. And while there are some theories about what might be causing the change, none have really caught on more than any other.

Resnick uses this study as a jumping-off point to examine variations in body temperature other than the historical:

As it stands, comparing an individual’s body temperature to a single number doesn’t make much sense.

“You can’t say there is a temperature you should be at, because it depends on who you are,” [Stanford researcher Catherine] Ley says.

Women tend to run a little hotter than men, and their body temperature can fluctuate with the menstrual cycle. Age matters too. The older we get, generally, the colder we are.

A 2017 study in BMJ of 35,500 patients found that our average body temperature declines around 0.03°F every decade (maybe due to the loss of fatty tissue under the skin). So it makes sense if Grandpa complains about being cold over time. People with a higher body mass index tend to run hotter than thinner people (as people with a higher body mass are more insulated). Overall, some people may run half a degree hotter or colder than “average” and that’s fine for them.

However, neither Resnick the journalist, nor Ley the researcher, go far enough. Here’s Ley again:

Instead of a set number like 98.6, Ley says it would be better if there were a sliding temperature scale for individuals to figure out what’s normal for their demographic group.

“If I go into the doctor and I’m 30 years old, and I have a BMI of 20, and it’s 2 o’clock in the afternoon, there will be a normal for that series of characteristics,” she says. “It would be so nice if we could boil health down to simple rules, but biology is more complex.

Wrong! Leaving aside all of the problems with BMI, that’s still an attempt to define “normal” in a way that makes simple individual variations abnormal. There might be a mean, or a median, for that group, but the only possible “normal” of any value would have to be a range — and that range probably wouldn’t look very different from a general human variation range.

Even though this conversation is taking us further away from “one true number,” the article still doesn’t mention that having a fever (i.e., running hot enough for it to be a problem), or having a troublingly low temperature, is very likely to be coupled with other symptoms: am I shivering? am I sweating?

Based on this article, Ley still wants to rely on numbers provided by instruments, sliding scale or not. She wants to be able to say that if you’re not in the center of your group’s statistics you’re not “normal.” While I am interested in these variations over personal time and historical time, I still want to land on a place where “is this temperature abnormal for you at this time in your life?” is the question the doctor wants to answer.