Human body temperatures are falling

New data shows that body temperature has fallen since the 19th century...
06 February 2020

Interview with 

Julie Parsonnet, Stanford




What’s a “normal” human body temperature? Probably not what you’ll likely see documented in the text books, as - speaking from Antarctica to Chris Smith - Stanford’s Julie Parsonnet explains…

Julie - The number that most people use nowadays was established in the 19th Century by a German doctor named Carl Wunderlich, who took about a million temperatures on 25,000 people. And on little pieces of paper, wrote them down and did all these calculations, figuring out what is the right temperature for the time of day and for women and for men. And really, he did it absolutely stellar job. Just fantastic. Um, but things have changed since the middle of the 19th Century.

Chris - In what way?

Julie - We're a lot healthier now than we were then. We have a lot fewer infections. We live in much more comfortable circumstances. We do a lot less physical labour. We have much better homes and much better hygiene and we've gotten taller and we've gotten fatter and we've gotten colder. We took temperatures from civil war soldiers in the, from the 19th Century - so people who are born as early as 1800 actually - and then we compared them to temperatures from two other groups: one that was in the 1970s, and then our clinical data that we have from Stanford University with a lot of temperatures, and we found that the temperatures are declining over time and we couldn't explain it by just the thermometers that were used or the way they took their temperatures. We did a lot to try to piece through that. It really appears that people who live in the 19th Century just had a really harder life. They had a lot more infections, things like tuberculosis and syphilis and rheumatic heart disease, and they lived under harsher conditions. They had no air conditioning and heating and their lives were just harsh enough that they just had to rev up their bodies all the time to keep themselves safe, to keep themselves healthy. They had to fight all those infections that were taking a lot out of them by increasing the metabolism.

Chris - How much of a difference is there comparing people? When Wunderlich did his research in Germany in the 1850s and today

Julie - It's about 1.1 degrees Fahrenheit is about the difference that we see between then and now. But I do want to make the point, 1 degree Fahrenheit is a lot, but we could see a fairly big difference within a person almost that high even today. So if you take a temperature, compare the temperature of a young adult late in the day with an elderly person early in the morning, their temperature might be more than one degree Fahrenheit different. So that although 1.1 on average seems like a big difference, temperature does fluctuate from person to person and from time of day especially, and by age. And so, again, I do think that that we shouldn't all expect our temperature to be exactly 37.5. We're all going to vary and we need to start thinking about that a little bit.

Chris - It's an interesting hypothesis and it's biologically plausible that the lower burden of disease in the population is translating into a lower average body temperature. But are there any actually studies that would support that hypothesis?

Julie - There are not a lot of studies that look at why this would happen that would show the reasons that it might've occurred. There is one study that was done a few years ago that looked at the level of inflammation in people since even 20 years ago and they shown that our inflammatory levels - the rates of proteins that indicates inflammation - have dropped even over the last 20 years. So that supports our underlying hypothesis that this is, that we're becoming less inflammatory, that we are just having less things that are revving up our immune systems. And there are also some data that suggests that at least one other country that has a lot more infectious disease burden, Pakistan, their temperature is still high. It's still 98 six so again, it supports that there are things that correlate with inflammation that are also correlating with temperature.

Chris - If one looks back in history, you can see that in fact disease was concentrated at the bottom end of the social spectrum and there were different diseases, perhaps diseases of affluence at the top end of the social divide. If you stratify your data by social class, can you see a temperature effect in there from the early days because obviously people with a lower burden of disease would have had less inflammatory stimulus and therefore their body temperature, if you're correct, would have been a bit lower.

Julie - We haven't really looked at it. The people that we're looking at are all veterans of the civil war, which makes them a rather unique group and people who required getting their pension to move forward in life. So they're not people who are at the upper end of the spectrum who could get by without their small military pension. So we didn't really have a great diversity of population and wealth. But I, you know, it's an interesting question. I think it's worth going back and trying to see if there's a better way that we can piece through that information and, and look into that hypothesis. We did look parenthetically at individual infections and say, did these raise temperatures? And if we exclude people with these diseases, does it matter? And we did find that people who said they had tuberculosis, which is a very small number, and people who said they had bladder infections did have higher temperatures overall. So we could find signs that the temperatures we were reading were accurate and that these infections do affect things.


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