
Human body temperature has decreased in United States, study finds - elorant
http://med.stanford.edu/news/all-news/2020/01/human-body-temperature-has-decreased-in-united-states.html
======
flyingfences
Previous HN discussion:
[https://news.ycombinator.com/item?id=22002734](https://news.ycombinator.com/item?id=22002734)

------
swebs
>The decrease in average body temperature in the United States could be
explained by a reduction in metabolic rate, or the amount of energy being
used. The authors hypothesize that this reduction may be due to a population-
wide decline in inflammation

Let's not ignore the elephant in the room. A sedentary lifestyle with minimal
exercise has become the norm.

~~~
yk
That seems pretty straight forward to measure, you just take the temperature
of people who exercise regularly (say three times a week) and compare that to
people who work physically demanding jobs, and to people who work in office
jobs, and don't exercise.

(I would almost expect, that the study authors know of something like that,
and would have mentioned it, if it were the case.)

~~~
dangerboysteve
I imagine apple has this data already with their apple watch data collections.

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LeSaucy
There is no body temperature measurements in Apple Health from the Apple
Watch, but that would definitely be an interesting data point were it
available.

~~~
jjeaff
Yes, but also a potential privacy concern since if you have temperature data
for a female, you could use it to pinpoint when ovulation may be occurring. I
can only imagine that due to some of the recent privacy issues with cycle
tracker apps that information might already be exploited for some nefarious
advertising campaign.

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BoiledCabbage
So the summary is they aren't certain why, but it seems like it's due to
people being a lot more sedentary, having lower metabolisms and being in
climate controlled buildings a lot more.

Unfortunately, it seems pretty predictable given the changes in our
lifestyles...

~~~
plopz
Why is it unfortunate? The article didn't seem to say whether this change was
good or bad, although I would assume a decrease in inflammation would be good?

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kempbellt
> I would assume a decrease in inflammation would be good

I'd be careful with that assumption. Context means everything. It depends on
what caused the inflammation, and what is causing its decrease.

~~~
jonas21
Here's the relevant context from the study:

> _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. For example, in the mid-19th century, 2–3% of the
> population would have been living with active tuberculosis (Tiemersma et
> al., 2011). This figure is consistent with the UAVCW Surgeons ' Certificates
> that reported 737 cases of active tuberculosis among 23,757 subjects (3.1%).
> That UAVCW veterans who reported either current tuberculosis or pneumonia
> had a higher temperature (0.19°C and 0.03°C respectively) than those without
> infectious conditions supports this theory._

I would assume that less tuberculosis is a good thing (though, this being
Hacker News, someone will probably show up to question that assumption too).

~~~
kempbellt
Inflammation is one of the body's natural defense mechanisms to issues like
infection, or injury. Treating inflammation like it is the "bad thing", while
ignoring (or not understanding) the underlying cause of it is not good, and
potentially hazardous. People do this all the time by taking ibuprofen to
treat headaches or minor pains that are natural indicators that something is
wrong (dehydration, infection, etc).

Tuberculosis is a disease, and widely accepted as detrimental to human health.
I agree, less tuberculosis = a good thing.

This being a medically related post, more information, and less assumptions is
better for everyone. Thank you for adding context.

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moralsupply
This has obviously been planned by the cold-blooded Reptilians

------
CamelCaseName
I found this passage interesting (about the impact of climate controlled
environments on resting metabolic rate)

> Changes in ambient temperature may also explain some of the observed change
> in body temperature over time. Maintaining constant body temperature despite
> fluctuations in ambient temperature consumes up to 50–70% of daily energy
> intake (Levine, 2007).

> Resting metabolic rate (RMR), for which body temperature is a crude proxy,
> increases when the ambient temperature decreases below or rises above the
> thermoneutral zone, that is the temperature of the environment at which
> humans can maintain normal temperature with minimum energy expenditure
> (Erikson et al., 1956).

> ... Thus, the amount of time the population has spent at thermoneutral zones
> has markedly increased, potentially causing a decrease in RMR, and, by
> analogy, body temperature.

Source:
[https://elifesciences.org/articles/49555](https://elifesciences.org/articles/49555)

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hopia
The latest temperature data was measured from patients visiting hospital.
Older people are naturally more prone to health problems and would therefore
be visiting hospital more than younger people, on average.

Our average lifespan is longer than it used to be. I'd assume older people
have a lower base temperature. This would skew the results statistically.

~~~
RhodesianHunter
I would assume that a well constructed study would account for this.

~~~
cosmotic
In my experience, studies are typically not well constructed.

See the numerous studies that cannot be replicated, the below-the-surface
iceberg of unpublished (unfavorable) studies paid for by for-profit
institutions, or the shockingly small sample set or biased sample set in most
studies. Most studies expose this information implicitly but it's up to the
reader to determine the bias or replicate the study.

~~~
hwillis
The study compared people born after 1990, so if there were any old people in
that cohort this study would be much bigger news.

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pandaman
I was always confused how 37 C was a normal temperature in the US. In the USSR
where I grew it was a fever and the normal was 36.6. Could it be the way it's
measured?

~~~
elric
Not sure about any US/USSR differences, but there can be pretty big
differences between people individually, between measurements in the same
person throughout the day (or month for women), and between how it's measured.
37°C is an average.

Axillary (armpit) measurements, for example, are a lot colder than oral or
rectal measurements. Perhaps the Russians favoured the former, while the
Americans preferred the latter?

I suspect there's a nice little bell curve with 37°C in the middle.

~~~
pandaman
Yes, Russia measures temperature in the armpit. Is it really that much colder
than in the mouth?

Edit: A 37 temperature would automatically give you an excuse from school when
I was a kid and, obviously, children always were finding ways to cheat. If
just putting the thermometer in your mouth worked then either I was in the
dark or nobody has discovered this.

~~~
elric
Yes, the armpit is roughly 0.5°C colder than the mouth.

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mechhacker
The original average was set in 1868.

Isn't it likely that this is just a measurement/calibration/methodology error?

~~~
pseudoramble
They do mention this in the article:

> As part of the study, the authors investigated the possibility that the
> decrease could simply reflect improvements in thermometer technology;
> thermometers used today are far more accurate than those used two centuries
> ago. “In the 19thcentury, thermometry was just beginning,” Parsonnet said.

> To assess whether temperatures truly decreased, the researchers checked for
> body temperature trends within each dataset; for each historical group, they
> expected that measurements would be taken with similar thermometers. Within
> the veterans dataset, they observed a similar decrease for each decade,
> consistent with observations made using the combined data.

~~~
mikedilger
I'm unconvinced by their method to assess whether temperatures truly
decreased. Why wouldn't a methodological error not also apply to the veterans
dataset? E.g. all thermometers of an era reading slightly hot (rather than
them all being less accurate in any direction)

~~~
pseudoramble
Yeah, fair point! I'm not well versed in this kind of stuff. So I'm not sure.

Maybe the original paper discusses the point though? Perhaps they have a way
to normalize temperatures to a baseline across all data sets and evaluate that
way.

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mattkrause
Link to the actual study:
[https://elifesciences.org/articles/49555](https://elifesciences.org/articles/49555)

Protsiv, M., Ley, C., Lankester, J., Hastie, T., & Parsonnet, J. (2020).
Decreasing human body temperature in the United States since the industrial
revolution. _eLife_ , 9, e49555.

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dorfsmay
Completely unscientific observation:

When working in offices, I have noticed that people who drink pop/soda daily
prefer lower room temperature than people not consuming them at all. I might
have "noticed" it because of a bunch of biases, but I'd love to see a proper
study on that.

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mrfusion
This could simply be a result of nationwide iodine deficiency.

Our main source is fortified salt and everyone is cutting back on their sodium
or using fancier salts that don’t add iodine.

I make sure to put a pinch of iodized salt in everything I cook.

~~~
hwillis
This study is from before iodine was added supplementally (1924 in the US).
You are correct that iodine deficiency leads to low body temperature due to
hyperthyroidism.

Dairy and grain are the biggest sources of iodine. To get your iodine from
salt alone would require over 3 grams per day, which is quite a lot. The
average American consumes 3.4 g, DV recommendation is 2.3 g.

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trhway
i'd guess that given the antibiotics, better nutrition, wide availability of
warm clothes/shoes, etc. natural selection pressure for higher temperature has
naturally decreased (in such historically short period that decrease mostly
manifested itself in much lower child mortality).

People also became bigger during the last century which results in better
thermodynamics, ie. surface/volume ratio, thus less need for burning of the
internal fuel, i.e metabolic rate can be lower. Compare - elephant's 96.6 vs.
cat's 101.5 body temp.

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jl2718
A lot of comments here blame obesity/sedentary lifestyles/junk food for low
body temperature. Let’s look at this:

    
    
      Obesity: raises temperature
      exercise: lowers resting temperature
      junk food: raises temperature
    

What if it’s the reverse? What if your desire to eat and not exercise had a
lot to do with being cold?

Frankly I’m not buying the arguments in the article. Occam’s razor says it’s a
common nutritional deficiency or environmental toxin affecting the thyroid.

~~~
jl2718
Anecdote: I’ve been getting noticeably colder every year for no natural
reason, so I’ve tried many things: fish oil, iodine etc. The only thing that
worked was iron. I took 600mg ferrous gluconate with 5g vitamin C, and I was
sweating hot and felt amazing for the next 24 hours. Iron overdose can be
deadly and irreversible, so I have not repeated this and do not recommend it.

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julius_set
I’m not sure how much this matters but I’ve been on TRT therapy for a while
now and I remember before I started my limbs would get so cold, and hands
would be so cold to touch.

After therapy my significant other tells me I’m like a body heater constantly
radiating heat possibly due to high synthesis and metabolism.

I’m curious but there was a chart I saw as well regarding men’s testosterone
levels declining over the last century I wonder how related that is to lowered
body temperatures

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pattusk
Could some of it be explained by demographic changes (immigration)? iI
remember always being taught growing up than 37.5°C was the average human body
temperature. However after moving to East Asia I met plenty of people who
consider 37.5 a light fever and 36 the normal temp. On my phone so no data to
back it up for now though...

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rhacker
I wonder if they simply looked at the antibiotic use. For example when that
was introduced specifically, and more so when we started "over-prescribing"
it. That's got to have had an effect of persistent low grade fevers that exist
in just about everyone.

~~~
hwillis
Body temperatures are currently lower, so the opposite of what you're
suggesting with low grade fevers from overprescription of antibiotics.

However, fevers are caused by a subset of the interleukin family, which as a
whole causes inflammation. Inflammation was the author's suggestion.

Also, low grade fevers aren't a thing. Fever is caused by the immune system.
You can only have a continuous infection with diseases that avoid or supress
the immune system. There are a small number of brain-affecting infectious
diseases like meningitis that can raise body temperature on their own.
Chronically recurring stuff will also give you intermittent fevers, but
there's no low-grade. You have it or you don't.

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danbr
WSJ article referencing this research:
[https://www.wsj.com/articles/98-6-degrees-fahrenheit-isnt-
th...](https://www.wsj.com/articles/98-6-degrees-fahrenheit-isnt-the-average-
any-more-11579257001)

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dmitryminkovsky
No mention of high prevalence of cold beverage and ice consumption. Couldn’t
possibly be related?

~~~
yakshaving_jgt
I don’t think that’s how the human body works.

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nostrademons
Interesting. 100F is supposed to be normal human body temperature, by
definition. (Fahrenheit set 100F as normal human body temperature and 0 as the
coldest temperature he could achieve, using a mixture of ice, water, and
ammonium chloride.) For years I'd heard that the reason normal body
temperature is 98.6 is because Fahrenheit was running a slight fever at the
time he made his scale, but this article suggests that it might've been
accurate, but body temperature has been decreasing. If you extrapolate the
numbers in the article out (a drop of just over a degree since the 1800s) to
when Fahrenheit made his scale in 1724, it's possible that normal body
temperature actually _was_ very close to 100F then.

~~~
DenisM
> Fahrenheit set 100F as normal human body temperature

Nope:
[https://en.wikipedia.org/wiki/Fahrenheit#History](https://en.wikipedia.org/wiki/Fahrenheit#History)

~~~
tomjakubowski
Much more here, with references. An early version of the scale set 96F to
human body temperature, apparently because 96 divides evenly in half five
times, so it's easy to make graduated markings between the reference points on
your mercury thermometer.

[https://antoine.frostburg.edu/chem/senese/101/solutions/faq/...](https://antoine.frostburg.edu/chem/senese/101/solutions/faq/zero-
fahrenheit.shtml)

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therealx
I've noticed that my temp is always 1 degree lower than it should be. It's
never been a problem and has been that way since birth.

~~~
hwillis
The normal range of resting body temperatures is anywhere from 95 to 100 F.
People vary a lot.

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bobloblaw45
I'm leaning strongly on the side of thermometers were less accurate and gave a
higher reading in the early 1800s.

~~~
hwillis
The study was on measurements taken in the early 1900s, not 1800s. Also, it's
not like technology suddenly started in the 30s or something. Liquid nitrogen
was first produced in 1883. The periodic table was published in 1869. Cells
were discovered in 1665. Science was a lot more than banging rocks together.

You would expect the opposite of what you're suggesting, if anything. If water
was used for calibrating temperature then there would be fewer good ways to
control the impurity level, which would lower its freezing point. Not to
mention Fahrenheit originally defined body temperature as 96 F, and it was
later corrected higher.

All of the above is totally moot because we _have_ plenty of medical
thermometers from the 1900s and earlier, and they are remarkably accurate and
the error is not particularly biased in either way. This is a highly
researched area because the accuracy of whether thermometers is important to
historical and climate research. Medical thermometers from the 1800s are good
to at least 1/5th of a degree and instrumentation thermometers were good to
hundredths.

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ZeroFries
When I shovel the driveway, I'm sweating wearing no jacket, but I freeze
sitting on the porch in a parka.

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redwood
Seems conceivable that higher proportion of body fat aka more insulation could
also contribute

~~~
hwillis
Fat would do the opposite. The core body temperature would be the same,
because otherwise it would be too hot. The distal body temperature would be
insulated from the sources of heat, so the inside of your mouth, forehead,
rectum etc would be cooler.

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juskrey
Just look inside any US pharmacy

~~~
freeone3000
For what, exactly?

~~~
hoka-one-one
Love

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cheschire
I would love to see some research that attempts to correlate this to other
species that have local declines in birth rate. Maybe it’s a mammalian
response to over population.

~~~
hwillis
No, that's silly. Unless mammals are magic, the only way a species could
possibly know about population is by the number of individuals it sees
locally, and the number they see. That would cause there to be a huge and
noticeable difference between eg rural and urban populations.

~~~
cheschire
Well, I didn't write an entire introduction paragraph for a research paper,
sure, but outright dismissing as "silly" seems rather closed minded.

