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Is our mean body temperature decreasing and why? What can we learn from it?

Image credit: Kelly Sikkema / unsplash

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J
Juran Nov 11, 2020
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Necessity

Is the problem still unsolved?

Conciseness

Is it concisely described?

Although the average human body temperature is considered to be 36.8 ± 0.5 °C (98.2±0.9 °F), the newest data show that it decreases in the last decades.

Questions:
Is there a valid biological/environmental reason underlying the mentioned decrease?
What could have caused the decrease?
What a decrease can further cause?
How does it affect our health?

Data

37 °C (98.6 °F) - 1861 first body temperature measurement (Carl Wunderlich)
36.61 °C (97.9 °F) - 2017 study on 35,000 UK adults
36.39 °C (97.5°F) - 2019 study in Palo Alto, California

Reasons

It could be due to different measurement methods
Human body temperature varies with the method used. It can be measured:
in the rectum, vagina, ear (37.5 °C / 99.5 °F),
mouth (36.8 °C / 98.2 °F),
under the arm (36.5 °C / 97.7 °F),
on the skin,
and using heat flux sensors.

It's (probably) not region-specific
The decrease in the average body temperature was also seen among the Tismane, an indigenous population of forager-horticulturists in the Bolivian Amazon. In 16 years, researchers observed an average body temperature decrease of 0.09 °F (0.05 °C) per year, finally reaching 97.7 °F (36.5 °C). The same was observed across the US in large samples of a few thousand individuals .

It's (probably) not due to a decrease in the number of infections
Due to a decreased rate of food and water contaminations, improved hygiene, vaccinations, and medical treatments, we would expect that the overall human body temperature mean reduced. Since the patient's temperature is measured during clinical diagnosis, scientists were able to check that hypothesis. Adjusting the data to a patient's diagnosis and biomarkers of infection (account for drug consumption, too), it was shown that infections do not account for the observed decrease in average body temperature.
The data from Bolivia also supports the fact that overall decrease in average body temperature is not due to a decreased rate of infection, because there was no comparable improvement in treatments, hygiene, or other factors in Bolivia, compared to the US .

Ideas

Could it be due to:
  • our "body thermostat" improved (by the inner/outer factors)
  • reduced physical activity in general?
  • population getting older (body temperature decreases as we age)


Update on session's contributions

Most probably because of:
  • the decrease in atmospheric O2 and an increase in atmospheric CO2, that cause a fall in body temperature (scroll down and for more info in the "Other environmental factors (if not global warming)" contribution
Could be because of:
  • Different measurement methods
  • Prostaglandins, which are influenced by environmental temperature and intrauterine devices
  • Obesity, through a hypothalamus regulation
  • Measurements were taken in different climate zones
Probably not because of:
  • Region specificity
  • A decrease in the number of infectious diseases
  • Population getting older (old ones having lower body temperatures)
  • Global warming

[1]https://www.sciencedaily.com/releases/2020/10/201028171432.htm

[2]https://en.wikipedia.org/wiki/Human_body_temperature

[3]https://advances.sciencemag.org/content/6/44/eabc6599

13
Creative contributions

Problems with Carl Wunderlich's temperature measurements

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Shubhankar Kulkarni
Shubhankar Kulkarni Nov 12, 2020
This one paper talks particularly about the problems with Carl Wunderlich's measurements.
  1. The device, thermometer, used by Wunderlich was cumbersome and required 15 to 20 minutes to equilibrate to make axillary (armpit) measurements. Thermometers, today, are more reliable and equilibrate more rapidly.
  2. The mouth and the rectum are the preferred sites for monitoring body temperature today instead of the armpit. Carl Wunderlich preferred axillary measurements.
  3. There have been recordings after Wunderlich of the body temperature of normal adult populations, which recorded temperatures closer to 36.6°C (98.0°F) but the one provided by Wunderlich is still considered normal, probably, because it was a milestone (the first record).
I don't know. We may have to consider Wunderlich's measurements with a pinch of mercury :)

[1]Mackowiak PA, Wasserman SS, Levine MM. A Critical Appraisal of 98.6°F, the Upper Limit of the Normal Body Temperature, and Other Legacies of Carl Reinhold August Wunderlich. JAMA. 1992;268(12):1578–1580. doi:10.1001/jama.1992.03490120092034 https://jamanetwork.com/journals/jama/article-abstract/400116

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J
Jurana year ago
I agree. Carl wunderlich also stated that he had a sample of 1 million adults. We definitely need to be careful.
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Povilas S
Povilas S10 months ago
😁
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A bit of everything

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Martina Pesce
Martina Pesce Nov 13, 2020
Often, biological outcomes are the results of a series of components that if would be considered alone could not create the observed phenomenon. Temperature regulation is tight to so many other regulation factors that it would not surprise me to be one of this phenomenon.
Still, really interesting to try to make an accurate list of which are these influencing factors.
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Shubhankar Kulkarni
Shubhankar Kulkarnia year ago
Martina Pesce I had the same thought. You beat me to it. :)
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Jurana year ago
I agree with you. Maybe I can do an updating list of proposed "maybe's and maybe-not's" on the bottom of session description, so that every newcomer has an overall picture what has been proposed or not.
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Other environmental factors (if not global warming)

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Povilas S
Povilas S Jan 05, 2021
I'm not very convinced that global warming can be ruled out so easily. If not the higher ambient temperature, could it be that higher CO2 concentrations in the atmospheric air are somehow influencing this?

One thing that Juran mentioned in the session particularly caught my attention. That is - the indigenous population in the Bolivian Amazon that has been studied and showed similar results of temperature decrease as elsewhere. Indigenous people have a very different lifestyle than rural populations and this traditional lifestyle, if changing at all, changes very slowly. Therefore if we see a similar decline in both indigenous and rural populations that are continent apart and assume that the causing factor has to be general for both of them we are left with very few similarities to consider as probable factors. Therefore global environmental changes stand as a highly probable cause.

According to that article, temperature measurements were performed during the years of socioeconomic change for indigenous communities, but it's not clear to what extent that change has been happening before. BT decline amongst Tsimane people was rapid (0.5 degrees in 16 years), however, it's unlikely that socioeconomic change for Tsimane populations was so profound in only a couple of last decades. It's also not clear if the change in BT only started at the beginning of the 21st century, because there was no data provided about BT of Tsimane people before 2002 and how or if it changed during the course of the 20th century. Human-caused climate change becomes evident around the 1980s, so if BT data since that time was available, it could be correlated.

It's also a possibility that local environmental differences could be reflected in BT change. It was mentioned in the article that those people who lived in more remote villages had higher BT than those who lived closer to a bigger town. Could this be directly related to the percentage of natural surroundings that people are immersed in? And if that socioeconomic change was really greater than ever could it be that it caused rapid deforestation or other environmental changes that lead to a decrease in BT?

It would be nice to find more similar data about indigenous communities covering larger periods of time. But of course, that kind of data is difficult to obtain.
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Shubhankar Kulkarni
Shubhankar Kulkarni8 months ago
Just adding a reference here, which might be useful while writing about the whole thing:
The global carbon project measures the greenhouse gas emissions and notes its causes (https://en.wikipedia.org/wiki/Global_Carbon_Project).
Particularly, this image is useful - https://en.wikipedia.org/wiki/Global_Carbon_Project#/media/File:Global_Carbon_Emissions.svg
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Darko Savic
Darko Savic9 months ago
If the rise of atmospheric CO2 is the cause, then many animal species should be affected as well. How is the body temperature of wild animals or a domesticated species that lives outdoors? Even cattle results would be viable.

In which situations is the body temperature of animals taken/logged on a regular basis, over years? Expensive racehorses, various famous endangered animals that are being tracked, etc.

How are sea animals doing? Dolphins? Whales? Do any of them have permanent monitoring devices that also report the body temperature?
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Shubhankar Kulkarni
Shubhankar Kulkarni10 months ago
Povilas S CO2 concentrations stood out when I read this. So I did some research and found that it might be true - an increase in the ambient CO2 concentration may decrease core body temperature. Here is an amazing paper (https://journals.physiology.org/doi/full/10.1152/japplphysiol.00010.2010) that demonstrates it (please look at Figure 1A). They made their participants exercise (cardio) for 60 minutes with and without CO2-enriched ambient air and monitored their core temperature (oesophageal temperature), blood pressure, and heart rate throughout. They found that although the blood pressure and the heart rate did not change, the rise in body temperature due to exercise was lower in the CO2-enriched air and the normal air.

Moreover, according to another paper, the solubility constant of CO2 in the blood depends on body temperature. The rise in body temperature decreases the solubility constant. Therefore, probably, to accommodate the increased CO2 from the ambient air in the blood, the temperature of the blood decreases. (https://www.jbc.org/content/44/1/131.full.pdf)

Also, the global average atmospheric carbon dioxide was the highest in 2019 (409.8 parts per million). Carbon dioxide levels then were higher than at any point in at least the last 800,000 years. (https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide#:~:text=Global%20atmospheric%20carbon%20dioxide%20was,%C2%B1%200.1%20ppm%20per%20year.) Therefore, even if certain geographical areas have a higher CO2 concentration than others, the difference is insignificant compared to the increase in the global average over time.

However, there is a small twist in this. Although the body temperature decreases to accommodate more CO2, it also decreases to compensate for the decreased oxygen levels in the atmosphere. There are two ways in which the body compensates for oxygen deficiency -
1. By increasing respiration (inhalation of O2) and by increasing the solubility of O2 in the blood.
2. Lowering oxygen demands by the body by decreasing the body temperature. (https://journals.physiology.org/doi/abs/10.1152/ajplegacy.1937.120.1.190?journalCode=ajplegacy) It was shown that mice exposed to air containing 8.1% oxygen showed a drop in their body temperature by about 2.5 to 3 deg C. Also, if CO2 is increased by 3%, the body temperature decreases further. Also, if the room temperature is elevated to 35 deg. C., the mice died when exposed to an oxygen pressure, which they could tolerate at normal room temperature (probably 25 deg. C.). To drive the point further, here is a reference that shows the fall in atmospheric O2 content over the years. (https://www.oxygenlevels.org/)

Taken together, these studies demonstrate that decrease in atmospheric O2 and an increase in atmospheric CO2 cause a fall in body temperature. As these conditions are currently observed at the global level, the global fall in the body temperature could be attributed to these.
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Not global warming

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Shubhankar Kulkarni
Shubhankar Kulkarni Nov 12, 2020
There is little doubt that ambient temperature affects body temperature (although not as greatly as in the poikilotherms) but in a narrower range. My guess was that to maintain the optimum body temperature even when the ambient temperature rises (such as due to global warming), the human body might maintain a lower temperature to combat the rise in the ambient temperature. The lowered body temperature might act as a buffer (or insurance) and compensate when the ambient temperature rises.

However, I found the opposite results. A warmer (than the body) environment increases the body temperature. It is easier for the body to store the heat (when the ambient temperature is higher) and use it when the ambient temperature is lower than the body temperature than maintaining a lower body temperature and generating heat when the temperature falls.

This indicates that the factors responsible for lowering body temperature are powerful enough to overcome the increase due to global warming.

[1]https://www.jci.org/articles/view/135006

[2]https://theconversation.com/how-rising-temperatures-affect-our-health-123016

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Povilas S
Povilas S10 months ago
"It is easier for the body to store the heat (when the ambient temperature is higher)" - what would be the mechanism of the body for doing this?
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Shubhankar Kulkarni
Shubhankar Kulkarni10 months ago
Povilas S
1. "Heat is stored in body fluids, muscles and bones, resulting in an increase in the core temperature."
2. Heat shock proteins are activated to protect enzyme systems and prevent cellular death. These proteins enable excess heat to be stored temporarily. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4515708/)
Another interesting study shows heat stored in muscles during exercise (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2269891/).
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Jurana year ago
Thank you for the contribution! I was thinking about this one, too, but I couldn't find something interesting.
For me, it also makes sense that the body temperature decreasing could be a potentially good strategy against global warming, but I didn't see any signs of this happening. I guess there really is something with much bigger effect that causes the drop.
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I don't think its population getting older

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Shubhankar Kulkarni
Shubhankar Kulkarni Nov 12, 2020
Body temperature decreases with age; that is true. I tracked the population growth curve back to 1860 when the human body temperature was first noted. Here is the graph of the population over 65 years of age:


The data on the population in the different age classes was available until 1950 and I had to extrapolate it to 1910 and 1861. When I extrapolated the graph, the population over 65 years of age in 1861 was 13,326,883. And the total population back then was 1,290,000,000. The current population over 65 is 727,610,000 and the total population today is 7,800,000,000. If you calculate the percent increase in the population over 65 from 1860 to 2020, the change is about 2*10^(-10) %.

Similarly, the current average body temperature, as Juran mentioned, is about 36.39 °C, which was 37 °C back in 1861. If you calculate the percent decrease in the average body temperature over the period, it is about 1.65%, which is 10 magnitudes higher than the change observed in the increase in the population over 65. The change in the average body temperature is far greater than the change in the older population. Also, as homeotherms, we are far more resistant to change in body temperature than poikilotherms.

Therefore, even though body temperature decreases with an increase in age, the current older population is not significant enough to bring down the average body temperature to the observed one.

[1]https://ourworldindata.org/grapher/age-dependency-breakdown

[2]https://ourworldindata.org/world-population-growth

[3]https://www.prb.org/2020-world-population-data-sheet/#:~:text=The%20world%20population%20is%20projected,as%20in%20the%20United%20States.

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Jurana year ago
I am impressed by the way how you solve problems! The data you extrapolated really says that older population temperature could not be responsible for the drop. I guess it could play a small part in it, but can't fully address it.

What could affect your extrapolated data is the fact that 65 years of age in 1861 was different from being 65 years old today. Since people's lifespan prolonged in this period, maybe being 65 then is more similar to today's 75/80.
But even if we adjust the data (e.g. that today's 65 is 45 in 1861), I think the rate of change would still not be big enough to independently address the problem.

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Maybe the hypothalamus?

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Martina Pesce
Martina Pesce Nov 13, 2020
So, the responsible for body temperature regulation is the hypothalamus.

May have we (human species) done something in the last decades that could have bothered our hypothalamus?

Now, the hypothalamus does a tonne of things, it would be fun to analyze them all, but for the moment I will consider just another thing apart from body temperature, which is hunger.

Obesity has been one of the bigger health beasts to fight in these modern times in which food is incredibly accessible for a good part of the Mondial population (unluckily not for all of it). It triggers a whole hormonal level setting for long periods that our body has never been used to. It sounds likely that it interferes with the temperature system which is overlooked by the same brain area.
Also, looks like that lower body temperature and obesity are related , even if the truth doesn't emerge so clearly due to many divergent opinions .



[1][1] D. Grimaldi et al., “Evidence of a diurnal thermogenic handicap in obesity,” Chronobiol. Int., 2015.

[2][1] F. Bastardot, P. Marques-Vidal, and P. Vollenweider, “Association of body temperature with obesity. The CoLaus study,” Int. J. Obes., 2019.

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Jurana year ago
Thank you Martina Pesce on your contributions! This could be on track of something very specific and easier to check, not only statistically, but also experimentally. I like the second paper you referenced. It's simple and clear.

But what they state is that body temperature is positively associated with obesity markers in men and postmenopausal women, what is different from our theory in the beginning of the session. That would mean that our temperature is decreasing because we are getting more fit.

But they also state some other interesting viewpoints that could help us:
- the increase body area of obese subjects does not influence significantly their temperature,
- adipose tissue is secreting hormones (leptin, adiponectin, and cytokines) that could affect other systems like cardiovascular and increase the temperature
- they suspect on indirect relationship between intestinal microbiota composition and thermal homeostasis in humans
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Martina Pesce
Martina Pescea year ago
Juran K.
the temperature is positively correlated in the second paper, yes, but in many more was the opposite. I also like more the clarity of the last one.


So, considering that the theme is discussed and sustained in both the opposites directions, it may actually not be the best indicator for our investigation.
This actually sounds to me familiar and typical for obesity-related topics.
It's a very complex phenomenon influenced by many different factors that itself influence many other factors.
edit: just after this I found that somehow obesity is side correlated, for that check my contribution "Great for the lifespan"

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Progressive increase in global iron deficiency

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Brett M.
Brett M. Jan 19, 2021
From what I've been reading, it is possible that a steady increase in the prevalence of anemia, or iron deficiency, may have influenced the body's ability to thermoregulate. The prevalence of anemia sits at roughly 24.8%. In other words, 1.62 billion people globally are reported to be anemic . This statistic was presented for the prevalence of anemia between 1993-2005. A study conducted in 2016 found that rates of anemia were even higher in 2011-2012 compared to 2003-2004, suggesting a significant increase in iron deficiency within a decade .

Now, this is important to consider given the important role of iron in thermogenesis. It was found that anemic piglets exhibited a blunted thermogenic response when exposed to cold temperatures . At the same time, a study in 2019 found that mice lacking the ability to sequester and metabolize cellular iron displayed atrophy of white and brown adipose tissue, and a decline in thermogenesis. The authors concluded that this was due to a dysfunction in mitochondrial respiration .

My thinking is, if this trend were true in terms of an increased prevalence of anemia from 1800s to 2000s, it may be possible that a steady decline in blood iron levels may play a role (in addition to the variety of environmental factors at play) in suppressing the body's ability to "generate heat" and thereby slowly decrease the body's basal temperature. In other words, I would expect a positive correlation between blood iron levels and body temperature over time. What are your thoughts on iron deficiency being one of the culprits involved?

[1]https://www.who.int/vmnis/anaemia/prevalence/summary/anaemia_data_status_t2/en/

[2]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5112924/

[3]https://www.nature.com/articles/pr19871174.pdf?origin=ppub

[4]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6531837/

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Shubhankar Kulkarni
Shubhankar Kulkarni8 months ago
Juran Povilas S Here is another study that supports the anemia hypothesis. Blood iron profiles of people from the hunter-gatherer communities and former hunter-gatherers from Dobe in the Kalahari desert of Botswana were measured between 1969 and 1987. Similarly, in 1988 and 1989, iron profiles were measured in people from Kutse, another place in the Kalahari Desert of Botswana, which had recently shifted to a sedentary lifestyle. Here, the effect of change in diet on the blood profile can be ruled out since these residents still relied primarily on wild animals for meat. The profiles from the older studies were healthier than those from the newer ones. The authors attribute the unhealthy iron profiles (anemia) to the transition from a hunter-gatherer lifestyle to a more sedentary lifestyle. Despite any change in diet, morbidity (chronic inflammatory diseases) is high because the residents now live in a relatively aggregated settlement pattern instead of their previous dispersed pattern.

The discussion section sheds further light on the topic. The authors mention that numerous studies have found that the anemia of chronic disease/ inflammation is an acute-phase response to infectious and inflammatory diseases. Reduced blood iron levels, fever, elevated erythrocyte sedimentation rate, and normal to elevated iron stores are effects of various cytokines, particularly interleukin-1 and tumor necrosis factor-alpha. Bacteria require iron but cannot store it themselves and cancer cells cannot store enough iron required for their propagation. Therefore, the body tries to protect itself by withholding the needed iron in case of disease, which is accomplished by removing iron from the circulation and putting it into storage where it is less accessible. The body tries to fight the pathogens and the cancer cells by starving them of iron in addition to other immune pathways. As another example, a decrease in the blood iron levels following surgeries may be a defensive mechanism by the body to prevent postoperative infections. [1]

Therefore, a change in lifestyle can be one of the reasons behind the reduction in body temperature via anemia. Another statement by Povilas S in his contribution "Other environmental factors (if not global warming)" also supports this hypothesis - "It was mentioned in the article that those people who lived in more remote villages had higher BT than those who lived closer to a bigger town." More and more people have immigrated to the cities in the last two centuries and even the cities have developed rapidly to ease human lifestyle. On the other hand, population density in the cities is on the rise (aiding the rate of infections) and also chronic non-infectious diseases.

I, therefore, revisited the reference from my contribution "Reduced number of infections or physical activity" and the paper indicates that there has been a reduction in the mortality due to infections and not a decrease in the number of infections. A reduction in mortality could have been achieved by improved health care over the years, however, the rate of infections is a different issue dependent on the density of population and individual immunity. To add to that, other inflammatory diseases like cancer and diabetes may be contributing to the increased levels of cytokines and anemic symptoms. Another study came to the same conclusion - "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." Using measurements from three cohorts — the Union Army Veterans of the Civil War (N = 23,710; measurement years 1860–1940), the National Health and Nutrition Examination Survey I (N = 15,301; 1971–1975), and the Stanford Translational Research Integrated Database Environment (N = 150,280; 2007–2017) — the authors determined that mean body temperature has decreased by 0.03°C per decade. [2]

Also, in studies where they measured temperatures of a large number of participants, they would not usually consider a person who has a fever for the study. I think it is safe to assume that a very high percentage of the participants did not have a fever during their measurement.

References:
1. Kent S, Dunn D. Anemia and the transition of nomadic hunter-gatherers to a sedentary life-style: follow-up study of a Kalahari community. Am J Phys Anthropol. 1996 Mar;99(3):455-72. doi: 10.1002/(SICI)1096-8644(199603)99:3<455::AID-AJPA7>3.0.CO;2-V. PMID: 8850185. https://pubmed.ncbi.nlm.nih.gov/8850185/
2. Myroslava Protsiv, Catherine Ley, Joanna Lankester, Trevor Hastie, Julie Parsonnet. Decreasing human body temperature in the United States since the Industrial Revolution. eLife 2020;9:e49555 DOI: 10.7554/eLife.49555. https://elifesciences.org/articles/49555
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Shubhankar Kulkarni
Shubhankar Kulkarni9 months ago
An interesting finding, Brett M. I am taking a different approach to confirm whether low iron levels can lead to a decrease in the global average body temperature. I want to sort this out mathematically. From the measurements displayed in the session text, we know that the average human body temperature is/ was 37 deg. C and we know that the current average is 36.39 deg. C. If 75% of the population (having normal iron levels) has a body temperature of 37 deg. C. and if the remaining 25% (having reduced iron levels/ anemic individuals) has a body temperature of X deg. C., (75*37 + 25*X)/100 should equal to 36.39 deg. C. Using this equation, the value to X is 34.56 deg. C.

A body temperature of 34 deg. C. is not impossible and it is a classic sign of hypothermia. However, I could not find the absolute decrease in the body temperature caused by iron deficiency in humans. If there are reports that anemic individuals present a body temperature of 34 deg. C. or lower, we might be able to prove the point.

Furthermore, I am not sure whether all forms of anemia cause a decrease in the body temperature but, in this regard, about 47% of the urban Indians have vitamin 12 deficiency, [1] which can also lead to anemia (Pernicious anemia). 47% is a sufficient proportion to cause a nation-wide decrease in the average body temperature. However, again, I could not find the net decrease in the body temperature caused by anemia.

Here is another study that concludes that a transition from the nomadic hunter-gatherer to the urbanized and sedentary lifestyle has increased the prevalence of anemia. The authors also mention that the diet of the studied people (Kalahari community in Africa) did not change with the transition and, hence, the drop in the iron levels cannot be attributed to diet. [2]

Reference:
1. Singla R, Garg A, Surana V, Aggarwal S, Gupta G, Singla S. Vitamin B12 Deficiency is Endemic in Indian Population: A Perspective from North India. Indian J Endocrinol Metab. 2019;23(2):211-214. doi:10.4103/ijem.IJEM_122_19
2. Kent S, Dunn D. Anemia and the transition of nomadic hunter-gatherers to a sedentary life-style: follow-up study of a Kalahari community. Am J Phys Anthropol. 1996 Mar;99(3):455-72. doi: 10.1002/(SICI)1096-8644(199603)99:3<455::AID-AJPA7>3.0.CO;2-V. PMID: 8850185.
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J
Juran9 months ago
Nice correlation data. I would agree that the correlation between iron and thermogenesis exists. Many sources confirm that low body temperature (BT) is one of the symptoms of low blood iron [1, 2].

Few things I would like to check before concluding anything.
The first one is the statistics of anemia from 2016 compared to 2011-2012, 2003-2004, and 1993-2005. Many historical measurements in medicine are significantly lower because of the increase in the number of BT measurements as a standard procedure (statistically more anemic persons will be measured and reported). Also, technology is becoming more and more sensitive and thus, more patients emerge every day.

The second thing that concerns me is the fact that in human experiments, they mostly correlated iron-deficiency and like you said, "a blunted thermogenic response when exposed to cold temperatures", but there were no data on the effect of iron deficiency on the BT when in normal conditions. Based on experiments on mice, scientists try to explain it like this:
Two possible mechanisms of how iron deficiency could affect thermogenesis are i) anemia and ii) the other involving tissue iron deficiency [3]. Iron-deficiency anemia results in decreased blood oxygen transport, which reduces the physiological response of the body temperature to cold. Low oxygen contents can result in hypoxia and further, hypothermia. The other mechanism involving tissue-iron deficiency accounts for the decreased activity of mitochondrial iron-containing enzymes that produce ATP, which reflects in, again, worse muscle response to cold.

Taking both mechanisms into account, I found strong evidence that the iron-deficiency affects the thermoregulation in cold, but not so significant in normal conditions (correct me if I am wrong). Believing that the conditions in the laboratory where the temperature was measured in previously mentioned papers [4] are normal, I am not so sure to what extent could the rate of anemia affect the observed reduction in BT. If we also take into account the Povilas S´s comment on likeliness, I think we need more research and statistics on this.

References:
1 https://consumer.healthday.com/circulatory-system-information-7/anemia-news-25/health-tip-symptoms-of-iron-deficiency-anemia-730529.html
2 https://www.medicinenet.com/iron_deficiency_symptoms_and_signs/symptoms.htm
3 https://www.nap.edu/read/5197/chapter/19#248
4 https://www.sciencedaily.com/releases/2020/10/201028171432.htm
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Measurements were taken in different climate zones

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Darko Savic
Darko Savic Jan 15, 2021
I haven't read through the entire thread so I'm not sure if this has been pointed out. I noticed that the measurements come from the UK in 2017 and then California in 2019.

The body acclimates to the environment. People living in California where there is always warm versus people living in the UK where the temperatures drop much lower and fluctuate more often might have different body "heating profiles". The study is comparing apples to nashi:) Could the normal temperature be slightly different for people living in different climates?
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J
Juran9 months ago
True, the measurements were done on people in different climate areas.

While researching on this, I found tons of material supporting a theory of the human body being an amazing machine that is capable of coping with different environmental temperatures, just so it's temperature could remain unchanged. Body functions perfectly on let's say, 36.6 degrees Celsius. When we say body, we mean organs and tissues made of cells, which are made of lipids, sugars, and proteins. To "work" on optimal turnover times, synthesize, degrade, or maintain the rate of chemical reactions, the body needs optimal conditions. Any small change in body temperature reflects heavily on cell and tissue mechanisms. That could be the reason why hot-blooded organisms, including humans, should have a constant body temperature. if the protein, sugar, and lipid structure or content are the same in all climate zones, then the body temperature should be, too.

On the other hand, why it could be true that body temperature change according to the climate? The problem is that we do not always measure the "body" temperature, but oral, tympanic, or rectal temperatures, from which the oral and tympanic vary the most. How could they vary? Some findings reported different body temperature measurements after the 15-min exposure to cold or hot temperatures. Shubhankar Kulkarni mentioned it in his contribution above and concluded that even much smaller temperatures could play a role in a temperature change. Therefore, it's not strange to conclude that people living in different climate zone could have slightly different "surface" body temperature - just because of the measurement method.

That could be the reason why your idea could be true (for this study). But are the "real" temperatures of the body changing according to the climate zones? I would say not.
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Povilas S
Povilas S9 months ago
Juran K. , Darko Savic This seems to be already accepted as know phenomenon, it couldn't be just from one study. The one with Tsimane people is another example, are there more?
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Povilas S
Povilas S9 months ago
https://elifesciences.org/articles/49555 It seems that there's enough proof.
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How about prostaglandins and IUD?

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Martina Pesce
Martina Pesce Nov 13, 2020
A quite direct responsible molecule for temperature regulation is prostaglandins .
What influence it? Are we somehow tackling them with something?
Seems that apart from the environment temperature, an intrauterine device can influence them . Intrauterine devices are surely something that some decades ago was inexistent. Could it be that? Do we have the data for the human body temperature also separated between females and males? Could it be that?

[1] "Eicosanoid Synthesis and Metabolism: Prostaglandins, Thromboxanes, Leukotrienes, Lipoxins". themedicalbiochemistrypage.org. Retrieved 2018-09-21.

[2] Mary Anne Koda-Kimble (2007). Handbook of Applied Therapeutics (8th ed.). Lippincott Williams & Wilkins. p. 1104. ISBN 9780781790260.

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Shubhankar Kulkarni
Shubhankar Kulkarnia year ago
Juran K. Only 10% of the women in the United States use intrauterine devices (https://www.cdc.gov/nchs/fastats/contraceptive.htm). Martina Pesce How do the IUDs affect body temperature - positively or negatively?
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Martina Pesce
Martina Pescea year ago
Shubhankar Kulkarni
UPS! I must have got confused: IUD increases the productions of prostaglandins, which increases the temperature. So, actually...not the IUD.
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J
Jurana year ago
Nice! I didn't remember to look at the problem from that perspective. It could be useful to look for inner regulators of body temperature and what affects them.

Also, since 49.584% of the world population are women (https://data.worldbank.org/indicator/SP.POP.TOTL.FE.ZS), the intrauterine device could affect the temperature on a larger scale. But we first need the data on how many females actually use those.

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Reduced number of infections or physical activity

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Shubhankar Kulkarni
Shubhankar Kulkarni Nov 24, 2020
This is more on a philosophical level and a thought, so cannot back up with references. Any references you find will be greatly helpful.

Imagine the mechanisms in the body to be like a circular pipe with water flowing through it. The pipe is the body and the water being the transfer of energy between the molecules that maintain homeostasis. The pipe is flexible and vulnerable to external impact (signals - both physical and chemical). The flow of the water changes on impact but is brought back to the optimum level after the shock (impact) resides. The flow of energy generates a certain amount of heat in the system. There is an exchange of heat with the environment, which maintains the balance.

Now imagine an impact that constricts the pipe at one point. There will be a build-up of water upstream of the constriction. This increases the heat since it cannot be easily dissipated. The constriction can be a microbial infection or something as simple as a bout of a sprint. The more the number of such bouts, the more is the frequency of the instances when the temperature is higher than normal. With this logic, either a reduced number of infections, or reduced overall physical activity, or both can be the reasons for the reduced average body temperature.

[1]https://jamanetwork.com/journals/jama/fullarticle/768249

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Shubhankar Kulkarni
Shubhankar Kulkarni9 months ago
An increase in the consumption of antibiotics may have led to a decrease in the type and number of microbes associated with the human body. The activity of the microbes generates heat. Lower microbial concentration generates a lower amount of heat. Hence, the drop in body temperature.

Reference: https://www.youtube.com/watch?v=1Scf7ZJcPUY&feature=youtu.be
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J
Jurana year ago
I understand the model you propose and yes, it could explain the observation of slightly reduced body temperature. It fits in "a bit of everything" theory.

But the problem with this is that the measurements are usually done at the hospital, in one moment, on one, usually sick individual, once. Statistically, it could be possible that the sample is nonbiased, but then the patients at the regular examination need to be taken into account, too. What they did in the study I mentioned in the session description is that they adjusted the data to the patient diagnosis (infections, general state) and didn't find a pattern.

Concerning physical activity, it is not very likely that someone will do a hardcore physical activity and then go to an examination or measure the body temperature and report it to a doctor. I don't know how we could fit that into the adjustment algorithm, but it could definitely contribute.
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Shubhankar Kulkarni
Shubhankar Kulkarnia year ago
Juran K. I agree with your comment concerning the effect of decreased infections on average body temperature.

Regarding physical activity, although it is unlikely that the participant had a run and then visited the clinic, the regular commute would be involving more physical activity then. For example, the participant would be more like to ride a cycle and climb up the floor to the clinic than travel in a car and use the lift. Since the year of the initial measurement is 1861, we cannot eliminate this possibility.
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Ambient temperature in the clinic might explain the differences in the average body temperature

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Shubhankar Kulkarni
Shubhankar Kulkarni Nov 26, 2020
Almost all clinics today have temperature regulation (air conditioning). On average, the indoor temperature maintained today is at 25 °C or less than that. I think it is safe to assume that there was no kind of air cooling in 1861 (the first air conditioner was born after 1900). The ambient temperature does affect the skin temperature. Moreover, we know that Carl Wunderlich preferred axillary (armpit) measurements, which can be definitely affected by the ambient temperature.

This study reports the changes observed in the temperature of the mouth, rectum, and ear. Please do check the figures from the manuscript. Twenty healthy subjects were exposed to hot (43.5 degrees C) and cold (-5 degrees C) temperature for 15 minutes. Rectal temperatures remained stable during the exposures. Oral temperatures rose significantly after hot exposure (max 0.4 degrees C) and decreased after cold exposure (max 0.5 degrees C). Tympanic temperature elevated after hot exposure (max 0.8 degrees C) and decreased only in male subjects after cold exposure (max 0.7 degrees C). Individuals demonstrated wide variability in their temperature responses. The authors conclude that "tympanic and oral temperatures taken within the first 20 minutes after exposure to outdoor temperature extremes may fail to accurately reflect the patient's true temperature."

The only problem I see with this report is the extreme temperatures (hot (43.5 degrees C) and cold (-5 degrees C)) that the participants were subjected to. Although there won't be any such drastic ambient temperature differences during measurements taken in 1861 and those taken today, the small change observed in the average body temperature can surely be due to the differences in the ambient temperature or can at least contribute to it.

[1]Mackowiak PA, Wasserman SS, Levine MM. A Critical Appraisal of 98.6°F, the Upper Limit of the Normal Body Temperature, and Other Legacies of Carl Reinhold August Wunderlich. JAMA. 1992;268(12):1578–1580. doi:10.1001/jama.1992.03490120092034 https://jamanetwork.com/journals/jama/article-abstract/400116

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J
Jurana year ago
This is an excellent find! I really like the simplicity of the study you linked. It could play a role in the mentioned changes in body temperature.

What I find a bit tricky is the sample size (41 participants), but that could be taken as a minor flaw.
Also, as you mentioned, the extreme temperatures seem a bit too radical. Plus, even after these extreme conditions, the temperature seems to get back to normal in the period of 20 mins, especially in females. If we take into account the average temperature differences inside and outside the hospitals and the average waiting time, the effects fall fairly below significant, I think.

What could support this idea is the integration of the thermometers into smartwatches or smartphones to follow the body temperature continuously. Also, the COVID-19 temperature measurement at airports, hospitals, pharmacies, shops, etc. could be used to test the theory.
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Shubhankar Kulkarni
Shubhankar Kulkarnia year ago
Juran K. Agreed! Even I loved the simplicity of the study, there was no confusion.

Yes, there will be a lot of data with COVID-19 testing that can be put to use for topics other than COVID-19.
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A decrease in the resting metabolic rate

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Shubhankar Kulkarni
Shubhankar Kulkarni Feb 23, 2021
Resting metabolic rate is the largest component, comprising around 65%, of a typical modern (sedentary individual) human’s energy expenditure. Heat is the byproduct of metabolic processes. Studies examining whether metabolism is related to body surface area or body weight suggest that metabolism is dependent on body weight. The authors mention that since US residents have increased in mass since the mid-19th century, the average body temperature should have increased. Thus, a decrease in body temperature indicates a decrease in metabolic rate independent of changes in anthropometry.

[1]https://elifesciences.org/articles/49555

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Great for the lifespan

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Martina Pesce
Martina Pesce Nov 23, 2020
Look like a lower basal temperature leads to a longer lifespan! At least in rodents .
These researchers created transgenic mice with an overexpressed protein (uncoupling protein 2) which elevates the temperature of the hypothalamus which consequently causes a response to lower the whole body temperature.
The female lifespan increased by 20% and the males one by 12 %. Really cool.
Funnily enough, the protein's genetic alleles in humans were associated with obesity, so after all, maybe there is something to do with obesity.

[1] Conti, B.; Sanchez-Alavez, M.; Winsky-Sommerer, R.; Morale, M. C.; Lucero, J.; Brownell, S.; Fabre, V.; Huitron-Resendiz, S.; Henriksen, S.; Zorrilla, E. P.; De Lecea, L.; Bartfai, T. (2006). "Transgenic Mice with a Reduced Core Body Temperature Have an Increased Life Span". Science. 314 (5800): 825–828.

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Shubhankar Kulkarni
Shubhankar Kulkarnia year ago
Martina Pesce Great find! The causality you mentioned in your suggestion (lower temperature leads to a higher lifespan) is the opposite of the one mentioned in another suggestion ("I don't think its population getting older") on this session. I think what you have suggested can very well be the (or one of the) reasons for the reduced body temperature. We were looking at it from the perspective of an "effect" but it can also be the "cause" of something. In the paper you cited, the authors report a reduction in the physical activity of the mice, which can also be one of the reasons. We do not know whether the reduction in physical activity was due to genetic modification (Heterozygous Hcrt-UCP2 mice).
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General comments

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Darko Savic
Darko Savic9 months ago
While this couldn't be the reason, it should be mentioned. Entropy is slowly but persistently decreasing the temperature of everything in the universe. We seem to be on the one-way road to the heat death of the universe https://en.wikipedia.org/wiki/Heat_death_of_the_universe.
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Juran9 months ago
Darko Savic Interesting theory. I didn`t hear about it before. That would mean that the temperature of the air, earth, sea, and all the other waters should be decreasing too? I guess the air is heavily influenced by global warming, but the other elements should respond to this, right? In our case, I guess the time frame of 20-30 years is definitely too small to "catch" the difference in temperature caused by the entropy. But if we had a much bigger time frame (since these events such as the Big Freeze are planned to occur in about 10^100 years), we could catch the change caused by the entropy.
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Darko Savic
Darko Savic9 months ago
Juran For us on earth, things will first get much hotter (sun expanding) before they get colder. But eventually, yes - entropy is coming for everything in the universe - https://youtu.be/80tdw-kBiu8
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