Measuring Body Temperature

The Significance of Declining Human Body Temperatures

While the planet warms up, our bodies are cooling down. That’s right. Declining human body temperatures are a fact of the last 200 years. A new study in eLife tells us that average body temperatures have cooled by 1.6% over the last 200 birth years.

If you thought that 37°C or 98.6°F is the norm for human body temperatures, you’re out of date. That was the number in the 19th century. Today, the correct number is more like 36.5°C or 97.7°F. The difference is small, but it’s important.

677,423 Measurements Spanning 157 Years

Any way you look at it, this is an impressive analysis. Myroslava Protsiv and colleagues crunched the numbers for more than half a million instances of taking a person’s temperature. The dataset covered a range of 157 years when the measures were taken. The people being measured represented birth cohorts as much as 197 years apart.

This huge dataset made it possible to find a small, but significant trend toward lower body temperatures.

Half a Degree – Who Cares?

You might have the impulse to say, “So what? How important could half a degree Celsius be?”

That’s the right question to ask. But the answer is that it might be quite important. That’s because it reflects a systematic change in human physiology happening over two centuries. These researchers suggest that this decline in body temperature reflects a decline in resting metabolic rate. One explanation for that, they say, might be a decline of inflammation across the population. Better health and hygiene, better dental care, and fewer infections could be factors.

HVAC might also be a factor. Over time, we have come to live in a more constant temperature environment. Better heating systems in more places was an ongoing trend throughout the timespan of this study. Air conditioning has become ubiquitous. With more constant temperatures, humans can maintain their body temperature with less energy expenditure. Thus, a lower resting metabolic rate is possible and bodies can settle down to a slightly lower body temperature.

Much to Learn

All of this leaves us with lots to learn. We note that other researchers have suggested ambient temperature control might play a role in obesity prevalence. On the other hand, a lower resting metabolic rate can be a factor promoting greater longevity.

New facts bring new questions. Those questions are the gateway to important new insights.

Click here for the study and here for further perspective.

Measuring Body Temperature, photograph © Elgin County Archives, St. Thomas Times-Journal/ flickr

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January 27, 2020

2 Responses to “The Significance of Declining Human Body Temperatures”

  1. January 27, 2020 at 10:38 am, Monica Reinagel said:

    This is absolutely fascinating. I had no idea. It seems that the medical industry still considers 37.0 the standard. How does that impact diagnosis and treatment of hypothyroidism, for example? Or using body temperature to track fertility?

  2. February 02, 2020 at 12:42 pm, David Benjamin said:

    Wow, Monica, what great questions. First of all, I am David Benjamin, Ph.D., Clinical Pharmacologist and Forensic Toxicologist. I think I can address some of your questions. But ask an MD about your diagnosis-related questions. 37 C or 98.6 F were derived by measuring the temperatures of a large population of healthy people and taking the average. Remember, there are at least 3 areas of the body that can be used to measure temp and temp is likely to differe from site to site.

    A “normal” temp is often cited as 37 C or 98.6 F. But what is really important is where does the heat come from. The cells and tissues of the body carry out energy-producing biochemical reactions that make hormones and neurotransmitters, or just keep the cell healthy. When you take your temp, you’re measuring the energy-generating capacity of the body which maintains our bodies at what docotrs call “homeostasis”, or keeping everything going in a normal way.

    Alterations in body temperature can tell a woman when she is ovulating. All you have to do is measure your temp every day, at the same time, with the same thermometer for 60-90 days and compare that to the date of the onset of your periods over the prior 2-3 months. Then you will know what is “normal” for you. Ovulation usually is on day 14 of a 28 day cycle, but is generally beleived to start 14 days before the onset of your next period. OK, that’s enough physiology for now! David