I’ll bet you didn’t know we’re all cooler than we used to be… I’m talking about our body temperature, of course. You always thought 37C (98.6F) is your normal temperature when you’re not sick, right? And that you should treat a fever that is higher than 38.5C (102F)? Well, both of these facts have been questioned by researchers recently.
According to newer studies, “normal” body temperature can actually range from 36.1 to 37.1C ((97 to 99F), depending on the time of day and what you’re doing. Your temperature changes slightly over the day, too, and gradually decreases as you get older. Because these are “averages”, some of us are often outside that range, even when we aren’t sick. I'm usually around 36 to 36.6C... cool, eh? The average human resting body temperature, often referred to as “basal body temperature”, has gradually been declining over the past 150 years.
The standard average body temperature of 37C or 98.6F was determined by a German physician, Carl Wunderlich, in the 1850s by measuring millions of axillary (underarm) temperatures from 25,000 patients and calculating the average.
First, note that he used axillary temperatures—the temperature of the armpit is actually 0.5C (1F) lower than when taken orally (under the tongue). It’s interesting that this measurement somehow, over the years, came to be used as a normal oral temperature when what he measured was actually equal to 37.5C or 99.1 if taken orally.
Researchers in the US recently compared recorded body temperature measurements from 3 sets of records: Union Army Veterans of the Civil War from 1862 to 1930; a large nutrition study, the National Health and Nutrition Examination Survey I (NHAINS survey); and the Stanford Translational Research Integrated Database Environment study (STRIDE study). On the graph below, you can see that body temperatures, on average, declined with age and were lower in more recent times. Note: the blue lines are temperatures taken in 1860-1940, the green lines are 1971-1975 and the orange are from 2007 -2017:
They noticed that body temperature rose slightly as the day went by and increased slightly in response to increased room temperature. But the average temperatures also decreased by about 0.03C each decade, even within each set of data, showing that the decrease was not simply due to more accurate thermometers. The trends were consistent in men and women, and different ethnic groups.
They calculated that men’s temperatures have decreased 0.59C since the early 1800s and women (who weren’t included in the army veteran study) had cooled by 0.32C since the 1890s. Given that the older temperatures were taken under the arm, while the more recent ones were taken orally, the total decrease in body temperature is actually greater than that. Since the 1850s our body temperatures have dropped, on average, by about 1C or 2F, and researchers suspect they’re still dropping.
What would cause body temperatures to drop?
Body temperature is an indication of your metabolic rate. A drop in average body temperature suggests our metabolisms must be slower than they once were.
Improved standards of living and sanitation, decreased chronic infections from injuries, improved dental hygiene, fewer cases of tuberculosis and malaria and the use of antibiotics for infections have also likely combined to decrease chronic inflammation since the 1800s. Inflammation and infections increase body temperature, so fewer chronic infections means less inflammation and lower average body temperature in the population. (Note that temperature measurements above 39C were not included in the study as these would indicate a fever associated with an acute illness.) A small study of heathy volunteers from Pakistan, a country that continues to have tuberculosis and other chronic infections, found temperatures that were closer to the averages recorded by Wunderlich in the 1850s.
Better dental care means fewer cases of periodontal disease, a source of chronic inflammation. Use of anti-inflammatory drugs may also be reducing inflammation, with many people taking daily aspirin to prevent heart disease or to treat diseases like arthritis that are inflammation based.
Our heating and cooling systems in homes and workplaces may also have had an influence in body temperature. Air conditioning is now found in 85% of homes in the US. Maintaining a consistent body temperature when the room is too cold or too hot requires energy and increases the resting metabolic rate (RMR). And remember that body temperature is a crude indication of metabolic rate. So, lower body temperature may have also partly resulted from needing to expend less energy simply to adjust to a warm or cold room.
But another factor, not discussed in this study of temperatures, is the possibility that thyroid dysfunction could be influencing average temperatures. Although we now have blood tests to measure thyroid function, low body temperature was once used to diagnose low thyroid. Thyroid hormone replacement is one of the most prescribed medications these days. Autoimmune disease, where the immune system attacks your own tissues, is the most common cause of hypothyroidism, or low thyroid. Could it be possible that some environmental factor is interfering with thyroid function in humans? Scientists believe autoimmune hypothyroidism could be caused by a combination of genes and an environmental trigger. Perhaps a study in the future will identify this cause more precisely and whether it could be contributing to our “cooling”…
So, what should we do with this information?
In spite of good evidence that lower body temperatures are the norm now, many doctors and hospitals continue to use normal values recorded in the 1800s. Normal body temperatures vary in individuals from 36.1C to 37.1C (or 97 to 99F) and sometimes beyond this. It might be a good idea to determine your basal body temperature when feeling fine, so you will know how much it has elevated when you become ill.
It’s best to measure your basal body temperature first thing in the morning, before eating or drinking anything or doing any exercise to determine your lowest normal healthy temperature. This would allow you to more accurately determine when your temperature has increased by more than 1.5C, the point considered to indicate a fever, and when a person should consider treating or at least to look for the cause. Doctors who are aware of temperature variations report they generally look for extremes in body temperature, such as a significant fever that means there’s something wrong or a very low temperature (below 34.5C/94F) that indicates hypothermia.
But should we treat fevers?
Given that fever is a natural response to infection, some have suggested we should let nature take its course, and that fever may even help us fight off an infection. Should we suppress a fever or let it ride? This has been debated since the 5th century BC when Hippocrates first identified fever as part of the immune system’s response to infection. Thomas Sydenham, in 1900, described fever as “nature’s engine which she brings into the field to remove her enemy.” Today we understand fever as a sign of a disease, not a disease itself. The decision of whether to treat a fever is controversial and not well supported by studies, surprisingly.
Should we suppress it or let it ride?
In the “suppress it” camp, it is assumed that fever is noxious, and treating it will make it less harmful and unpleasant. It is also suggested that since it requires energy to maintain the increased body temperature, at some point, the energy cost must exceed the benefit. But neither of these assumptions have been measured or proven. Even febrile seizures, seizures in children associated with high fever, have not been shown in studies to be reduced by preventing fever with medications.
Those who suggest we should “let it ride” when a patient has a fever, advocate that fever is a protective mechanism that enhances immune cell function and promotes antimicrobial activity. They recommend that fever should be left to run its course under most circumstances. And there are a few studies done in the past 15 years that support this approach.
But, despite this evidence, the choice of whether to treat a fever is not well proven scientifically, although one would think it should be after having treated fevers with medications for so many years (aspirin was first formulated in the 1890s). If we take an evolutionary approach, survival of the fittest, it could be that developing a fever may be helpful in fighting off infections, even though inflammation (which causes an increase in body temperature) is associated with many damaging chronic illnesses.
Perhaps the best approach is to treat what is causing the fever whenever possible, rather than the fever itself.
So, some quirky controversies for you this week… Medicine doesn’t always have the answers. As much as we try for evidence-based medicine, some treatments continue to be based instead on standard historical practice, habits, and opinions.
References:
Decreasing human body temperature in the United States since the Industrial Revolution – eLifeSciences.org
Body temperature norms – MedlinePlus
Fever: suppress or let it ride? – Journal of Thoracic Disease
Average normal body temperature isn’t 98.6 anymore and it’s getting lower, research shows – Medical Xpress
Hypothyroidism – Mayo Clinic
