Anyone will tell you that a normal body temperature is 98.6 degrees Fahrenheit. Well, it’s not that simple. Research has revealed that human body temperatures have dropped 0.05 degrees every decade since the mid-1800s.
What we consider normal body temperature was determined by a German physician named Carl Reinhold August Wunderlich, who took the axillary temperature, in the armpit, of 25,000 patients. He determined that a normal temperature was 98.6 degrees, with a range from 97.2 to 99.5 degrees. A review of 27 more recent studies puts normal body temperatures consistently lower.
In the latest study, scientists compared body temperature data from several groups: 83,900 Union Army veterans measured between 1862 and 1930; 5,998 temperatures taken between 1971 and 1975; and 230,261 temperatures taken between 2007 and 2017.
All three groups of data showed body temperature declines with age. Heavier people had higher body temperatures, taller people had lower temperatures, and the results were true for men and women of all ethnicities. The more recent two studies also showed people have higher temperatures later in the day.
The most interesting find from the study is body temperatures have been steadily decreasing by 0.05 degrees per decade since the 1800s. The authors of the study speculate this decline is the result of physiological differences.
Today, a sedentary individual uses about 65 percent of his or her energy on the basal metabolic rate (BMR). BMR is the amount of energy expended while at rest, and expending energy produces heat. The study’s authors argue the decreases in body temperatures are the result of a decrease in the BMR.
We’re less likely now to be burdened with chronic infections and inflammation than people in the 1800s, which would decrease the BMR and could contribute to the temperature decline. When body temperatures were taken in Pakistan, where there is a high incidence of tuberculosis and other chronic infections, the mean was 98.4 degrees, closer to the value Wunderlich found.
The continued decline from the 1970s to the 2000s could also be because of the decline in chronic infections such as gum disease during that time. There has also been an increase in the use of aspirin and non-steroidal anti-inflammatory drugs (NSAIDS), which would also reduce incidence of inflammation.
Other studies have shown that NSAIDS reduce body temperatures even in normal people. Further, a marker of inflammation that can be measured, the C-reactive protein, decreased by 5 percent between 1999 and 2010.
The environment in which we now live also has changed dramatically since the 1800s. Most people live in homes where temperatures are controlled, which would decrease the BMR because less energy is required to maintain proper body temperatures.
While some have assumed that Wunderlich’s determination of 98.6 degrees was because of poor measurements, this study suggests it’s accurate, and body temperatures have dropped 1.6 percent since the pre-industrial era.
We wonder what else has changed in us and how our modern environment is going to change us in the future.