In 1851, the German physician Carl Reinhold August Wunderlich obtained millions of axillary temperatures from 25,000 patients in Leipzig, thereby establishing the standard for normal human body temperature of 37°C or 98.6 °F. Modern studies, however, suggest that it’s too high.
Another examination investigates body temperature trends and concludes that temperature changes since the time of Wunderlich reflect an exact historical pattern, as opposed to measurement errors or biases. Scientists found that the average human body temperature in the United States has dropped since the 19th century. The found the average temperature of 25,000 British patients to be 97.9 F.
The study involved 677,423 temperature measurements. Then a linear model that interpolated temperature over time. The model affirmed body temperature trends that were known from past investigations, incorporating increased body temperature in younger people, in women, in larger bodies, and at later times of the day.
Scientists observed that the body temperature of men born in the 2000s is, on average, 1.06 F lower than that of men born in the early 1800s. Similarly, they observed that the body temperature of women born in the 2000s is, on average, 0.58 F lower than that of women born in the 1890s. These calculations correspond to a decrease in body temperature of 0.05 F every decade.
What might be the possible cause of colder body temperature?
According to scientists, it can be explained by reduced metabolic rate or the amount of energy being used. This reduction may be due to a population-wide decline in inflammation.
Julie Parsonnet, MD, professor of medicine and health research and policy, said, “Inflammation produces all sorts of proteins and cytokines that rev up your metabolism and raise your temperature. Public health has improved dramatically in the past 200 years due to advances in medical treatments, better hygiene, greater availability of food, and improved standards of living.”
The authors also hypothesize that comfortable lives at constant ambient temperatures contribute to a lower metabolic rate. Homes in the 19th century had irregular heating and no cooling; today, central heating and air conditioning are commonplace. A more constant environment removes a need to expend energy to maintain continuous body temperature.
Parsonnet said, “Physiologically, we’re just different from what we were in the past. The environment that we’re living in has changed, including the temperature in our homes, our contact with microorganisms, and the food that we have access to. All these things mean that although we think of human beings as if we’re monomorphic and have been the same for all of human evolution, we’re not the same. We’re changing physiologically.”
The study is published in the journal eLife.