Our body is no longer at 37°C, and that’s normal

We never question enough the truths we take for granted. Take the example of temperature. For more than 150 years, it has been fixed in a healthy adult at between 37 and 37.5°C. This barometer, we owe it to Karl Wunderlich, a German doctor of the XIX^and century, which in 1868 established the temperature curve to follow the evolution of the disease. If these results were correct at his time, researchers from the University of Health Barbara (United States) question these figures. The results of their study were published on October 28 in the journal Science Advances.

A drop of 1.7°C observed over the past twenty years

This drop in body temperature is observed everywhere. A 2017 study of 35,000 adults in the UK showed the average body temperature was 36.6°C. Another, this time in the United States, indicated an average temperature of 36.3°C.

For this new study, an international team of doctors, anthropologists and researchers focused on the Tsimane, an indigenous population living in the Bolivian Amazon. For 16 years, they observed a drop in the body temperature of this people of 1.7°C. Today, the temperature of Tsimane is estimated at 36.5°C.

“In less than two decades, we are seeing roughly the same level of decline that has been seen in the United States for about two centuries.”, emphasizes Michael Gurven, professor of anthropology at the University of Santa Barbara.

This new analysis is based on a sample of 18,000 observations made on 5,500 adults, and takes into account factors that could affect the temperature, such as ambient temperature or the body mass of individuals.

The path to a healthier lifestyle

The first large-scale temperature measurements date back to the American Civil War (1861-1865). At that time, the temperatures recorded corresponded to those used by Karl Wunderlich. However, they have only fallen since, without anyone understanding why, the fault surely of the tests carried out only on the American population, without any comparative reference. “Ione of the main hypotheses is that we have experienced fewer infections over time thanks to improved hygiene, clean water, vaccinations and medical treatments”, says Michael Gurven.

In the measurements carried out for this experiment among the Tsimanes and in other populations, the analysis always remains the same. “Even when we limited the analysis to the 10% of adults who were diagnosed by doctors as perfectly healthy, we observed the same drop in body temperature over time.”, completes Thomas Kraft, postdoctoral researcher within the unit of the integrative anthropological sciences of the university of Santa Barbara.

However, if the improvement of sanitary conditions can be seen as a serious track, Michael Gurven has some reservations. “While health has generally improved over the past two decades, infections are still widespread in rural areas of Bolivia. Our results suggest that the reduction in infections alone cannot explain the observed drop in body temperature..”

An adaptation to the modern world

The whole question is therefore to know why this drop in temperatures is observed everywhere. If no “miracle solution” clearly explains the phenomenon, several factors can add up. Among them, the temperature in which we operate is a good lead. “Another possibility is that our body doesn’t have to work as hard to regulate internal temperature, due to air conditioning in the summer and heating in the winter.explains Thomas Kraft. Although the Tsimanes’ body temperature changes depending on the time of year and weather conditions, they still do not use advanced technology to help them regulate their body temperature. However, they have more access to clothing and blankets.”

Despite a standard that sets body temperature between 37° and 37.5°C, most people agree that our body regulates itself throughout the day, but that multifactorial elements, such as taking medication such as that ibuprofen, age, physical activity and even the menstrual cycle can cause body temperature to vary by one degree.