Why Average Body Temperature has Dropped in Healthy Adults

Almost two centuries ago the standard “normal” body temperature in humans was established at 98.6 degrees Fahrenheit and has been used as the measure for assessing fevers. However, over time lower body temperatures have been widely reported in healthy adults.

A study conducted in 2017 among 35,000 adults in the U. K. found average body temperatures to be lower at 97.9F and a study in 2019 in the U. S. found normal body temperatures at about 97.5F.

Interestingly, recent studies in both the U. S. and the U. K. have found a similar decrease among the Tsimane which is an indigenous population of forager horticulturists located in the Bolivian Amazon. A team that has been observing this population have observed a rapid decline in body temperature of 0.09 degrees Fahrenheit such that today this population’s body temperatures are about 97.7F.

In less than two decades researchers are seeing the same levels of decline as that which has been observed in the U. S. over about two centuries. The analysis is based on a large sample of 18,000 observations of about 5,500 adults with adjustments for factors that can affect body temperature such as body mass and ambient temperature.

It is clear that something about human physiology has changed. The researchers leading hypothesis is that we have experienced fewer infections over time due to clean water, improved hygiene, medical treatment and vaccinations. In the recent study, they were able to test these ideas directly. They have information on clinical diagnosis and also biomarkers of inflammation and infection at the time each patient was seen.

Although some infections were associated with high body temperatures, adjusting for those did not account for the steep decline in temperatures over time. The same type of thermometer was used for most of the study so the changes were not due to changes in instruments.

No matter how they did the analysis, the declines were still there. Even when they restricted analysis to less than 10% of adults who were diagnosed as completely healthy, they still observed the same decline in body temperatures over time.

A key question became then is why body temperatures have declined in both Americans and the Tsimane population. Data available from the team’s long-term research in Bolivia addressed some of the possibilities. Declines could be due to the rise of modern health care and lower incidences of lingering mild infections compared to the past. However, while health has improved over the past two decades, infections are still widespread in rural areas of Bolivia. This then suggests that reduced infection alone isn’t why the decline in body temperatures.

It may be that people are in general in better condition which means their bodies could be working less to fight infection. Greater access to antibiotics and other medical treatments means that the duration of infections is shorter now than was in the past. The team did find that respiratory infection in the early period of the study led to having a high body temperature than having the same infection more recently.

Another possible explanation is that our bodies don’t have to work as hard to regulate our internal temperature due to air conditioning in the summer and heating in the winter. Although Tsimane body temperatures will change with time of year and patterns of the weather, these people do not use any advanced technology for helping to regulate their body temperature. They do however, have more access to blankets and clothes.

The research team was initially surprised to find no single magic bullet that would explain the decline in body temperature. They believe it is likely due to a combination of factors which all point to improved conditions.

Temperature as a vital sign is an indicator of what is occurring physiologically inside the body. One thing that has been known for a while is that there is not universal body temperature for everyone at all times. Despite the fixation on 98.6F, most clinicians know that normal temperatures have a range and throughout the day, temperatures can vary by as much as 1 degree Fahrenheit from lowest in early morning to its highest in the late afternoon. It will also vary following physical activity, across the menstrual cycle, and tends to decrease with age.

Through linking improvements in the broader epidemiological and socioeconomic landscape to body temperature changes, the study does suggest that information in regards to body temperature may provide clues to a population’s overall health along with other common indicators such as life expectancy. Since body temperature is easy to measure, it can easily be added to routine large-scale surveys that monitor the health of populations.

To view the original scientific study click below

Rapidly declining body temperature in a tropical human population.