No longer is 98.6°F the normal body temperature. Researchers now say that the new normal body temperature is 97.5°F, owing to environmental changes over the past 200 years.

The normal body temperature has reduced from 98.6° Fahrenheit, studies say. The current benchmark for the standard body temperature was published in 1868 by a German physician Carl Reinhold August Wunderlich. However, various studies in the past have claimed that the human body's normal temperature has changed to 97.7°, 97.9° and 98.2° Fahrenheit.

1. In 1992, a group of researchers from the University of Maryland had looked at oral temperatures taken on a glass thermometer from 148 healthy people and found their average reading to be 98.2 degrees. The doctors at the University of Maryland had said that the century-old belief that 98.6° Fahrenheit constitute a person's normal body temperature should be discarded.
2. In 2017, a study published on BMJ, a peer-reviewed medical journal, on 35,000 British individuals stated that the average body temperature of individuals was 97.9° Fahrenheit. The study also found that individuals have body temperature baselines that correlate with a range of demographic factors, comorbid conditions and physiological measurements.
3. In 2018, a researcher from Boston in the US used an iPhone app, Feverprints and collected 5,038 oral temperatures from 329 participants. He found out that the average normal body temperature in adults to be 97.7° Fahrenheit.
4. Dr Sylvia Karpagam, a Bengaluru-based public health expert, explained that one’s normal body temperature usually fluctuates at times of the day and seasons. So, now, they consider 97-99° Fahrenheit as the normal range.

Observations

The researchers used 6.7 lakh temperature measurements to develop a linear model that interpolated temperature over time. The model confirmed body temperature trends that were known from previous studies, including increased body temperature in younger people, in women, in larger bodies and at later times of the day.

The researchers concluded that the body temperature of men born in the early- to mid-1990s is, on average, 1.06° Fahrenheit lower than that of men born in the early 1800s. Similarly, they concluded that the body temperature of women born in the early- to mid-1990s is on average 0.58° Fahrenheit lower than that of women born in the 1890s. These calculations correspond to a decrease in body temperature of 0.05° Fahrenheit every decade.

Causes behind the decline

The decrease in average body temperature could be explained by a reduction in metabolic rate or the amount of energy being used. The authors hypothesise that this reduction could be owing to a population-wide decline in inflammation. “Inflammation produces all sorts of proteins and cytokines that rev up your metabolism and raise your temperature,” Parsonnet told the media.

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 hypothesise that comfortable lives at constant ambient temperature contribute to a lower metabolic rate. Homes in the 19th century had irregular heating and no cooling, while today, central heating and air conditioning are common. A more constant environment removes a need to expend energy to maintain constant body temperature.

The author Kapil Kajal is a Mumbai-based freelance writer and a member of 101Reporters.com, a pan-India network of grassroots reporters.