In today's open access paper, the authors argue that a downward trend in normal human body temperature recorded by physicians over the past 150 years is real, rather than being an artifact of changing approaches to measurement. Taking that as settled, though I'm sure there is plenty of room left to debate the point, one might then ask why this trend exists and what it might imply.
Over the past few centuries, both life expectancy at birth and adult life expectancy have risen steadily, the former more profoundly than the latter due to sizable reductions in childhood mortality. The majority of these gains in adult life expectancy have been the result of improved control over infectious disease, reducing the burden placed on the immune system over the long term by both chronic and passing infections. The authors of this paper pull from a number of sources to suggest that this burden of chronic infection is the source of raised body temperature, due to inflammation.
Does a lowered body temperature in and of itself cause differences in human longevity, or do the effects of chronic inflammation on the pace of immune aging far outweigh it? Human data is supportive of the idea that lower body temperature correlates with greater longevity, but not definitively so. The practice of calorie restriction lowers core body temperature in the course of slowing aging in mammals. But is body temperature actually an important mechanism in comparison to the others involved in chronic infection and calorie restriction? I would guess no, given what I've seen of the literature on these topics.
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. A compilation of 27 modern studies, however, reported mean temperature to be uniformly lower than Wunderlich's estimate. Recently, an analysis of more than 35,000 British patients with almost 250,000 temperature measurements, found mean oral temperature to be 36.6°C.
In this study, we analyzed 677,423 human body temperature measurements from three different cohort populations spanning 157 years of measurement and 197 birth years. We found that men born in the early 19th century had temperatures 0.59°C higher than men today, with a monotonic decrease of -0.03°C per birth decade. Temperature has also decreased in women by -0.32°C since the 1890s with a similar rate of decline (-0.029°C per birth decade). Although one might posit that the differences among cohorts reflect systematic measurement bias due to the varied thermometers and methods used to obtain temperatures, we believe this explanation to be unlikely.
The question of whether mean body temperature is changing over time is not merely a matter of idle curiosity. Human body temperature is a crude surrogate for basal metabolic rate which, in turn, has been linked to both longevity (higher metabolic rate, shorter life span) and body size (lower metabolism, greater body mass). We speculated that the differences observed in temperature between the 19th century and today are real and that the change over time provides important physiologic clues to alterations in human health and longevity since the Industrial Revolution.
Resting metabolic rate is the largest component of a typical modern human's energy expenditure, comprising around 65% of daily energy expenditure for a sedentary individual. Heat is a byproduct of metabolic processes, the reason nearly all warm-blooded animals have temperatures within a narrow range despite drastic differences in environmental conditions. Over several decades, studies examining whether metabolism is related to body surface area or body weight, ultimately, converged on weight-dependent models. Since US residents have increased in mass since the mid-19th century, we should have correspondingly expected increased body temperature. Thus, we interpret our finding of a decrease in body temperature as indicative of a decrease in metabolic rate independent of changes in anthropometrics.
Although there are many factors that influence resting metabolic rate, change in the population-level of inflammation seems the most plausible explanation for the observed decrease in temperature over time. Economic development, improved standards of living and sanitation, decreased chronic infections from war injuries, improved dental hygiene, the waning of tuberculosis and malaria infections, and the dawn of the antibiotic age together are likely to have decreased chronic inflammation since the 19th century. For example, in the mid-19th century, 2-3% of the population would have been living with active tuberculosis. Although we would have liked to have compared our modern results to those from a location with a continued high risk of chronic infection, we could identify no such database that included temperature measurements. However, a small study of healthy volunteers from Pakistan - a country with a continued high incidence of tuberculosis and other chronic infections - confirms temperatures more closely approximating the values reported by Wunderlich.
In summary, our investigation indicates that humans in high-income countries have changed physiologically over the last 200 birth years with a mean body temperature 1.6% lower than in the pre-industrial era. The role that this physiologic 'evolution' plays in human anthropometrics and longevity is unknown.