American scientists have calculated that every year the average body temperature of people drops by 0.004 degrees. From the middle of the 19th century to the present, it has decreased from 37 to 36.6 degrees. Researchers believe the reason for this cooling is the development of hygiene and medicine, which have reduced the intensity of inflammation in human bodies. The work was published in the eLife magazine.
In 1851, the German physician Karl Wunderlich measured the body temperature of 25,000 patients in Leipzig and found it to be on average 37 degrees Celsius. However, today it is believed that the body temperature of a healthy person is usually 36.6 degrees - this is indicated by the results of many modern measurements.
It is still unclear what caused this difference. Perhaps the point is in the measurement technique, and over a century and a half, thermometers have simply become more accurate. Or maybe it's a matter of the sample, and Wunderlich's patients were, on average, less healthy than the participants in modern studies. But it may also be that people in the middle of the 19th century had a higher body temperature than it is now.
To understand what happens to human body temperature, a team of scientists led by Julie Parsonnet at Stanford University analyzed data from three independent studies. The first took place from 1862 to 1930 and included 83,900 veterans of the American Civil War, the second was from the National Health and Nutrition Survey (5998 people, 1971-1975), the third was another independent study at Stanford, in which 230261 participated men from 2007 to 2017. In addition, the scientists collected data on the temperature of women: 9303 from the second study and 348006 from the third.
The authors of the work noticed that the average temperature in the first sample was higher than in the second, and in the second - higher than in the third. From these data, it follows that the temperature should have decreased by about 0.003-0.0043 degrees every year. The trend continued even after the researchers took into account body mass index, height, gender and race in their calculations.
Scientists have also suggested that body temperature may be affected by the time of day or time of year. They confirmed this assumption in the second and third samples. However, even taking this factor into account, the trend towards “cooling down generations” persisted.
To eliminate the influence of the measurement method on the result, the scientists focused on the first sample, which was collected over many decades and measured in the same way. However, even inside it, the temperature dropped every decade. Therefore, the researchers concluded that the observed effect does not depend on the method and accuracy of measurements.
Thus, the researchers confirmed that people gradually cool down from generation to generation. And as long as this is not a mistake of method or sampling, then they believe that it is a matter of lowering the base level of metabolism, that is, the amount of energy that a person spends on everyday life. The researchers cite, firstly, the controlled ambient temperature as the reasons for this "cooling". They believe that people are more likely to cool rooms so as not to overheat, and this can affect their metabolic rate. The second reason they call the state of health. A century and a half ago, infectious diseases were much more common, and inflammation developed more often in the human body, which required energy expenditure and increased body temperature. Now we manage to cope with it with the help of hygiene and anti-inflammatory drugs, and this "extra" temperature has disappeared.
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Scientists have previously found that exposure to cold changes the composition of mammalian gut microbiome and promotes weight loss, while living in the cold forces people to drink more alcohol. In addition, it turned out that the settings of modern air conditioners are outdated and are designed mainly for men in warm three-piece suits, which makes women and lighter dressed men freeze in offices.
Polina Loseva
