Why there’s no one ‘normal’ body temperature

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If you think 98.6°F is the “normal” body temperature for everyone, you might need to think again.

Researchers at Stanford Medicine have found that body temperature is more personalized than we previously believed.

Not only does it depend on individual factors like age, sex, weight, and height, but it also changes at various times throughout the day.

What the Researchers Discovered

Dr. Julie Parsonnet, a leading author of the study, dispelled the common belief about the “normal” body temperature of 98.6°F.

According to her, it varies significantly from person to person and even from one time of the day to another.

For example, an 80-year-old skinny man might have a lower body temperature in the morning compared to a 20-year-old woman who is obese in the afternoon.

The researchers analyzed over 618,000 oral temperature measurements taken from adults at Stanford Health Care between 2008 and 2017.

They also used machine learning techniques to filter out data that might be affected by certain illnesses or medications, providing a more accurate average temperature.

They found that a so-called “normal” body temperature is actually closer to 97.9°F.

Men generally had lower temperatures than women, and body temperature tended to decrease with age and height but increased with weight.

The time of day was a significant factor as well, with temperatures being lowest in the early morning and highest around 4 p.m.

How the 98.6°F Myth Started

The widespread belief in 98.6°F as the standard “normal” body temperature dates back to a German study from the 1860s.

Even then, the study noted variations in temperature among different people and at different times of the day.

However, over time, this nuanced understanding was reduced to a simple average of 98.6°F, which became widely accepted as the standard for everyone.

Why It Matters: Rethinking ‘Normal’

Personalized Medical Care

Understanding that body temperature is not one-size-fits-all can have significant implications for healthcare.

Dr. Parsonnet recounted her own experience with her elderly mother-in-law, whose heart infection was not diagnosed for weeks because her temperature never reached what is traditionally defined as a fever—above 100.0°F or 100.4°F.

The Need for Further Research

About a quarter of the variation in body temperature from person to person could be attributed to factors like age, sex, height, weight, and time of day.

That leaves a lot of unexplained variation, which could be influenced by a range of factors not yet studied, such as clothing, physical activity levels, menstrual cycles, and even weather conditions.

New research could help establish personalized benchmarks for body temperature, making it a more accurate and useful vital sign.

Studies could also investigate how having a consistently higher or lower “normal” temperature could impact one’s life expectancy.

In summary, the Stanford Medicine study challenges long-held beliefs about what constitutes a “normal” body temperature.

The findings pave the way for a more personalized approach to healthcare, potentially affecting how we define fever and other critical health markers. It’s a reminder that when it comes to our bodies, one size definitely does not fit all.

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The research findings can be found in the JAMA Internal Medicine.

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