Commenting on our review, Nelson writes that “extensive cohort studies indicate that [body temperature] is closer to 36.5°C” than 37°C.1 The review indeed specified that “normal human core body averages about 37°C,” and that “there is a superimposed circadian rhythm with roughly a 1°C range.”2 “Normal core body temperature thus varies from ≈36.5°C (usually about 3:00 am) to ≈37.5°C (usually about 3:00 pm). In premenopausal women, there are also superimposed changes in temperature with the menstrual cycle, with core temperature being about 0.5°C greater during the luteal phase.”
Nelson cites two studies to support an average core temperature of 36.5°C. In one of them, Gurven et al.3 present a cohort study in which average core temperature somewhat implausibly decreased ~0.5°C over just 16 yr. However, about a third of the way through, temperature measurement changed from mercury-in-glass oral thermometers, which are reasonably accurate, to infrared aural canal thermometers, which are not. The other study cited, Protsiv et al.,4 is more convincing and describes a large cohort, 2007 through 2017, in which core temperatures were estimated with digital oral thermometers. The average was 36.7°C in men and 36.8°C in women. Importantly, though, electronic oral temperatures average 0.25°C below true core temperature.5 Protsiv’s report is thus perfectly consistent with average core temperature being very close to 37°C.
There have also been several studies in which core temperature was measured over time with ingested capsules which accurately report core temperature. Core temperatures reported in these studies were 37.1 ± 0.1°C,6 36.8 ± 0.3°C,7 and 37.1 ± 0.1°C.8
There is not a single “normal body temperature.” Instead, core temperature varies over the circadian cycle, between men and women, with age, and over the menstrual cycle. However, available evidence indicates that 37°C is a reasonable estimate of normal core temperature in humans.
The authors declare no competing interests.