Let’s look at the study in question. Nine “semi-nude” men cycled non-strenuously on a stationary bike for 75 minutes in a warm-ish room (24°C), and had their temperatures measured in their oesophagus (via a probe in their nose) and rectally. They were linked up to a calorimeter, which calculated their heat production from the amount of oxygen they were using and the carbon dioxide they were breathing out. They were also weighed with a very sensitive scale to measure sweat loss.

Just before, and then a few times during, the exercise they were given a controlled amount of water, adding up to just under a litre across the whole experiment, at various temperatures: 1.5°C, 10°C, 37°C, and 50°C. Each participant did four sessions, one at each of the water temperatures.

To note a few things already: n = 9; only males; doing exercise for quite a long time; in a not-particularly-hot room; rapidly drinking water, not sipping a cup of tea. They also had a big fan pointed at them to help their sweat to evaporate away. These are just a few of the differences between this experiment and perhaps the average person’s situation in a heatwave - and I haven’t even mentioned the fact they were doing all this while they had a thermometer “inserted to a minimum of 12 cm past [their] anal sphincter”.

https://stuartritchie.substack.com/p/hot-drinks?r=5rny&s=r&utm_campaign=post&utm_medium=email

(http://www.autoadmit.com/thread.php?thread_id=5156096&forum_id=2:#44872399)

https://www.basugasubakuhatsu.com/blog/wp-content/uploads/2006/04/it.gif

(http://www.autoadmit.com/thread.php?thread_id=5156096&forum_id=2:#44873603)

https://www.uwindsor.ca/kinesiology/1035/bain-anthony

And it turns out he has a lot of experiments that require the participants to stick things in their asses:

https://journals.physiology.org/doi/full/10.1152/ajpregu.00257.2011?rfr_dat=cr_pub++0pubmed&url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org

Large differences in peak oxygen uptake do not independently alter changes in core temperature and sweating during exercise

"Fourteen males ... cycled for 60-min

Rectal temperature (Tre) was measured using a pediatric thermocouple probe (Mon-a-therm General Purpose Temperature Probe, Mallinckrodt Medical, St. Louis, MO) inserted to a minimum of 12 cm past the anal sphincter."

https://pubmed.ncbi.nlm.nih.gov/21127900/

Does summer in a humid continental climate elicit an acclimatization of human thermoregulatory responses?

"Eight young healthy male volunteers cycled at 60% VO(2max) for 90-min in a temperate environment before (mid-May) and at the end of (start of September) summer. Core temperature [measured in the esophagus (T (es)), rectum (T (re)) and aural canal (T (au))], mean skin temperature (T (sk)), forearm skin blood flow (SkBf), upper back sweat rate (LSR) and heart rate (HR) were measured throughout exercise. "

(http://www.autoadmit.com/thread.php?thread_id=5156096&forum_id=2:#44875644)