Rectal thermometers.

Children dread them. And most adults probably do, too. They’re something one figures will be nicely tucked away, in the rear view, so to speak, never to be seen or, rather, felt once capable of holding an oral thermometer under the tongue. However, taking a subject’s core temperature during hard physical exertion has, until recently, always required the insertion of a rectal thermometer connected to a long wire. [See Exhibit A below.]

Fortunately, thanks to NASA and its Space Shuttle astronauts, including then-77-year-old Senator John Glenn, there is now a digestible transmitting thermometer that serves as an alternative. [See Exhibit B.] The size of a giant vitamin pill, or about three Tic Tacs, the transmitter is swallowed and, after a couple hours from ingestion, it reaches the intestinal tract and is able to send a wireless transmission, providing real-time core temperature readings. The transmitter then passes through the digestive system and is eliminated within a normal cycle of 18-30 hours. It’s one of the many advancements that have helped improve the study of human performance in athletics, including and especially endurance sports.

UConn’s New Facility

On September 22, 2017, the University of Connecticut’s Korey Stringer Institute (KSI) opened its $700,000 “Mission Heat Lab,” a state-of-the-art laboratory designed to facilitate research and education for maximal performance, optimal safety and the prevention of death from exertional heatstroke (EHS). The lab was made possible by donations from, among others, the NFL—Korey Stringer was an offensive tackle for the Minnesota Vikings who died in 2001 from EHS—Gatorade, Camelbak, and the namesake thermo-regulating active wear company, Mission.

I was fortunate enough to visit the lab and serve as an early test subject less than two weeks after it had opened. I was even more fortunate to have the option of using the digestible thermometer, thus foregoing the rectal alternative. Taking the subjects’ core temp readings is an absolute requirement of the lab. If the core rises above 103 degrees F, the test ends.

A glimpse inside the University of Connecticut’s Korey Stringer Institute Mission Heat Laboratory.

Other test participants were cyclists, including two-time Olympic mountain biker Lea Davison, and runners, such as Gediminas Grinius, winner of this year’s Ultra Trail World Tour. As a runner, I was put on a treadmill and told to go at an exertion level I could maintain for 90 minutes. The room measured 94 degrees F and the humidity was set at 60 percent. The highly-trained lab technicians, PhDs and doctoral students, guided by KSI’s CEO and UConn’s director of athletic training, Dr. Doug Casa, a passionate trail runner, performed the gold standard “body washdown” test on us. Very few labs are equipped to do this test, designed to measure your sweat content, down to the drop.

Before and after the exertion we were carefully weighed and then, while we ran or rode, we mopped up all the sweat and the technicians kept the wet towels. After we finished we were washed down inside of a giant trash bag, with a set amount of purified water so that the lab techs could later remove that control from the sample. Any water we consumed during the test was also factored into the results. Before the test, we had to follow a careful protocol of washing, including our exercise clothes, using only water, as detergent, lotion or deodorant would alter the results. Similarly, we were instructed not to drink alcohol or coffee for 24 hours before the testing.


Once we were washed down with the pre-measured purified water (see above), the technicians placed the sweaty towels and our drenched exercise clothing into the “sweat soup” to be measured for various electrolyte levels. My sweat loss was 3.8 percent of my body mass and my sodium loss was 980.0mg, which was in the normal range (0.18 to 1.5) for exercise in heat.

After exercising, all test participants were washed down with the pre-measured purified water.

Based on our before and after weight and the amount of water consumed during the exertion, KSI determined our sweat rate, measured as liters per hour. Normal sweat rates are 1.5-2.5 L/hr for men, 1.3-2 L/hr for women. I lost almost exactly 5 pounds during my test and had a high sweat rate of 2.31 L/hr. My core rose to the high of 103.1F, running a max speed of 9mph and I consumed a measly 35g of water.  My heart rate went from 90 to 160 and, according to my follow-up anaerobic threshold test, spent the last 15 minutes above that heart rate level.

Alberto Salazar was tested at UConn’s predecessor lab, run by Dr. Lawrence Armstrong, a leading expert on hydration and who now serves on KSI’s Medical and Science Advisory Board, and he had the outlier sweat rate of 3.7 L/hr. Salazar would lose as much as 8 percent of his body weight during his races and he put great emphasis on training for heat tolerance before the 1984 Olympic Marathon in Los Angeles. Humans are only able to take in 1.3-1.4 L/hr so, while a high sweat rate helps to cool the body during exertion, it isn’t all good because the loss of hydration can be a real detriment if you lose too much. According to my measured sodium loss, I’d need to have consumed 4.5 salt capsules or drank enough Nuun hydration mix for 2.5 tabs.

Exhibit A (left): an old-school rectal thermometer used to record internal body temperatures; Exhibit B (right): a modern pill-sized digestible transmitting thermometer.

Clear and Copious

Dr. Armstrong invented the urine color chart and, with that simple educational measure, has probably done more for more individuals than any other campaign to address dehydration. The chart has been helpful to many and was even converted to resemble beer types at one of the Western States 100 aid stations, where they used it to illustrate the command of avoiding stouts and sticking to lagers. Clear and copious urine is usually a sign of adequate hydration.

We learned from Dr. Armstrong that, without excess, caffeine doesn’t have an impact on hydration. But he and Dr. Casa were quick to point out that one needn’t be dehydrated to suffer heat stroke and that if you wait until you are thirsty, you are already late because thirst isn’t triggered until you dehydrated by 1-2 percent. In contrast, however, there are schools of thought, namely, Dr. Tim Noakes of South Africa, who professes that runners are over-hydrating and need to listen to their bodies and let thirst be their guide. Dr. Casa agreed with Noakes that, for purposes of avoiding hyponatremia, a sodium imbalance suffered by recreational runners who drink excessive water, thirst is a safe guide.

The key determining factors of one’s sweat rate are intensity of exertion, body size and environmental factors, such as heat and humidity. The body is an incredible system for trying to thermoregulate and what you drink can show up in your sweat as soon as 10 minutes after you consume it.

The takeaway from KSI’s Mission Heat Lab was that there are considerable performance benefits to be gained from knowing your sweat rate, sweat content, how, when and with what to rehydrate. As an easy rule, if you use your morning weight, thirst and urine color as basic guiding indicators, you can stay on top of your hydration needs.

Dr. Casa, with the help of many of his assistants and PhD students, is publishing a book in early 2018, “Sport and Physical Activity in the Heat,” as a full resource for serious athletes, coaches, trainers, sports medicine practitioners, kinesiologists, concerned about performance in hot conditions.

“Since we do not currently have an accurate wearable technology that can provide real-time assessment of hydration status, it is essential for high-level athletes to have an understanding of factors that influence sweat rate (amount of fluid/electrolytes lost in a given amount of time in a given environmental conditions at a given intensity),” Casa said. “This knowledge can allow an athlete to train hydration replacement and develop an appropriate race-day strategy.

“We will eventually have an accurate real-time wearable for hydration status that athletes can perpetually monitor so tweaks can be made—although lab data would still establish the general plan. But until that time, the lab work helps establish a plan and race-day thirst, environmental changes, intensity, pre-event hydration, and etc. need to be considered to make hydration decisions.”