sweating bike

Ask The Expert: Sweating, Weight Loss, and Fitness

Josephine asks: I have one rider that wants the fan directed to her face while Spinning and I noticed after class she hardly is sweating. What is the disadvantage of not sweating while working? Does sweating relate to weight loss or does it not play a role in weight loss?

Ah, sweating. In yoga we are exhorted to “sweat out the toxins” as we bend and stretch in a heated room. In life, to “never let them see you sweat.” Overly revered in fitness and reviled in regular life, sweating is one of the critical ways to keep our bodies from overheating when we work, exercise, or play in the heat(1).

As has been discussed in the past, sweating is one of three mechanisms by which humans can release excess heat, the other two being radiation and convection. Radiation only works if our environment is cooler than we are. We walk into the studio at least 37 (~98.7) and go up from there, each participant adding to the overall heat load of the room, and also contributing humidity and CO2.

In order to benefit from convective heat loss, we need decent airflow. Riding outdoors provides all we need, but indoors, it gets trickier. Air-conditioning provides cooling and dehumidification and is an important start. Fans that generate air movement around the room are also important. But even with decent fans and air cooling, effective convection may not be possible, leaving us with our sweat(1, 8).

Now here’s the thing most of us forget about sweat: it has to evaporate to do anything(3). If it just drips off the skin (usually after getting in the eyes) or into our moisture-wicking, super-duper fitwear, it has done nothing for us. We are not cooled by sweat that sits on our skin; we are cooled when the heat of our skin vaporizes those little droplets. Unfortunately, a very humid environment (i.e., one where the air is already heavily water saturated) is not conducive to vaporization. This is one of the reasons heat and humidity are a deadly combination when they reach a certain index. If the vaporization of our sweat is our last defense and we don’t have that, we are cooked.

So back to our studio, let’s discuss the intricacies of sweating while exercising.

Jennifer Klau, Ph.D. did her master’s thesis on sweat, but she still keeps a fairly straight face in hot yoga class. Klau is the Director of Training and Certification at Fitscript, LLC in New Haven, CT.


  1. Adams WC, Mack GW, Langhans GW, Nadel ER. Effects of varied air velocity on sweating and evaporative rates during exercise. J Appl Physiol 73: 2668–2674, 1992.
  2. Armstrong LE, Hubbard RW, Jones BH, Daniels JT. Preparing Alberto Salazar for the Heat of the 1984 Olympic Marathon. The Physician and Sportsmedicine 14: 73–81, 1986.
  3. Armstrong LE, Klau JF, Ganio MS, McDermott BP, Yeargin SW, Lee EC, Maresh CM. Accumulation of 2H2O in plasma and eccrine sweat during exercise-heat stress. Eur J Appl Physiol 108: 477–482, 2009.
  4. Bouchama. 062002 Heat Stroke.1. Bouchama A, Knochel JP. Heat stroke. N Engl J Med 346: 1978–1988, 2002.
  5. Jeffery E, Wing A, Holtrup B, Sebo Z, Kaplan JL, Saavedra-Peña R, Church CD, Colman L, Berry R, Rodeheffer MS. The Adipose Tissue Microenvironment Regulates Depot-Specific Adipogenesis in Obesity. Cell Metab 24: 142–150, 2016.
  6. Quinton PM. Sweating and its disorders. Annu Rev Med 34: 429–452, 1983.
  7. Schliess F, Häussinger D. Cell hydration and insulin signalling. Cell Physiol Biochem 10: 403–408, 2000.
  8. Shirreffs S, Casa D, Carter R III. Fluid needs for training and competition in athletics. J of Sports Sc 25: 83–91, 2007.
  9. Shirreffs SM, Armstrong LE, Cheuvront SN. Fluid and electrolyte needs for preparation and recovery from training and competition. J of Sports Sc 22: 57–63, 2004.
  10. Thornton SN. Increased Hydration Can Be Associated with Weight Loss. Front Nutr 3: 1678–8, 2016.


  1. Dr. Klau:

    you mentioned that wiping away the sweat does not let the body cool as it is not vaporizing.

    However, in a class that gets warm and humid and the initial vaporization occurs but then sweat just stays on the skin as the exercise continues and the saturation point, as it were, is reached- would this not inhibit further perspiration from reaching the skin surface, therefore would not a wipe with the towel to let new sweat reach the skin and vaporize until the buildup of sweat begins again be helpful rather than letting the skin stay saturated without any vaporization occurring.

    1. Author

      You are correct that the presence of sweat on the skin inhibits further sweating. Wiping, in this case, would permit sweating to continue, but if the sweat can’t vaporize, the thermoregulatory situation isn’t improved. But neither is it made worse, so if it’s more comfortable, wipe away.

      My suggestion about not wiping sweat was specific to the situation of an instructor telling her students that they might reabsorb the purported ‘toxins’ and ‘impurities’ it contained. In that case I was cautioning against wiping off sweat that could, theoretically, vaporize. The bigger picture is that you can’t ‘infect’ yourself with something from your own sweat and the fear of it as some sort of contaminant is unfounded and counter-productive.

      Sweat that is collecting on the skin isn’t likely to be of cooling help unless environmental conditions are suddenly improved, so wiping it away is not problematic. The bigger concern is an exercise room where there is inadequate cooling and air flow.

  2. Dr. Klau, I have a question for you! It’s one I *think* I can answer, but not sure if I’m 100% correct or if there’s more to it.

    I sweat much more in lower intensity but sustained workouts such as subthreshold endurance/tempo rides. HIIT classes, even very high intensity, not nearly as much.

    Is it simply the “sustained” part of the equation? My internal temperature is higher when there’s no recovery? Are there any other factors at work?

    1. Author

      We can’t know your core temperature without measuring it, but it probably doesn’t drop below your sweat onset threshold during a given workout. However, it can certainly go up, depending on how hard you’re working, how hot your environment is, air flow, humidity, etc. High intensity efforts don’t last very long, so they may not really boost your core temperature very much, while sustained efforts will. Although humans are considered remarkably efficient at converting chemical energy (food) to mechanical energy (muscle contractions), we also generate a lot of heat in the process. If your HIIT classes are like many, the process of blasting hard for a shortish period is followed by recovery at a much lower intensity. The area under the intensity curve is going to be markedly smaller for HIIT than for a steady-state workout.

      Also, just throwing this out there: you are aware of more sweat on your sustained efforts, but we really have no way of knowing if you are sweating more, or if less of it is evaporating or some combination of the two. Sweating is affected by sympathetic nerve activity both at the skin and centrally, and your body and brain may have different ‘stress’ reactions to these two types of workouts. So there are many factors at work (as usual), so, ‘it depends’ (and yes, the hypothalamus, too)

      If you want to be your own lab rat (and who doesn’t?), weigh yourself (no clothes) before and after each type of workout. Add any water consumed to your losses (i.e. if you lose 500g during a workout and you drank 1 liter, you lost 1.5l) and see what the difference is for each. Standardize your sweat rate to ml/min so you can account for different workout lengths. One more set of data points for your training log! 🙂

  3. Great article…especially for us avid sweaters lol ….

  4. So glad we’re talking about this topic. I’ve often heard instructors tell you to wipe the sweat away so that your body doesn’t absorb the toxins or impurities that have been released while working out. Do we really sweat out toxins or is it mainly water and salt? Do our bodies have the ability to reabsorb or is this false? TIA!

    1. Author

      Karen, you are correct that sweat is primarily water and electrolytes like sodium, potassium and chloride. The sweat gland, well below the skin’s surface, reabsorbs as much sodium as it can before the sweat is released, but once sweat hits the skin surface, reabsorption is over. Substances can be absorbed through the skin (thinking of medications or even poisons), but our own sweat does not pose a risk to us, nor to most other people.

      Although there have been reports of sweat containing small amounts of certain viruses (and it’s not clear if it is enough to infect someone else), we sweat as a cooling mechanism. It can feel purifying and cleansing, but that’s a feeling, not science.

      We do not sweat out toxins (I’d like to know what the instructor thinks those toxins are) – it is the liver and kidneys that are tasked with detoxifying and excreting, not the lowly sweat gland.

      Wiping away sweat before it has a chance to vaporize inhibits its cooling function; if it wasn’t vaporizing (high humidity or lack of air movement,etc) then it’s not harming anything, but should definitely be wiped up before the next class comes in.

      It’s not easy to disabuse people of certain notions, but I guess we can try. Thanks for your question!

      1. Thanks so much Jennifer! This really helps a lot and makes a lot of sense now.

  5. Loved this article! Thanks Jen!

  6. Loved reading this article. I sweat within 5 minutes after starting my indoor ride. Always have a puddle of sweat under the bike.
    Good to know that I’m OK.

  7. Is it good idea to have Spin Class on rooftop when weather is hot?

    1. It’s probably not a good idea to have a Spinning class outdoors in the direct sun when it’s really hot. If it’s cool and shaded, excellent. I love outdoor rides. I got to present at Challenged Athletes event in San Diego at La Jolla Cove years ago. It was very sunny and pretty warm, but there was a nice breeze off the ocean nearby.

      Those who plan big outdoor events may not have a choice, however. If you plan an outdoor event (many months in the making) and it turns into a very hot day, I would advise to take precautions. Ask people to hydrate, and not go as hard as they might normally. Ask them to take breaks. I think most people are pretty wise about things like this.

      But where they are not necessarily wise is when the instructor or facility sets the studio temperature artificially higher than it should be and tells the riders that it’s “good for them” to sweat more. Participants often believe what they are told and trust the instructor & studio owner. This could be far more dangerous than riding outside.

  8. What about Spinning in rooftop will it help on weight loss and sweat?

    1. rooftop, outside, near the beach, hot humid room…wherever. This article does a great job of explaining the reasons why sweat does not play a role in weight loss. But it does play a role in cooling the body.

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