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I'm riding indoor (zwift), and apparently when I don't use fan, I sweat much more. Does that mean I lose more weight -> if this is my main goal then I should NOT use a fan?

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    You cannot achieve weight loss with zwift, and really with exercise in general. You have to do it in the kitchen. It's still very good to get exercise, but it is far too easy to regain calories burned if you aren't careful with your diet. Jan 15 at 0:52
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    @whatsisname: This gets repeated all the time, but it depends hugely on what works for the individual person. If your calorie intake is “predetermined” (e.g. because you go for lunch with the colleagues, there is cake and coffee in the afternoon etc. and it’s really hard to avoid those eating activities) the easiest way to avoid a surplus is with exercise. Not to mention all the other benefits you get from exercise.
    – Michael
    Jan 15 at 8:31
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    @whatsisname I recently completed a 10 week training program in Zwift. I wasn't concerned with weight loss and ate more than I normally do during this period (whatever I wanted, to be exact). I lost 5kg. I know it's just a single case of anecdata, and I agree that you have to be careful with your diet', but it works in both directions. Jan 15 at 18:48
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    Side note: Most of the weight lost when we lose fat leaves our bodies as CO2 exhaled by the lungs. Jan 15 at 20:35
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    You absolutely can lose weight with exercise, it just takes a lot more exercise than most people think and you are generally better off fixing your diet first. One other issue is that a lot of high intensity cardio exercise will tend to make you hungry and you'll eat all those calories back, which is why ironically one of the best exercises for losing weight is walking.
    – Turksarama
    Jan 16 at 4:01
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No.

Weight lost by sweat is water weight. Your body needs water, and you should replenish lost water as you ride and/or immediately after. People have reduced their weight through sweating to meet an arbitrary weight cutoff for an event (eg boxing or wrestling), but attempting to keep water weight off ultimately would be counterproductive.

Furthermore, riding in hot conditions limits your performance: your heart rate goes up as your body tries to use blood circulation to cool you. If you're interested in making the most of your workout, you want your body to be cool enough that it isn't doing extra work to keep you cool.

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    I once lost 4.3 kilograms of body weight in one long hot ride. Next morning I weighed more than at the start of the ride, because it was mostly water weight and I drank it all back.
    – Criggie
    Jan 15 at 5:08
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    Also, you still sweat when a fan blows on you - it's just better able to evaporate, so you don't notice it accumulating and beading up.
    – J...
    Jan 15 at 12:26
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    @J... the quicker it evaporates, the more effectively it cools you. So it (the sweat) is doing its job more efficiently, so you don't need to sweat as much. So OP is correct that they will lose more sweat-weight without a fan.
    – Brondahl
    Jan 15 at 12:52
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    @Brondahl Yes, but it's an easy trap to correlate the quantity of sweat lost to the amount of sweat ostensibly seen accumulating on the person, which is not accurate. You will sweat more when it doesn't evaporate efficiently, this is true, but the difference in water loss will not be as dramatic as the difference in sweat accumulation on the skin. We lose water to sweat all day without ever necessarily accumulating any.
    – J...
    Jan 15 at 13:02
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    ...you want your body to be cool enough that it isn't doing extra work to keep you cool... - Can you add info to your answer as to why this isn't helpful for weight reduction? After all, the point of exercise is largely to create extra work. Jan 16 at 3:55
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My answer is essentially the same as Adam's, but phrased a bit differently. The less adequate your cooling, the lower the power you'll be able to sustain. Power directly correlates with calories burnt. Obviously you shouldn't make every session a maximum effort, but all else equal, being able to sustain a similar power as you could outdoors (where cooling is generally not a problem) will help you with your weight loss goals.

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    Oh, cooling is a problem outdoors, all right: I've made the experience that a dew point temperature (measure for absolute water content in air) above 15°C is sufficient to make biking less comfortable, and above 18°C it's bad enough to force me to slow down. Simply because my body gets into trouble removing the heat fast enough. I guess that people living in more humid climates than I do will have developed a higher tolerance against humidity, but cooling can definitely become an issue even outdoors. Jan 15 at 12:40
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No, it does not mean that. Losing water is not weight loss, unless it's the night before a fight (judo, boxing) and you need to get your weight into your weight class, then happily gain it back.

Your body needs water and will maintain as much water as it needs (unless you have some disease that makes you keep too much water). Being dehydrated is not healthy and not helpful to weight loss.

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If anything, cooling your body may increase the metabolic rate because the body tries to maintain its core temperature by transforming chemical energy into heat, e.g. by reflexively tensing the muscles when shivering.

If you are exercising to the point that you are sweating you are of course far from shivering so the cooling will not have much if any impact, except perhaps after you stop exercising but while you are still sweaty.

By the way, it is conceivable that even with a fan you still sweat a lot, in the sense that salty water is secreted and evaporates from the skin for cooling; it's just that it is less noticeable because the air flow from the fan evaporates it before it accumulates so that the skin feels mostly dry.

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What they said --^. Two points here... first, as others have pointed out, sweating will lose weight but not in any way that helps with health and you'll just gain it back once you've rehydrated. Better to run the fan and be able to possibly ride harder, burning more calories. Second, depending on your build you may lose fat but not lose much if any weight -- muscle weighs! I'd consider measuring both weight and waist (or belly) size -- that way, instead of being like "It's been a month, how did I only lose 1 pound 8-( " it's like "It's been a month, I only lost 1 pound but 2 inches off my waist, must have built up some serious muscle 8-) ".

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