Sometimes, science can seem slightly at odds with the world as we experience it. Just a little digging, however, can often reconcile the two with the minimum of fuss. And if you don't know where to dig, who better to ask than the SExperts?
While watching people exercising one day, steve-paul1990 was struck by a thought...
I heard somewhere that if you are fit, then you sweat after less exercise than if you are unfit. But how come fat people sweat a lot if they exercise?
A fair question. So, how to reconcile these two seemingly contradictory observations? First to weigh in was Orcus:
Bigger mass-to-surface area ratio means that large people lose heat less easily through standard radiation, conduction and convection. Sweating is more efficient at cooling than these and so large people sweat more as they need more heat loss than smaller people.
To further explain this, Arnie Appleaide decided to apply a little maths to the problem:
To use a 'circular' analogy, where 'R' is the radius of the circle:
Surface area is proportional to R^2
Volume is proportional to R^3
So, a change in mass means a change in volume1. However, the corresponding change in surface area is not linear, it would go up as R^(2/3).
As larger mass = larger volume = larger R, the person would, in absolute terms, have more cooling surface. But, depending how you define it, their efficiency should be worse, as they have a lower ratio of surface to mass.
Now, this all assumes that the larger person and the smaller person are doing the same amount of work. This may not be the case, as our SExperts were quick to point out:
If someone very thin walks a mile at 5 mph, and someone very fat walks the same distance at the same speed, isn't the actual work done by the fatter person more? They have to move a heavier load, in effect. This could lead to feeling hotter and sweating more, too.
- King Bomba
If I strapped 130 pounds to my back, stood up from a chair and walked forty feet, I'd be feeling the strain. As a result of the extra strain, my body would burn more calories and create extra waste heat, and I might sweat a little. A 300-pound man does that same amount of work every time he gets up to go to the bathroom. This is partly why obese people always seem to be tired and out of breath.
- Blatherskite the Mugwump
Well, this seems to explain why larger people sweat more, but why should fit people sweat more than the rest of us too? Blatherskite the
Mugwump had this to add:
A fit person sweats more easily than a sedentary person because, like any other system in the human body, the more often you use your sweat glands, the better they function. Exercise triggers the mechanism because as you increase your efforts, you burn more fuel in order to do so. The extra waste heat must be disposed of.
To quote Skankyrich, 'So circumferencially challenged people sweat more because their sweat glands are used more often and are therefore more efficient? Makes sense'. And Rich also provided yet another possible contributing factor:
Sweating regulates body temperature by cooling the skin, which cools the rest of the body by heat transfer. Presumably a thick layer of fat would act as a kind of insulation blanket, meaning the skin needs to be cooler for longer to cool the rest of the body, thus necessitating more sweat2.
So there you have it. Pulling everything together, Blatherskite the Mugwump summed up the conversation perfectly:
I don't think anyone so far has offered a theory which is invalid. I think they all come together to explain certain aspects of the question. It's a combination of all these factors... insulating properties of fat, decreased surface ratio, extra workload, and increased efficiency of sweat systems due to regular use caused by the previous factors.