[vhfpmr (OP)]

It occurs to me that if oxygen consumption is directly proportional to energy metabolised, then water produced must also be. Wikipedia just gives typical levels, not the relationship to metabolic rate.

[Bored chemist]

The air leaves your lungs practically saturated with water (at 37 ^oC) regardless of your metabolic rate.
The excess water mainly leaves via the kidneys.
I think that actual value is about 6% by volume or 4% by weight.

[alancalverd]

Intriguingly, the moisture content of exhalate does seem to increase as a response to stress. I notice this particularly as a tendency of aircraft windscreens to fog during times of high cockpit workload - no increase in physical exertion but a lot of mental arithmetic and decisions. And by Sod's Law, these are exactly the times when you most need to see where you are going!

[Bored chemist]

Intriguingly, the moisture content of exhalate does seem to increase as a response to stress. I notice this particularly as a tendency of aircraft windscreens to fog during times of high cockpit workload - no increase in physical exertion but a lot of mental arithmetic and decisions. And by Sod's Law, these are exactly the times when you most need to see where you are going!

That looks like confirmation bias to me.
Surely a more plausible explanation would be that you breathe more and thus exhale more water rather than that the air exhaled has a water content that's not "pretty close to saturated at 37C".

[paul cotter]

Stress will, in general, cause an increase in respiration and hence more water vapour emitted.

[vhfpmr (OP)]

Surely a more plausible explanation would be that you breathe more and thus exhale more water rather than that the air exhaled has a water content that's not "pretty close to saturated at 37C".

You've just demolished your previous argument there.

If you breath faster/deeper, then you'll still be producing more water when your metabolic rate is higher, even if the water content of each breath is the same.

during times of high cockpit workload - no increase in physical exertion but a lot of mental arithmetic and decisions.

But the brain accounts for 20% of energy consumption.

[Bored chemist]

You've just demolished your previous argument there.

No, I didn't.
Why do you think I did?
Did you not realise that you would be exhaling more air and also exhaling more water so the concentration would be the same?

[Bored chemist]

But the brain accounts for 20% of energy consumption.

Almost independently of whether or not you are thinking about stuff or sleeping.
https://time.com/5400025/does-thinking-burn-calories/

But the rest of your body changed its metabolism significantly.

[alancalverd]

Mental stress does indeed increase brain metabolism, as evidenced by functional MRI. Or is experimental psychology all bunk?

[Bored chemist]

Mental stress does indeed increase brain metabolism, as evidenced by functional MRI. Or is experimental psychology all bunk?

You seem to have missed this bit.

Almost independently

[bhawnaggy]

The human body is made up of approximately 60% water, and this water is constantly being renewed. Every time we breathe out, we expel water vapour. In fact, the average person exhales around 1 litre of water vapour per day. Interestingly, the amount of water vapour in our breath varies depending on the air temperature and humidity. When it's cold outside, we exhale less water vapour because the air is already saturated with moisture. Conversely, when it's hot and humid, our bodies have to work harder to get rid of excess moisture, so we tend to breathe out more water vapour. So next time you're feeling a bit parched, take a deep breath and know that you're also releasing a refreshing mist of water into the air.

[Origin]

When it's cold outside, we exhale less water vapour because the air is already saturated with moisture

That does not seem correct.  Whether the temperature is 40 C or 0 C, our internal temperature remains (relatively) constant.  So the amount of moisture we exhale per breath should be the same.

[Bored chemist]

When it's cold outside, we exhale less water vapour because the air is already saturated with moisture

When the weather is cold, the air, even if it is saturated, can't hold much water vapour.
So when we breathe it in, there's not much water in it but, by the time we breathe it out, it's pretty much at 37C and saturated.
So we have to add more water to get it saturated in cold weather.
The concentration is pretty much the same, but the amount of water we lose is bigger in cold weather.

Conversely, when it's hot and humid, our bodies have to work harder to get rid of excess moisture, so we tend to breathe out more water vapour.

Nope.
We are really quite good at getting rid of water- that's what your kidneys are for.

[William Hardy]

It depands on your heart rate. How much you breathe.

[Bored chemist]

It depands on your heart rate. How much you breathe.

How does the air, getting saturated with water in your lungs at pretty close to 37C know how fast your heart is beating?

[wolfekeeper]

The pilots breath faster when under stress, but I think a bigger effect would be that pilot sweat more, and I think that's where most of the moisture comes from.