humidity is not the "poor man's altitude." Heat is the poor man's altitude.
Clueless people who think heat and humidity are synonymous have apparently confused the old maxim.
In hot weather - which only rarely has humidity above 50% - you have to cool by massively sweating. That provokes an increase in blood volume to adapt. That results in an increase in RBC's, or your blood would get too thin.
In humid weather - not generally hot - you can't cool rapidly by sweating. The sweat won't evaporate fast enough. So all you can do is limit how much heat you generate, by going slower or more efficiently. That can be useful training too, but it won't give you a blood boost.
- Until recently, the consensus was that there is no increase in RBCs from heat training. Your blood does, in fact, get thinner. It has been suggested that there may be some increase in hemoglobin mass in response to the higher plasma volume, but more research needs to be done. But even if that happens, the blood would still get thinner because the plasma volume increase is so massive. Most of the heat training research has been on a timeframe too short for RBC production to have an impact, so if it's happening and helping, it's on top of what's already been documented.
- The research on heat training uses different levels of humidity, and it has been assumed that humidity simply amplifies the effect because it increases sweating and core temperature, which are probably the signals for adaptation. I'm not aware of any research specifically comparing the training effect of wet heat vs. dry heat, but wet heat does cause you to sweat more, specifically because it's less effective. The body doesn't say, "there's no point in sweating because it's humid." It says, "oh man, we're sweating a bunch and still cooking, let's sweat more."
- Also, I'm not sure where you live, but in much of the country it is, in fact, both hot and humid. Typical Mid-Atlantic weather in the summer is 90 with a dew point in the low 70s, so that's relative humidity over 50. Right now in DC it's 98 with a dew point of 75 (RH 55%).
I've run 102F. I did not love it but I'd take it any day rather than one of the really humid 80 days we get here on the Massachusetts coast. But it's an individual thing I suppose. Lydiard used to say running in heat was no problem as long as it was also humid.
My evidence on this issue is all anecdotal based on a study of one... me.
I have to train every summer in the heat and humidity of TN. It really is disheartening, it makes every run into a much dreaded slow sweaty slog. My pace is so far off what my normal pace that I dont even bother to look at it any more. (It just messes with your head.)
I also know that when the weather finally breaks in October I feel like a weight has been lifted off my back.
I dont know if there was really a "training benefit" so much as a training hinderance that has now been removed. (Like running w/ankle weights vs not)
If heat really were a big training aid then Gainesville FL, Austin TX, etc.. would be the mecca for endurance athletes.. and I think we all know the verdict on that. We all seek the cool confines of the NW, the West Coast, and mountain air of Flagstaff.
That is my anecdotal evidence as well. I feel like I can train normally sometime in late september or october. The only way to really test it would be to fly to somewhere cooler like the pac northwest in summer for a race. My gut is that I would perform slower than I would if I flew there for a race after a month of training in cooler temps, but so many people swear by heat training that I don't know any more.
I know I can do so much more volume and run significantly faster in cooler temps and I don't think that is some training benefit from running in the heat.
What I have seen is the heat effect some individuals less than others - like a 104 half marathon guy runs 108 in the mid 80s. Or a guy only loses like a second a lap at 5k.
I grew up in Florida and ran there in highschool and college. I'd take on anyone from altitude anyday if they showed up in Florida and they would probably say the same to me if I showed up in their backyard. With that said, I never found running at altitude to be an issue as it is usually dry and infinitely cooler than Florida! I don't live there anymore and would never move back.
humidity is not the "poor man's altitude." Heat is the poor man's altitude.
I guess it depends on what one considers "hot" or "humid. Have you ever heard the phrases "It's not the heat, it's the humidity" or "but it's a dry heat"?
What the body does to cool itself isn't subject to interpretation. When it is hot (90s and above) it is near or above body temperature, and sweating (or otherwise putting water on your skin) is the only way to cool off, even if you are lying flat on your back not exercising at all. Fortunately, air that hot can hold incredible amounts of moisture, so is rarely too humid to sweat efficiently.
Those unused to triple digits may be unaware how much they are sweating, because it's so efficient it doesn't collect in liquid form.
In the southern US, the nights that drop to the 70s or 80s will then be humid, but that's not hot, in the sense that you have to sweat to stay cool. Which is good, because sweating doesn't work in humidity. Fortunately, it's far enough below your body temperature that you don't have to sweat. Just don't overheat.
Training can be similar in both situations - go slow, don't overheat - but only one will boost your blood.
- The research on heat training uses different levels of humidity, and it has been assumed that humidity simply amplifies the effect because it increases sweating and core temperature, which are probably the signals for adaptation.
Why do so many people think this? Because you see more sweat? That's because it didn't evaporate. Sure, that will increase your core temperature quicker than if it does evaporate. But that will just shorten your workout.
Why would core temperature be the trigger for increasing blood volume? If that were true, you could get it done on intensity alone, a series of short sprints on short recovery would work. The more plausible trigger is chronic high-volume sweating from long, low intensity workouts in low humidity heat.
Admittedly this isn't my specific field of study as a physiologist, but I'll try and break down some factors that very likely contribute.
1. Like others have mentioned, water vapor will displace some of the oxygen molecules in humid air, which ultimately reduces the partial pressure of O2 in that air, at the same atmospheric pressure and temperature as dry air. Your lungs cannot discriminate between the gas fractions of the air they inspire, so you end up with the same full lung volume, but with a smaller oxygen content. Therefore, there is a lower O2 gradient into the capillaries (and thus, a reduced influx of O2 into the blood stream), ultimately promoting adaptations that are relevant to improving your ability to transfer and uptake the oxygen you DO have. I'll skip out on the downstream effects of this for now, but if anyone asks, I can certainly elaborate.
2. Humid air also makes it more difficult for your body to thermoregulate using sweat. At the very least, this will necessitate you vasodilating more blood vessels that DO send blood to the skin to maximize your ability to thermoregulate this way, in a compromised state (your body won't downregulate this mechanism (at least, to my knowledge) in a humid environment even though it isn't as useful). Because of this, your body will need to rely on other mechanisms, such as respiration (breathing out hot air) to thermoregulate, more than normal. Collectively, these mechanisms pull even more oxygenated blood (especially for respiration; you can look up something called the "respiratory stealing of skeletal muscle blood flow" if you're interested, it's actually quite fascinating, and it seems obvious after your read about it, but we don't typically think about it) away from the skeletal muscles that are being used for your running workout. This results in additional "pressures" to promote cardiovascular adaptations.
3. Increase in blood volume is an acute adaptation observed in just about any stimulating cardiovascular exercise, not just at altitude or in humid environments. It is a product of multiple factors that are present in varying amounts with many types of aerobic stress.
NOTE: All of this is simplified in a vacuum of course. I've never personally trained at altitude, but I've done a ton of high intensity workouts and long runs in hot and humid conditions. Personally, I find it hard to really focus super effectively on high efforts, particularly high speeds, in hot weather, especially, because even my feet get hot in my shoes and that just sucks haha. So, ultimately, outcome studies comparing these environments, even when well controlled, are just hard to truly be well controlled enough to know that if the effect size between the two treatments is actually a result of JUST the atmospheric factors. But yeah, technically you can see similar adaptations in both types of weather.
4. "Increase in anaerobic capacity" really? Go on then, explain that, I challenge ya.
Then explain how an increase in blood volume with a cocommitant decrese in hemoconcentration increases aerobic capacity? You will really have to work hard on that one, but don't give up.
i challenge you to get on pubmed and read a few of the hundreds of studies that have proven this. it's actually pretty interesting stuff and seems a lot of otherwise knowledgeable people are unaware of this.
And I challenge you to do the same. Hemodynamics of altitude training has been studied for decades. What groundbreaking discovery do you believe the experts have missed?
- The research on heat training uses different levels of humidity, and it has been assumed that humidity simply amplifies the effect because it increases sweating and core temperature, which are probably the signals for adaptation.
Why do so many people think this? Because you see more sweat? That's because it didn't evaporate. Sure, that will increase your core temperature quicker than if it does evaporate. But that will just shorten your workout.
Why would core temperature be the trigger for increasing blood volume? If that were true, you could get it done on intensity alone, a series of short sprints on short recovery would work. The more plausible trigger is chronic high-volume sweating from long, low intensity workouts in low humidity heat.
All of those stresses can help increase blood volume.
There is an established answer to this question, but buried on page five, I doubt many will see it.
In hot/humid weather, your body diverts more blood towards the skin surface to make the most of sweating. As water evaporates, it cools the skin, which cools the blood just below the surface. That slightly cooled blood can then circulate through the body. This helps thermoregulate the body in warm conditions. It also functionally lowers the amount of oxygen available to the muscles, just like altitude training, because more blood is diverted to the skin, and away from the muscles.
When cool temps return and the muscles get their full complement of oxygen again, it is much like the runner who trained at altitude coming back to sea level. The muscles suddenly have much more oxygen than they are used to.
There is an established answer to this question, but buried on page five, I doubt many will see it.
In hot/humid weather, your body diverts more blood towards the skin surface to make the most of sweating. As water evaporates, it cools the skin, which cools the blood just below the surface. That slightly cooled blood can then circulate through the body. This helps thermoregulate the body in warm conditions. It also functionally lowers the amount of oxygen available to the muscles, just like altitude training, because more blood is diverted to the skin, and away from the muscles.
When cool temps return and the muscles get their full complement of oxygen again, it is much like the runner who trained at altitude coming back to sea level. The muscles suddenly have much more oxygen than they are used to.
No one should choose to train in high heat and humid conditions, unless you’re doing a short heat adaptation cycle for a hot race. Just the same as you are way better off living high and doing workouts low. Unless your key race is at high altitude. Heat adaption happens on the order of 10 days. You are not getting some great benefit from running in hot and humid conditions all summer. Sorry.
Been a while since I've read up on this but to add on to what coach ewe mentioned, several physiological adaptations from training in heat/humidity are similar to those seen at altitude...I remember seeing some stuff regarding increases in aerobic capacity which seems to be primarily through increases in blood volume and cardiac output. Off a quick search, came across this one, but I believe there are several others as as well.
there are multiple advantages to moving/training at altitude (for responders): 1. increase in rbcs 2. increase in blood volume 3. increase in lactate buffering capacity 4. increase in anaerobic capacity (not often talked about, don't get hung up on this plz) 5. there are others, but those are the big 4 if memory serves me right
advantage to training in humid conditions: 1. increase in blood volume 2. there are others, but that's the main one (lot of people about heat shock protein for instance, but i honestly forget it's impact rn)
an increase in blood volume, even without any increase in rbcs (thus hematocrit goes down even if global hemoglobin levels are effectively static) increases aerobic capacity
if you do all your training mid-day in a hot/humid environment of course your results are likely to be bad. that would be like living at 6500' but choosing to drive further up the mountain to do all speed workouts. hard workouts generally need to be early morning or later in the evening, while the more moderate/easy workouts can be performed with more sun exposure
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