Science Says: Take Hot Baths

This is my most important article ever. In it, I will give you permission to take more hot baths. WORLD-CHANGING STUFF.

I’m always looking for possible interventions that can improve the health and performance of athletes I coach. It usually starts with a problem to solve. Here, I’m writing this article in December because the problem is that cross-population injury rates seem to go slightly up and overall performance seems to go slightly down in winter. There aren’t great studies to validate that hypothesis, but a 2014 master’s thesis found higher rates of musculoskeletal injury in winter months, attributing that increase to cold, icy and hard surfaces (cue Frosty the Snowperson saying, “That’s what she said”). And, in coaching, I rarely see the same levels of rapid progression in winter as in the other seasons, even controlling for training approach and background.

So we have the problem, and it boils down to this: screw winter. 

There are plenty of possible reasons for performance and health reduction during winter. Some options include the cold’s impact on blood flow, slippery surfaces affecting form, reduced Vitamin-D levels, lower motivation, fatigue from past training and inadequate ice-cream intake. I’m sure you can think of a bunch of others. But in thinking about this topic way too much over the years, I kept coming back to another explanation. Maybe it’s related (at least partially) to blood volume.

There are plenty of possible reasons for performance and health reduction during winter. Some options include the cold’s impact on blood flow, slippery surfaces affecting form, reduced Vitamin-D levels, lower motivation, fatigue from past training and inadequate ice-cream intake. … I keep coming back to another explanation. Maybe it’s related (at least partially) to blood volume.

A reduction in blood volume (primarily via plasma volume, consisting mainly of the water content in blood) during winter has been observed for nearly 100 years (see this 1940 article in the American Journal of Physiology). A 1950 article in the Journal of Physiology pegged the seasonal blood-volume difference at 8.6%, and subsequent estimates have varied around that number depending on where you look. While there is no set number for every person, that 1950 article is titled “Seasonal Changes in Human Body Fluids,” so I know I trust it.

There are a few principles related to blood volume that could be important here. 

One, as outlined by a 2010 article in the Journal of Applied Physiology, heat acclimation and increased blood volume can be associated with improved performance in both hot and temperate conditions. Two, there is growing evidence that hemoglobin mass may be affected by blood-plasma volume changes. A 2019 study in Frontiers of Physiology and a 2020 study in Experimental Physiology found that hemoglobin mass increases after plasma volume increases in athletes training in the heat. Three, both plasma volume and hemoglobin mass may impact how the body performs, heals, and adapts. The physiology of how it works gets complicated and is not 100% direct (incorporating seasonal changes in hematocrit, or the ratio of red blood cells in total blood volume), so the exact relationships are not always certain.

The million dollar question follows: how the heck would those principles apply in winter? Each of those blood-volume concepts is based on beneficial adaptations to heat, so when thinking about winter we are extrapolating the inverse—negative adaptations to lack of heat. Perhaps the body adapts to cold weather with time and none of the issues are relevant when that equilibrium is reached. There are studies on seasonal hematological variation (see this 2013 review in Sports Medicine) that do not provide a definitive answer for athletes. In addition, there is no settled evidence that cold-weather performance is significantly affected by heat adaptation (2014 review in Sports Medicine), so that part may not be relevant at all.

Either way, we do know that blood volume usually decreases in winter and increases in summer. I have written articles on heat acclimation to improve performance and blood volume’s role for fitness, but there are no perfect studies looking at the training question I can’t stop thinking about. If blood volume is important in summer, might it also be something to think about (at least a little bit) in winter?

I think it might be.

That sentence is powerful prose, I know. The conclusions to come rely on the physiological theory that blood volume matters for athletes even in cold conditions (which is debated), and that natural, seasonal reductions in blood volume could affect injury rates and performance. Perhaps that is part of why Scandanvian athletes are so famous for their saunas (fun side note: there are 2.2 million saunas in Finland for a population of just over 5 million people). And all that prelude brings us to the topic of the article: hot-water immersion via baths or tubs could play a role in winter health and performance.

Let’s take a step back first. We have known for a long time that exposure to heat causes rapid expansion of blood volume. As outlined in a 2000 review article in the Medicine & Science in Sport & Exercise journal, total blood volume is primarily composed of the sum of erythrocyte volume (red blood cells) and plasma volume (mostly water plus some dissolved proteins, glucose, clotting factors, electrolytes, hormones and other components). Plasma volume can adapt to environmental changes rapidly. For example, a 2015 study in the European Journal of Applied Physiology found a 17.8% increase in plasma volume in highly trained cyclists after just four exposures to post-exercise sauna. Plenty of other studies back that up. And recent studies tie those plasma volume increases to red-blood-cell-volume increases too.

Right now, though, I wouldn’t recommend that you go to a gym sauna unless you are participating in a COVID-19 research study testing the efficacy of a new vaccine. So what other options are there?

Right now, though, I wouldn’t recommend that you go to a gym sauna unless you are participating in a COVID-19 research study testing the efficacy of a new vaccine. So what other options are there? Training impacts blood volume, and it could be as simple as doing some of your activities in warm clothing, or doing indoor doubles. But more and more studies are using hot-water immersion to induce heat acclimation, which could be an option for more athletes.

Hot-Water Immersion Studies

A 2015 study in the Scandinavian Journal of Medicine & Science in Sports had 17 non-heat-acclimated males do a 40-minute easy run every day for six days, followed by hot-water immersion in 104 degrees F water (40 degrees C) for 32 to 40 minutes. The hot water caused heat acclimation similar to what would be expected from traditional sauna protocols—lower skin temperature and perceived exertion in temperate and hot running, improved performance in hot running and a non-significant improvement in temperate running. The study did not measure blood volume, but those adaptations are likely at least partially associated with increased plasma volume.

A 2018 study in Frontiers of Physiology followed up on those findings, based on the assumption that the principles may apply differently to trained endurance athletes since there is some amount of heat training in normal running at baseline. The study used the same protocol, but with eight trained athletes and eight untrained athletes. The findings of the 2015 study were reproduced in both groups. A 2019 study in Frontiers of Physiology found that athletes following the protocol retained the heat acclimation benefits for at least two weeks. Yet another 2019 study in Frontiers of Physiology found that delaying the hot-water exposure from right after exercise to eight hours later still had similar benefits, though of slightly less magnitude.

Now we get to the good stuff.

We have the problem: winter can suck for running health and performance. We have a hypothesis: that suckitude may at least partially be related to natural changes in blood volume. We have data that may be relevant: studies show heat acclimation is partially associated with increased blood plasma volume, and that hot-water immersion improves heat acclimation, likely increasing blood plasma volume. Now let’s conclude with another unproven (possibly wrong) hypothesis: hot-water immersion could make winter running more fun by giving blood volume a helpful nudge when it wants to go to sleep for the season.

Now let’s conclude with another unproven (possibly wrong) hypothesis: hot-water immersion could make winter running more fun by giving blood volume a helpful nudge when it wants to go to sleep for the season.

That’s all conjecture, so the exact mechanism isn’t the most important thing in the world. What matters more is the general principle that some passive heat exposure could be helpful even in winter months.

If you have a hot tub, consider spending some time in it. I wouldn’t suggest mimicking the study protocols unless you have a doctor and a crash cart on site, since 30 to 40 minutes at 104 degrees F could cause a deadly case of prune fingers and an even deadlier case of heart attack. But all of these benefits likely happen on a spectrum, so some time in the tub will likely have some benefits, even if they’re not as rapidly undeniable as the six-day protocols in the study.

Most of us will not have a hot tub, though, unless that student debt forgiveness plan goes through. So what are we to do? In the last few years, a few athletes I coach have sworn by hot baths for the same purpose. Their anecdotes are powerful enough that my wife Megan and I both take hot baths each day.

Most of us will not have a hot tub, though, unless that student debt forgiveness plan goes through. So what are we to do? In the last few years, a few athletes I coach have sworn by hot baths for the same purpose.

Does it make a difference? I have no freaking clue. My routine is to fill up the bathtub with some soothing hot water and meditate for 10 to 20 minutes (by meditate, I mean listen to a bumping playlist), until I break a nice sweat. The first week I did it this year, my heart rate after 10 minutes was 10 to 20 beats per minute higher than it is now, possibly indicative of some acclimation effects over time, but that’s subject to tons of confounding variables.

Long story short: There is some physiological basis for benefits from hot baths or hot tubs (or even extra hot showers), both in winter and summer. However, the evidence is by no means definitive or even overly persuasive outside of the extensive study protocols. But I do know this for a fact: Some time in a hot bath is awfully relaxing.

And whether it’s winter or not, we could all use a little bit of extra time to relax.

David Roche partners with runners of all abilities through his coaching service, Some Work, All Play. With Megan Roche, M.D., he hosts the Some Work, All Play podcast on running (and other things), and they wrote a book calledThe Happy Runner.