Study says 40 minutes in a 104F bath matched zone 2 work
Hot water immersion matched moderate aerobic exercise on one blood-vessel measure after 12 weeks, and the post posting it says a bath still does not replace running.
Sitting in a hot bath improved vascular function as effectively as zone 2 aerobic exercise (steady easy effort), according to research shared by Dr. Rhonda Patrick of FoundMyFitness.
Neither is a replacement for exercise, but their ability to produce similar improvements in endothelial function is remarkable.
The setup was simple. Participants did three sessions a week of either 40 minutes in 104F (40C) water or 40 minutes of moderate-intensity aerobic exercise. After 12 weeks, both groups showed comparable improvements in flow-mediated dilation, which Patrick describes as a measure of how effectively blood vessels widen in response to increased blood flow.
What it actually means
This is one marker, not fitness. Flow-mediated dilation says something about how your arteries respond, and it is not a VO2 max test, a threshold test or a race result. Patrick's own post makes the limit explicit, saying neither passive heat nor a bath replaces exercise, and that the notable part is that the two produced similar improvements in endothelial function.
She also notes that research supports similar benefits for other passive heat methods such as sauna, so the bath is not uniquely magic. It is heat exposure doing heat exposure things.
Why runners are paying attention
The obvious reading for a marathoner is the injured week, the travel week, or the week when easy running is off the table. On this one measure, a long hot soak was not nothing. Brady Holmer, posting about the same wave of interest, put it plainly.
The less obvious reading is the one to resist. Nobody in these posts claims a bath builds endurance, raises your threshold, or gets you fitter for a fall marathon. Three 40-minute soaks a week is also a real time commitment, roughly the same block of time the exercise group spent training, and the exercise group got the vascular result plus everything else running gives you.
So treat this as a small, specific finding. Heat exposure moved one vascular number as much as moderate aerobic work did over 12 weeks. That is interesting. It is not a training plan, and it is not a reason to skip the run.
If you are already using a sauna or hot baths for recovery, this is a point in favour of keeping the habit. If you are thinking about swapping easy miles for bath time, the person who posted the study is the one telling you not to.
Hot baths are having a moment.
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