Do Hot Baths Improve Circulation? 20 Studies
Do hot baths improve circulation? While you are in one, yes, measurably: in a 2026 review that pooled 20 hot-water immersion studies in healthy adults, heart rate was about 28 beats a minute higher and diastolic blood pressure about 5 mmHg lower by the end of a session than in people who stayed dry. Whether weeks of hot baths change the blood vessels themselves is far less settled than the headlines suggest; the same review calls that evidence inconclusive. Here is where the hot-bath idea came from, what the trials measured, what a single soak does and how long it lasts, what eight weeks did and did not change, the risks the researchers list, and where daily support for healthy blood flow and circulation from a nattokinase supplement fits beside the bath.
In this article: Where the hot-bath idea came from · What the 20 studies actually measured · What a single hot bath does · What eight weeks of baths did, and didn't, change · How the trials did it, and the risks they list · Where Great Billiom Nattokinase 10,800 FU fits
Where the hot-bath idea came from
The modern interest starts in Finland. In 2015, researchers writing in JAMA Internal Medicine reported on 2,315 men aged 42 to 60 from Eastern Finland, followed for a median of 20.7 years: the men who used a sauna more often, and for longer, fared better on cardiovascular outcomes over those two decades. It was an observational study of men in a country where the sauna is a way of life, and it could not say why. It did make physiologists curious about whether heat alone, without exercise, does something to the vessels.
The study that turned curiosity into a headline came a year later from the University of Oregon. Twenty young, sedentary adults spent eight weeks in a lab protocol, four to five sessions a week, sitting in 40.5 °C water long enough to hold their core temperature at or above 38.5 °C for an hour, roughly 90 minutes in the bath; a matched group sat in 36 °C water as a sham. In the Journal of Physiology, Vienna Brunt and colleagues reported that flow-mediated dilation, a standard measure of how well an artery widens when blood flow rises, went from 5.6 to 10.9 percent in the heated group; aortic pulse wave velocity, a measure of arterial stiffness, improved from 7.1 to 6.1 meters per second; and mean arterial pressure fell from 83 to 78 mmHg. The authors called the changes "on par or greater than what is typically observed in sedentary subjects with exercise training." For what a time-efficient exercise itself does to resting pressure, and how quickly the effect fades when the sessions stop, our article on wall sits and blood pressure covers the 100-adult trial published this fall.
That is a striking result, and it is also ten people per group. Nearly every "a hot bath is as good as a workout" story since traces back to it. The 2026 review asked what the whole literature says.
What the 20 studies actually measured
The review, published in Physiological Reports by Bruno Sotomaior and colleagues at Queensland University of Technology, searched six databases and found 20 laboratory studies of hot-water immersion in healthy people, 15 with data that could be pooled: 384 participants in all, 338 of them young adults aged 20 to 27 and only 46 older adults aged 57 to 69. Sixteen studies were randomized.
The baths varied widely. Water ranged from 37.5 to 45 °C and sessions from five minutes to two hours, most lasting 45 to 60 minutes. Ten studies immersed people to the chest, shoulders or neck; nine used a lower-body bath to the waist, the knees or just the feet; one warmed only the hands. Fourteen tested a single session and six tested repeated sessions, from five up to the Oregon group's 36, against a thermoneutral bath at 33 to 36.5 °C or the same time spent dry.
Two honest limits come with that picture, and the review states both. Every study was a supervised lab protocol, not a bath at home, and most vessel measurements exist in only two or three studies each, so the pooled estimates for those outcomes carry wide uncertainty, a width that reflects "the uncertainty of the effect and not necessarily a wide variation in the effect."
What a single hot bath does
Pool the single-session studies and the direction is consistent. By the end of a hot bath, heart rate was 28 beats a minute higher than in the control condition (95 percent confidence interval 19 to 36), diastolic blood pressure 5 mmHg lower (−9 to −1) and mean arterial pressure 7 mmHg lower (−12 to −1). Systolic pressure, the top number, did not change meaningfully.
The mechanism is the body shedding heat. Warm the skin and core, and the skin's vessels open wide to carry heat to the surface; the heart speeds up to push more blood through them; and blood moves faster along the artery walls, raising the friction physiologists call shear stress. Our article on how to improve circulation while sitting explains why shear stress is the signal arteries respond to and why a desk day removes it. A hot bath supplies it without moving a muscle, which is the appeal.
How long does it last? A 2026 randomized crossover study from Bangor University in Experimental Physiology answered that directly. Twenty-three healthy adults, average age 26, spent 60 minutes in 40.4 °C water one morning and, on another day, 60 minutes in 24.7 °C air, then wore an ambulatory blood pressure monitor for 24 hours. Core temperature reached 38.8 °C in the bath. Mean arterial and diastolic pressure were lower during the bath and for about 20 minutes afterward, then returned to normal; across the full 24 hours, daytime and nighttime pressure matched the control day. The one trace that survived was at night: the normal overnight fall in diastolic pressure was deeper after the morning bath, 19.1 percent versus 15.4 percent, with a healthy dip generally defined as 10 to 20 percent. So the effect lives mostly in the tub and the 20 minutes after it, with a faint echo at bedtime.
That 20-minute window matters for one practical reason. Vessels wide open from heat behave much like vessels wide open after a run; our article on whether you need to cool down after exercise covers why standing up abruptly with dilated vessels can leave you light-headed. The same caution applies to climbing out of a hot bath.
What eight weeks of baths did, and didn't, change
The repeated-session studies are where the headlines and the pooled data part ways. Five studies measured resting heart rate after a course of hot baths: 3 beats a minute lower than in controls (−6 to −1), with no disagreement between studies and every measurement taken at normal temperature, well after the last bath. A modest, consistent change in the direction you would expect from any training-like stimulus.
Everything else was inconclusive. Resting blood pressure after repeated baths was not significantly different from control: systolic +0.5 mmHg, diastolic −2. Flow-mediated dilation pooled to a 2.5-unit improvement, but the confidence interval ran from −18.3 to +23.2, because the Oregon group's large gain sits beside a 2025 footbath study that found much less. Pulse wave velocity edged toward improvement (−0.3, interval −0.6 to 0.1) without reaching significance; shear rate showed no effect; and the only study to measure cardiorespiratory fitness, eight weeks of footbaths, found no difference from control. The review's conclusion: the findings "indicate inconclusive beneficial effects of HWI on cardiovascular health markers and further research is needed."
Sauna research tells the same acute story. In a 2018 study in the Journal of Human Hypertension, 102 Finnish adults, average age 52, each with at least one cardiovascular risk factor, took a single 30-minute sauna at 73 °C: pulse wave velocity fell from 9.8 to 8.6 meters per second immediately afterward, and blood pressure from 137/82 to 130/75 mmHg, with systolic pressure still lower 30 minutes later. A clear effect of the session; what months of sessions do to the vessels is the open question.
Study | Who | Protocol | What changed |
Brunt et al. 2016, J Physiol | 20 young sedentary adults | 8 weeks, 36 sessions, 40.5 °C, core held ≥38.5 °C | FMD 5.6 → 10.9%; aortic PWV 7.1 → 6.1 m/s; MAP 83 → 78 mmHg |
Laukkanen et al. 2018, J Hum Hypertens | 102 adults, mean age 52, ≥1 risk factor | One 30-min sauna at 73 °C | PWV 9.8 → 8.6 m/s; BP 137/82 → 130/75 right after; systolic still lower at 30 min |
Leaney et al. 2026, Exp Physiol | 23 healthy adults, mean age 26 | One 60-min bath at 40.4 °C, then 24-h monitoring | Diastolic and mean pressure lower for ≤20 min after; 24-h BP unchanged; night diastolic dip 15.4 → 19.1% |
Sotomaior et al. 2026, Physiol Rep (single session) | 10 studies pooled | One hot bath vs control | Heart rate +28 bpm; diastolic −5 mmHg; mean pressure −7 mmHg; systolic unchanged |
Sotomaior et al. 2026, Physiol Rep (repeated) | 5 studies pooled | 5 to 36 sessions | Resting heart rate −3 bpm; blood pressure, FMD and stiffness not significant |
The pattern down the right-hand column is plain: the single-session effects are real and short, and the long-term vessel changes rest on a handful of studies that do not yet agree.
How the trials did it, and the risks they list
The lab protocols are more demanding than a bath at home. Most held the water at 40 to 40.5 °C for 45 to 90 minutes with a core-temperature probe in place, and the review notes that although 38.5 °C is "generally suggested" as the core temperature needed for heat adaptations, "the existence of a threshold for cardiovascular adaptations remains unknown." Nobody should reproduce a probe-monitored protocol in their own bathroom; the numbers show what the research tested, not a target to hit.
The risks are spelled out in a 2026 review of the whole heat-therapy field in Comprehensive Physiology, written by the Oregon group and colleagues: overheating, dehydration, low blood pressure and orthostatic intolerance, meaning the dizziness that comes with standing up, and burns from hot surfaces. The main causes of injury during sauna bathing were falls and fainting, and slips and falls were the largest category of hot-tub and spa injuries in the United States between 1990 and 2007. Alcohol, they write, "greatly increases the risk" and is a contributing factor in the 20 to 25 sauna-related deaths reported in Finland each year. Early pregnancy calls for particular care with frequency, temperature and duration, and anyone on blood-pressure medication or with a heart condition belongs in a conversation with their doctor first. The practical version: drink water, skip the wine, stand up slowly, and keep the first few baths shorter and cooler than the trials.
There is also a seasonal footnote. A 2026 study in Physiological Reports tested the same ten healthy young adults in summer and again in winter: the vessels in their skin dilated less in response to local warming in the cold months, with the nitric-oxide part of that response falling from about 62 to 38 percent. Our article on cold hands and feet covers why those skin vessels close down in the first place. A warm bath in November is working against that, which may be why it feels like it does more than it did in July.
Where Great Billiom Nattokinase 10,800 FU fits
A hot bath is an event: it moves heart rate and blood pressure for an hour and a bit, and the longer-term vessel changes are still being worked out. Nattokinase belongs to a different part of the picture, the daily one. It is an enzyme from natto, the fermented soybean dish, and its research concerns healthy blood flow and circulation over months, not what happens in a tub. No nattokinase trial has measured anyone's response to heat, hot baths or saunas, so this article makes no claim that the enzyme adds to or changes what a bath does. What Great Billiom builds the formula for is what the product page says: daily support for your body's natural ability to maintain healthy blood flow, healthy blood viscosity and daily circulation, for men and women over 40 who want to stay active and make heart health a daily priority alongside regular movement, good sleep and a balanced diet.
Each two-capsule serving delivers 10,800 FU of nattokinase. That matches the higher dose arm of the largest nattokinase study to date, 1,062 adults followed for 12 months in Frontiers in Cardiovascular Medicine, whose authors state plainly that it "represents a retrospective study and not a randomized control trial." The serving also carries 180 mcg of vitamin K2 (MK-7) and 240,000 SPU of serrapeptase in delayed-release vegetarian capsules: 90 capsules, 45 servings, a 45-day supply. The label directs two capsules daily on an empty stomach, at least one hour before or two hours after a meal, and many people take theirs before bed. Nattokinase contains soy. Anyone taking an anticoagulant, antiplatelet or blood-pressure medication, anyone pregnant or nursing, and anyone with surgery scheduled should talk to their doctor before starting it.
Key takeaways
A single hot bath changes circulation while you are in it: across 20 healthy-adult studies pooled in 2026, heart rate was about 28 beats a minute higher and diastolic pressure about 5 mmHg lower by the end of a session (Sotomaior 2026).
The blood-pressure effect lasts about 20 minutes after getting out; 24-hour pressure was unchanged, with only a slightly deeper overnight dip in diastolic pressure after a morning bath (Leaney 2026).
After repeated sessions, resting heart rate was about 3 beats a minute lower; blood pressure, flow-mediated dilation and arterial stiffness were not significantly changed when pooled, and the review calls the vessel evidence inconclusive (Sotomaior 2026).
The "as good as exercise" headline comes from one 8-week trial of 20 young sedentary adults that held core temperature at 38.5 °C for an hour per session, four to five times a week (Brunt 2016); the only study to measure fitness found no difference from control.
The researchers' own risk list is falls, fainting on standing, overheating, dehydration and alcohol; early pregnancy and blood-pressure medication call for a doctor's advice first (Richey 2026).
Enjoy the bath for what the evidence says it is: a warm hour that opens the skin's vessels, raises your heart rate and settles your blood pressure for a short while, with the long-term story still being written. For the daily side of healthy blood flow and circulation, Great Billiom Nattokinase 10,800 FU gives you the research-matched dose with K2 MK-7 and serrapeptase in one two-capsule serving, taken as the label directs.
Frequently asked questions
Do hot baths improve circulation?
During the bath, yes. In a 2026 review pooling 20 hot-water immersion studies in healthy adults, heart rate was about 28 beats a minute higher and diastolic blood pressure about 5 mmHg lower by the end of a session. Whether weeks of baths improve the vessels themselves was inconclusive: only two or three studies measured each vessel outcome, and they disagreed.
How long does a hot bath affect blood pressure?
About 20 minutes. In a 2026 randomized crossover study of 23 healthy adults, 60 minutes in 40.4 °C water lowered diastolic and mean arterial pressure during the bath and for up to 20 minutes afterward; 24-hour, daytime and nighttime pressure matched the control day. The one lasting trace was a deeper overnight dip in diastolic pressure, 19.1 versus 15.4 percent.
Is a hot bath as good as exercise?
That claim rests on one 2016 trial of 20 young sedentary adults, in which eight weeks of hot baths improved flow-mediated dilation from 5.6 to 10.9 percent, a change the authors said was on par with exercise training. When the 2026 review pooled every healthy-adult study, the vessel effects of repeated baths were inconclusive, and the only fitness measurement found no difference from control.
How hot and how long were the baths in the studies?
Water ranged from 37.5 to 45 °C and sessions from 5 to 120 minutes, most lasting 45 to 60 minutes. The most-cited protocol used 40.5 °C water for about 90 minutes to hold core temperature at 38.5 °C, with a probe in place, four to five times a week for eight weeks. Those are supervised lab conditions, not instructions for home.
Does nattokinase do what a hot bath does?
No, and no nattokinase trial has measured anyone's response to heat, so this article makes no such claim. A hot bath changes heart rate and blood pressure for an hour or so. Great Billiom Nattokinase 10,800 FU is daily support for healthy blood flow and circulation, taken as the label directs: two capsules a day on an empty stomach.
Megan Elliott, Content Editor at Great Billiom. Every claim in this article is sourced from the primary research linked above. Read our editorial standards → thegreatbilliom.com/editorial-standards
Sources
Sotomaior BB, Stewart IB, Rodrigues P, Osiecki R, Minett GM. "The effects of hot-water immersion on cardiovascular and cardiorespiratory health of healthy adults: A systematic review and meta-analysis." Physiological Reports, 2026;14(2):e70668.
Brunt VE, Howard MJ, Francisco MA, Ely BR, Minson CT. "Passive heat therapy improves endothelial function, arterial stiffness and blood pressure in sedentary humans." The Journal of Physiology, 2016;594(18):5329–5342.
Leaney SF, Ponce FAF, Owen TF, et al. "Acute hot-water immersion augments the diastolic blood pressure nocturnal dip in healthy adults." Experimental Physiology, 2026.
Laukkanen T, Khan H, Zaccardi F, Laukkanen JA. "Association between sauna bathing and fatal cardiovascular and all-cause mortality events." JAMA Internal Medicine, 2015;175(4):542–548.
Laukkanen T, Kunutsor SK, Zaccardi F, et al. "Acute effects of sauna bathing on cardiovascular function." Journal of Human Hypertension, 2018;32(2):129–138.
Richey RE, Hyldahl RD, Kaiser BW, Geiger PC, Halliwill JR, Minson CT. "Heat Therapy: Targeting Health, Disease, and Disability." Comprehensive Physiology, 2026;16(1):e70089.
Bangle JF, Jennings WE, Guadagni AJ, Albino GR, Grundstein AJ, Wolf ST. "Cutaneous microvascular function is reduced in winter compared with summer in healthy young adults." Physiological Reports, 2026;14(4):e70784.
Chen H, et al. "Effective management of atherosclerosis progress and hyperlipidemia with nattokinase: A clinical study with 1,062 participants." Frontiers in Cardiovascular Medicine, 2022;9:964977.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Individual results may vary.
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