Hot Water Immersion May Improve Blood Vessels More Than Moderate Exercise

A soothing hot bath after a demanding day can be an excellent way to relax, yet its effects may extend well beyond easing tension.

Prolonged immersion in hot water can deliver some unexpectedly notable health benefits, and fresh research has identified another striking consequence of a lengthy soak.

A study published in the European Journal of Applied Physiology found that spending 30 minutes immersed in hot water can cause substantial changes in the blood vessels beneath the skin. In certain respects, these changes were greater than those generated by moderate exercise.

Hot water immersion also reduced participants' blood pressure more than 30 minutes of moderate-intensity cycling.

This was a small study, however, and the findings are not a reason to replace exercise with extra hot baths.

For people who cannot exercise, though, hot water immersion and other approaches known as passive heat therapy may offer a valuable means of obtaining some of the health benefits associated with physical activity.

Hot water immersion and vascular health

"Exercise training is a well-established stimulus for improving vascular health, in part by enhancing endothelial function, arterial compliance, and microvascular responsiveness," researchers explain in a new paper, led by first author and exercise physiologist Kristanti Wigati from the University of Western Australia.

"However, recent global estimates indicate that approximately 31 percent of adults worldwide do not meet physical activity guidelines, supporting the need to explore adjunct or alternative interventions for individuals who may struggle with regular exercise participation."

Like exercise, immersion in hot water increases blood flow and the shear stress exerted along the inner walls of blood vessels. This prompts adaptations that can enhance vascular function.

Earlier studies involving healthy young adults have indicated that repeated hot water immersion can improve blood-vessel function, lower arterial stiffness and help blood vessels dilate through nitric oxide.

"These findings suggest that hot water immersion may act as an 'exercise-like' vascular stimulus, but evidence in older individuals with risk factors for cardiovascular disease who are unable to engage in conventional exercise is lacking," the researchers note.

Comparing a hot bath with moderate cycling

To address this evidence gap, the team recruited 25 adults aged 45–70 who had overweight or obesity. Their average age was 59.

Each participant undertook two interventions during the experiment, assigned in a random sequence and separated by a minimum of 72 hours.

For one intervention, participants remained seated chest-deep in water heated to 40 °C (104 °F) for 30 minutes.

For the other, they completed 30 minutes of moderate-intensity exercise on a stationary bicycle. The cycling intensity equated to 60–70 percent of their maximum heart rate.

Throughout the tests, the researchers monitored participants' heart rate, mean arterial pressure and core body temperature. They also evaluated the tiny blood vessels in the skin using optical coherence tomography (OCT), a non-invasive imaging method.

Compared with the exercise session, hot water immersion produced significantly greater increases in blood-vessel diameter, blood flow, blood velocity and the density of microvessels with detectable blood flow.

This effect is visible in an OCT-derived image of participants' forearms, where the microvascular network shows noticeably greater blood-flow activity after hot water immersion.

"These pronounced microvascular responses were accompanied by a larger elevation in core body temperature and greater reductions in systolic and diastolic blood pressure following hot water immersion, whereas moderate-intensity exercise elicited a higher heart rate response," the researchers write.

"Collectively, these findings suggest that the larger microvascular responses observed after hot water immersion likely reflect the combined effects of direct local skin heating and greater systemic thermal strain imposed by passive heating."

Why passive heating affects blood vessels

According to the researchers, hot water immersion encourages blood vessels to widen, a process called vasodilation. This helps the body release heat by redirecting blood flow towards the skin.

Although the results indicate that hot water immersion may have potential as a complementary method for delivering a vascular stimulus, the researchers stress that significant uncertainties remain.

In particular, the two interventions were matched by duration, at 30 minutes each, but not by the degree of thermal strain placed on the body.

"Accordingly, the present findings should not be interpreted as indicating that hot water immersion is intrinsically superior to moderate-intensity exercise per se; rather, they reflect the greater thermal strain achieved during hot water immersion, which elicited substantial shear-mediated vascular stimuli in the absence of active muscle contraction," the authors explain.

"This may be particularly relevant for clinical populations who are unable to engage in regular exercise, suggesting that hot water immersion could serve as a complementary or alternative strategy to induce beneficial vascular stimuli."

The findings are reported in the European Journal of Applied Physiology.

This article was fact-checked and edited by Rebecca Dyer. Although we take pride in our process, we are only human. If you notice an error, please let us know.