A new study, published online in the journal Heart in March 2020, reports that with hot water, it seems to be a case of washing away health problems – literally. As a result, the more a person uses hot tubs, the better in terms of cardiovascular health. In other words, soaking in hot water daily improves heart health more than having a hot tub once or twice a day.
However, an editorial accompanying the research points out that experiencing sudden death during or after a hot tub bath is a relatively common phenomenon in Japan, which is where the research originates. About 19,000 people are estimated to die in association with hot baths, which is three times more than the estimated number of deaths due to car accidents, for instance.
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Tub bathing provides water pressure that increases stroke volume and cardiac output and reduces total peripheral vascular resistance. It is also associated with good sleep quality and self-rated health.
However, there is little agreement as to how this might impact the risk of developing cardiovascular disease, including heart attack, sudden cardiac death, and stroke.
Again, there is a possible association with sudden death in older people, due to accidental drowning, rapid alterations in body temperature leading to a heart attack, or a heatstroke in which the increased body temperature fails to be controlled by profuse sweating.
The current study was meant to fill in this gap. The researchers retrieved data on more than 61,000 middle-aged adults (45 to 59 years) who were part of Study Cohort 1 of The Japan Public Health Center, a population-based tracking study.
At the start of the study in 1990, about 43,000 participants completed a detailed questionnaire on their bathing habits and other factors that might potentially explain or affect cardiovascular health. These include lifestyle factors such as exercise, diet, alcohol consumption, weight (BMI); the average number of hours of sleep; a history of medical illnesses, and current medication use.
The researchers monitored each participant until death or the end of December 2009, whichever was earlier.
In the end, the researchers had data on about 31,000 people. In this group, there were almost 2,100 episodes of cardiovascular disease, ranging from 275 heart attacks, through 53 sudden cardiac deaths to almost 1770 strokes.
After the researchers adjusted for the presence of confounding factors, they found that a daily hot tub reduced the overall risk of cardiovascular events by 28%, and stroke by almost the same amount (26%). There was a 23% reduction in the incidence of infarcted stroke, and 46% for intracerebral bleeding.
On the other hand, there was no increase in the risk of sudden cardiac death, or with subarachnoid hemorrhage, which causes one type of stroke called a hemorrhagic stroke. This is associated with bleeding into space immediately around the brain.
Now we come to another big question: how much is hot enough when it comes to a hot tub? The researchers analyzed the water temperature preferred by the large group. The answer showed that warm water lowered the risk of cardiovascular disease by 26% compared to 35% lower for hot water. However, there were no broad areas of linkage between the overall risk for stroke and the water temperature.
The associations also became weaker, though still significant, when the investigators adjusted for already existing cardiovascular risk processes by excluding people who developed cardiovascular disease within 5 and 10 years of the start of the study, respectively.
The observational nature of the study makes it impossible to say that the hot tub bath caused a decrease in cardiovascular death. Also, the study doesn't measure changes in the number of hot baths taken over time, as lifestyle habits change. Moreover, in Japan, a hot tub involves being immersed up to the shoulders, which could explain a lot of the protective effects. Immersion to the level of the right atrium is found to be beneficial in earlier small studies.
However, according to earlier studies, there is an association between the exposure of the body to heat and the reduced risk of cardiovascular disease. It is thought that being exposed to heat acts somewhat like exercise in the way it promotes cardiovascular fitness. The authors say, "Heat exposure increases core body temperature, cardiac contractility, heart rate, and blood flow, and decreases vessel endothelial shear stress."
The scientists say, "We found that frequent tub bathing was significantly associated with a lower risk of hypertension, suggesting that a beneficial effect of tub bathing on risk of CVD may in part be due to a reduced risk of developing hypertension."
On the other hand, they point out the potential risk of taking a hot bath at too high a temperature. This danger is dealt with by Dr. Andrew Felix Burden in a linked editorial.
"There can be no doubt about the potential dangers of bathing in hot water, and the occurrence of death from this increases with age, as well as with the temperature of the water," he writes. The most common reasons for this risk do not include cardiovascular disease but may include overheating due to very hot water, which in turn can produce confusion and even drowning-related death.
Burden says, "Investigations into the potential cardiovascular benefit of heat-free immersion in warm to hot water are needed. In the meanwhile, caution is needed because of the higher mortality associated with such bathing in an unselected population."
- Burden AF. Tub bathing and heart disease. Heart Epub ahead of print [25 March 2020]. doi:10.1136/heartjnl-2019-316187