Hot Water Immersion: Better Than Cardio?

Twenty-two healthy, recreationally active adults completed two of three training "interventions" (12 training sessions over 14-16 days, with a 4-week "washout" period in-between). The interventions were as follows:

• 60 minutes of steady-state cycling at 70% of maximum heart rate (HRmax).
• 30 minutes of steady-state cycling at 70% HRmax followed by 30 minutes of warm water (about 102 degrees Fahrenheit – the temperature of the average hot tub) immersion.
• 30 minutes of steady-state cycling at 70% HRmax followed by 30 minutes of cold water (about 52 degrees Fahrenheit) immersion.

In each intervention employing water, participants sat with their legs fully extended, immersed to their belly buttons.

Before and after each training intervention, subjects were required to complete a 20-minute bout of cycling where the researchers measured their V02 max and glucose tolerance (where indirect calorimetry was used to measure how well they burned up carbs).

The exercise alone (no water immersion) improved V02 max better than 30 minutes of exercise followed by cold water immersion, but there were no significant differences between exercise alone and when an additional 30 minutes of exercise was substituted with 30 minutes of WARM water immersion.

Lest you think this is some standalone, unicorn experiment that no sober researchers will ever be able to duplicate, there's a decent amount of already-completed related research that gives credence to the current study. At least two other studies (Gupte et al., 2009 and Sjoberg et al.) found that warm water immersion alone improved fasting glucose and glucose uptake, while a couple of others (Hoekstra, et al., 2018 and Green, et al., 2010) found that being a human teabag improved vascular function and structure.

The scientists involved in the cycling study summed it up this way:

"Collectively, these studies indicate that warm water immersion might be used as a viable substitute in achieving some of the benefits of exercise training (e.g., reduced risk of diabetes, improved heart health, improved cardiovascular fitness), particularly for those who cannot or will not exercise sufficiently."

There's also evidence, at least a little, that combining exercise and warm water immersion might also have some benefits to muscle and bone. A previous experiment (Tamura, et al., 2014) used such a protocol and found additive increases in both metabolic enzyme and protein adaptations in skeletal muscles, albeit those of mice.

It comes down to this: A lot of key adaptations to exercise are simulated by elevating muscle and body temperature. This increase in temp results in increased blood flow and volume, increased metabolic rate, and even the birth of new mitochondria.

So, will cutting your workout short so you can jump into a warm bath substitute for all the benefits of exercise? Unfortunately, no. While the combo of exercise and warm water has some eye and pore-opening benefits, it's highly unlikely that it will improve bone density, calcium handling, neuromuscular junction maintenance, or building 19-inch biceps.

Neither will it prepare you for some hardcore competitive event better than straight training. Regardless, the results are compelling for people who don't like to, or are unable to, exercise for long(er) periods of time.

It does make you wonder, though, what the effects of sitting totally immersed (not just up to the belly button, but up to the neck) in, say, a hot tub would be, either alone or with naked women.

1. Brooke M. Russell, et al., "Post-exercise Warm or Cold Water Immersion to Augment the Cardiometabolic Benefits of Exercise Training: A Proof of Concept Trial," Frontiers in Physiology, 03 November 2021.