Water wins. That's the uncomfortable finding buried inside this American Journal of Physiology study, and it matters if you care about which heat modality is actually doing what you think it's doing. Twenty participants cycled through all three — hot water immersion at 40.5 degrees Celsius for 45 minutes, traditional sauna at 80 degrees for 10 minutes, infrared at 65 degrees — and when the data came back, only the hot tub reliably pushed core body temperature above the critical 38-degree threshold. Traditional sauna got close. Infrared didn't.
The mechanism is physics, not magic. Water conducts heat roughly 25 times more efficiently than air. When you're submerged, your entire body surface area is in direct contact with the thermal medium. A sauna heats the air around you and lets your body absorb it. A hot tub heats you. That difference shows up in the cytokine data, the cardiovascular strain markers, and the immune activation cascade.
Here's where it gets interesting. Most of the landmark longevity research — the Finnish epidemiological data Rhonda Patrick has spent years translating for a general audience, the 40-year Laukkanen studies showing reduced cardiovascular mortality — was conducted with traditional saunas. And those studies are extraordinary. Four to seven sessions per week at 174 to 200 degrees Fahrenheit, and all-cause mortality drops significantly. So the traditional sauna is clearly doing something profound. But the proposed mechanism in much of that research involves cardiovascular adaptation: elevated heart rate, plasma volume expansion, vascular flexibility. Core temperature elevation is a part of it, but not necessarily the whole story.
What this study isolates is specifically the thermal dose — how efficiently each modality delivers heat to the body's core. On that single metric, hot water immersion is in a different category.
There's broad consensus that passive heating works. The disagreement is about mechanism, dose, and whether the Finnish sauna data translates to other modalities at all. Some researchers argue the benefits in those long-term studies are partly attributable to the social ritual of regular sauna — stress reduction, community, behavioral consistency. Others focus purely on the thermal physiology. This study can't settle that debate. What it does is clarify that if your specific goal is raising core body temperature — for interferon production, heat shock protein activation, or immune priming — the modality you choose matters enormously.
If you have access to a hot tub and you're using it for wellness rather than relaxation, treat it as a serious protocol. Forty-five minutes at around 104 degrees Fahrenheit is a meaningful thermal stressor. Get out, cool down deliberately, hydrate. Don't combine it with alcohol. And if you're building a contrast therapy session, consider that a hot tub followed by cold immersion may deliver more physiological stimulus than a traditional sauna followed by cold — the thermal delta is larger and the starting core temperature is higher.
The 38-degree threshold the study keeps referencing isn't arbitrary. It's the temperature at which interferon production begins to spike significantly — your body's frontline antiviral signaling protein. MedCram covered this in an earlier video on hydrotherapy and COVID, and it's one of the more quietly remarkable pieces of immunology in the heat therapy literature. Most people think of saunas and hot tubs as recovery tools. They are. But they're also, when dosed correctly, immune training. The hot tub gets you there. The infrared sauna, in this study at least, did not. That doesn't make infrared worthless — it has its own profile of benefits, particularly for muscle recovery and localized tissue effects — but if you're chasing the immune cascade, you need to hit the threshold. Know your tool. Know what it's actually doing inside your body.