Four figures, each tied to its study.
What the evidence does not say.
The research is genuinely promising. It's also widely overstated. These are the lines we won't cross, because the data doesn't support them.
Cold doesn't burn fat
Cold recruits brown fat and raises heat production, but no study here shows weight or fat loss. Anyone selling plunges as a fat-loss tool is ahead of the evidence.
Cold doesn't "boost immunity"
The shower trial found fewer sick days called in — but the same number of actual illness days, and no immune markers measured. That's resilience, not a stronger immune system.
Cold isn't a depression treatment
The popular reference is an untested hypothesis. Trials show, at most, a brief dip in stress — and no improvement in mood. We won't claim more.
Sauna isn't a workout
The heart rate looks like exercise, but it's passive heat. It won't build muscle or fitness. It complements training; it doesn't replace it.
Sauna studies are associations
The headline numbers come largely from one Finnish cohort of middle-aged men. Strong and dose-dependent — but observational, so they show a link, not proof of cause.
These numbers are for traditional dry heat
The sauna research used hot Finnish dry saunas (~80–100°C). It doesn't automatically transfer to infrared cabins, and the cold figures are for cold-water immersion specifically.
Not all evidence is equal.
So we grade every claim above with one of four labels — and we never let a weaker tier borrow the confidence of a stronger one. A hormonal spike in a lab is not a health outcome; an association in a cohort is not proof of cause.
Randomised trials and systematic reviews. The strongest tier — though even here, sample sizes can be small.
Large cohorts followed over time. Shows strong associations, but cannot rule out confounding or prove cause.
Small physiology studies showing how something works in the body. Useful, but not an outcome you would feel.
A proposed idea with little or no trial backing. Interesting, but not evidence — and labelled as such.