Summary
Andrew Huberman interviews Dr. Susanna Soberg, the researcher behind the landmark Cell Reports Medicine study that established minimum thresholds for cold and heat exposure to boost metabolism. Soberg explains the cold shock response, the three parallel pathways by which cold activates brown fat (hypothalamus signaling, direct skin-to-fat neurons, and muscle shivering via succinate), and why cold water immersion is far more potent than cold air or cold showers due to the density of cold receptor activation. The conversation covers the biology of brown fat in depth, including its plasticity and ability to grow with regular cold exposure, how it declines with age but can be maintained through deliberate practice, and its critical role in insulin sensitivity and metabolic health. Soberg introduces her principle of ending on cold to force the body to reheat naturally, maximizing the "after drop" effect that drives shivering and brown fat thermogenesis. She draws parallels between cold exposure and exercise, framing deliberate cold as cellular training through hormesis that builds cold shock proteins and improves cardiovascular markers like blood pressure and heart rate over a winter swimming season.
Key Points
- Cold water immersion activates the sympathetic nervous system and catecholamines (dopamine, norepinephrine, epinephrine) far more potently than cold showers or cold air
- Three parallel pathways activate brown fat: hypothalamus signaling, direct cold receptor-to-fat neuron pathways, and muscle shivering releasing succinate
- The "after drop" phenomenon: core temperature continues to decrease after exiting cold water as blood vessels dilate and cooled blood returns to the core, triggering shiver
- Brown fat is plastic -- it can grow with repeated cold exposure and shrink without it, and tends to decline after age 40 alongside rising obesity rates
- Winter swimmers showed lower blood pressure, lower resting heart rate, and improved insulin sensitivity after one season of regular practice
- End on cold (the Soberg principle) to maximize metabolic benefits by forcing the body to reheat through its own thermogenesis
- Shivering should not be avoided -- it is cellular training through hormesis that builds cold shock proteins and increases insulin sensitivity
- Cold exposure parallels exercise: the acute stress response (elevated inflammation, blood pressure, catecholamines) drives long-term adaptation and improved baseline health markers
Key Moments
Three parallel pathways from cold to brown fat activation
Dr. Soberg explains the three pathways by which cold activates brown fat -- through the hypothalamus, direct cold receptor-to-fat neuron connections, and muscle shivering releasing succinate -- showing why the body has redundant systems for this evolutionarily critical function.
"the co-receptors will send a signal to our temperature regulating center in the brain, so hypothalamus. And that's going to be taking in this message. And we have so many co-receptors in the skin. So it's going to be very fast, as you can say, if you immerse the body into cold water, this is going to be so rapid."
The after drop and why shivering matters
Soberg explains the "after drop" phenomenon -- how core temperature continues to fall after exiting cold water as blood vessels dilate and cooled blood returns to the core -- and why shivering should be embraced as cellular training through hormesis that builds cold shock proteins and improves insulin sensitivity.
"the after drop is when your core temperature decreases even after you get out of the cold water. And it always does that, your body, because as soon as you get into the cold water, all your blood vessels is going to constrict because you need to keep your blood in your core and keep your vital organs warm"
Brown fat is plastic and can grow with cold exposure
Soberg describes how brown fat is highly plastic -- it can grow with repeated norepinephrine stimulation and shrink without it. Studies of pheochromocytoma patients proved this plasticity, and research shows brown fat declines after age 40 alongside rising obesity, but outdoor workers who expose themselves to cold maintain higher levels.
"So the brown fat is very plastic, so it means that it can grow and it can decrease. And this is proven in studies where we have seen people with a fair cryocytoma"
Long-term health benefits of winter swimming
Soberg describes how regular cold exposure creates lasting fitness adaptations -- more efficient brown fat, better capillary constriction, reduced catecholamine response -- and cites a study showing winter swimmers achieved lower blood pressure, lower heart rate, and improved insulin sensitivity after one season.
"So you will have more activation of your brown fat. The mitochondria in the brown fat cells are going to be, you'll have more of those and they will be more efficient at heating you up because it expects, the body expects you to do this again"
Cold shock as uncomfortable but essential -- the cold shock response
Soberg explains what happens physiologically during the cold shock response, how adaptation reduces hyperventilation over time, and why the discomfort is the point -- if you enjoy it, something is wrong, because the stimulus drives the beneficial adaptation.
"you should do hard things, right? It's not something that we, you shouldn't think about cold water and cold water immersion as something that is comfortable. It should be hard because that's the point of it, right? If you enjoy it, then yeah, then I'm thinking something is wrong"
