How to Use Cold & Heat Exposure to Improve Your Health | Dr. Susanna Søberg
Dr. Susanna Søberg discusses her landmark research on deliberate cold and heat exposure, explaining how these practices activate brown fat thermogenesis, improve insulin sensitivity, and lower blood pressure. She outlines minimum thresholds for health benefits—11 minutes of cold and 57 minutes of sauna per week—and introduces the 'Søberg Principle' of ending sessions on cold to maximize metabolic adaptation.
Summary
Dr. Susanna Søberg, who completed her doctoral thesis at the University of Copenhagen, joins Andrew Huberman to discuss the science behind deliberate cold and heat exposure. The conversation begins with the physiological response to cold immersion: activation of the sympathetic nervous system, release of catecholamines (dopamine, norepinephrine, epinephrine), and the 'cold shock' response characterized by hyperventilation and gasping. Søberg explains that the discomfort is intentional and necessary, drawing parallels to exercise-induced stress that ultimately leads to beneficial adaptations.
A central topic is brown adipose tissue (brown fat), which Søberg describes as the body's primary thermogenic organ and 'first responder' to cold. Brown fat is activated via three parallel pathways: cold receptors in the skin signaling the hypothalamus, direct skin-to-brown-fat neural connections, and muscle shivering releasing succinate. Unlike white fat (energy storage), brown fat is rich in mitochondria and burns glucose and fatty acids to generate heat. Søberg notes that brown fat is plastic—it can grow with cold exposure and shrink with sedentary, temperature-neutral living—and that decreased brown fat is associated with obesity and aging.
Søberg describes her landmark study published in Cell Reports Medicine, which observed winter swimmers in Denmark who performed alternating cold water immersion (1-2 minutes) and sauna sessions (10-15 minutes) two to three times per week over a winter season. Key findings included improved insulin sensitivity, faster blood glucose clearance despite lower insulin production, and lower blood pressure. The study calculated minimum effective thresholds: approximately 11 minutes of cold exposure and 57 minutes of sauna per week, divided across multiple sessions.
The 'Søberg Principle'—ending sessions on cold rather than heat—is explained as a mechanism to force the body to self-regulate temperature post-exposure, extending thermogenic activation of brown fat and muscles for hours afterward and sustaining elevated catecholamine levels. Søberg also explains the 'after drop' phenomenon, where core body temperature continues to fall after exiting cold water due to cold peripheral blood returning to the core, which triggers shivering and further thermogenesis.
The discussion covers practical nuances including: cold showers vs. immersion (immersion is more potent but showers still activate brown fat); the importance of not over-adapting through excessively long sessions (beyond ~30 minutes in sauna or extended cold immersion can exhaust cells and counteract benefits); sleeping in cooler rooms (~19°C) as a low-barrier method to activate brown fat; and safety considerations for children and smaller-bodied individuals who lose core heat faster. Gender differences are noted—women have more brown fat than men and a higher thermoneutral comfort temperature (24°C vs. 22°C for men)—though Søberg's published study focused only on men, with a forthcoming study including both sexes. The conversation also touches on inflammation reduction, potential mental health benefits (serotonin, dopamine elevation), and the relationship between cold exposure and improved sleep quality.
Key Insights
- Søberg's study found that the minimum effective dose for metabolic benefits is approximately 11 minutes of cold exposure and 57 minutes of sauna per week, divided across 2-3 sessions—demonstrating that very short exposures are sufficient and that exceeding ~30 minutes per sauna session yields no additional cardiovascular benefit and may be counterproductive.
- Søberg argues that ending deliberate cold or heat sessions on cold—the 'Søberg Principle'—forces the body to self-generate heat via brown fat and muscle thermogenesis for hours afterward, extending metabolic and neurotransmitter benefits well beyond the session itself rather than terminating them with a warm shower.
- Søberg explains that brown fat activation occurs via three parallel physiological pathways: cold skin receptors signaling the hypothalamus, a direct skin-to-brown-fat neural pathway, and a muscle shivering-to-brown-fat pathway—indicating that brown fat's temperature regulation role was evolutionarily critical enough to warrant redundant mechanisms.
- Søberg describes an outlier subject in her study who had zero detectable brown fat, and observed that this individual shivered earlier and more vigorously than adapted winter swimmers, had less stable thermal comfort ratings, and showed insulin and glucose clearance levels more similar to the sedentary control group—providing rare human evidence linking brown fat absence to impaired metabolic regulation.
- Søberg cites research showing that submerging the head in cold water increases core heat loss rate by 36% compared to neck-level immersion alone, and notes that cold water immersion up to the neck at near-freezing temperatures reduces cerebral blood flow by 30-40%, providing physiological rationale for keeping the head out of cold water during deliberate exposure.
Topics
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