How Cold and Heat Exposure Activate Brown Fat to Improve Metabolism: Dr. Søberg's Research Explained

Core Logic: Cold-Heat Contrast Matters More Than Just Freezing

On the Huberman Lab podcast, Dr. Susanna Søberg, a researcher at the University of Copenhagen, systematically laid out the mechanisms by which cold and heat exposure affect metabolic health. She repeatedly emphasized a point that is easily misunderstood: what matters is not the "cold" itself, but the change in temperature.

When the skin senses cold, cold receptors rapidly transmit signals to the hypothalamic thermoregulatory center and simultaneously activate brown fat through a direct pathway—this heat-generating adipose tissue distributed above the collarbone is the body's "first responder" to temperature fluctuations. Muscle shivering is the second heat-generating mechanism. Using infrared thermography in her own research, Søberg observed that just 4 minutes of immersion in 4°C cold water caused the temperature of brown fat in the collarbone region to rise markedly, showing that this response is extremely rapid.

She offers an intuitive intensity benchmark: the gasping "cold shock" response upon entering cold water indicates the stimulus is sufficient; as you adapt, this response gradually diminishes. She states plainly: "If you enjoy cold water immersion, there might be a problem." This doesn't mean enduring extreme cold—water at around 12°C can also effectively activate brown fat.

Brown Adipose Tissue (BAT) is a special type of adipose tissue in mammals. Unlike white fat, which stores energy, its cells are rich in mitochondria and use uncoupling protein 1 (UCP1) on the inner mitochondrial membrane to short-circuit the proton gradient, releasing energy as heat rather than generating ATP—a process called non-shivering thermogenesis. Human brown fat is mainly distributed in the neck and supraclavicular regions and is regulated by the sympathetic nervous system and cold stimuli. Techniques such as PET-CT and infrared thermography can assess its activity in vivo. The level of brown fat activity is closely related to energy expenditure, insulin sensitivity, and glucose metabolism, making it a potential target for metabolic health.

"After Drop" and Shivering: The Misunderstood Heat-Generating Components

Søberg explained in detail the "after drop" phenomenon following cold water immersion: upon entry, blood vessels constrict to protect core organs; after exiting, blood vessels dilate again, and cooler blood returns to the core, causing core body temperature to drop further and triggering shivering. Shivering is not a side effect to be feared—it further activates brown fat through signals such as succinate, helping the body rewarm.

This is precisely the physiological basis for her "Søberg Principle": end with cold. By ending with cold exposure, the body is forced to continue generating heat on its own for hours afterward, keeping brown fat and muscle cells active, with mitochondrial quantity and efficiency increasing accordingly. In her research, she found that winter swimmers had higher skin temperatures at rest than controls, indicating that their heat-generating capacity had been "trained."

11 Minutes of Cold Water Plus 57 Minutes of Sauna per Week: An Effective Low-Threshold Dose

Søberg's doctoral research recruited subjects with two to three winters of swimming experience. The protocol was 2 to 3 times per week, with 1 to 2 minutes of cold water immersion each time (3 rounds total), 10 to 15 minutes in an 80°C sauna, and always ending with cold water. Converted, this amounts to approximately 11 minutes of total cold water time per week and about 57 minutes of sauna.

The results pointed to clear metabolic improvements: winter swimmers secreted less fasting insulin and cleared blood glucose faster after oral glucose—that is, achieving better blood glucose control with lower insulin. In addition, systolic blood pressure and heart rate decreased, and inflammatory markers such as IL-6 showed changes, linked with IL-10.

Søberg specifically warns about the upper dose limit: excessive exposure can deplete cellular stress reserves and may actually increase cardiovascular risk. This is consistent with the logic of exercise training—sufficient stimulus is enough, and recovery is equally important.

Hormesis refers to the dose-response phenomenon in which low-dose stressors produce beneficial adaptations in organisms while high doses are harmful. In areas such as exercise, caloric restriction, and cold-heat exposure, moderate cellular stress can induce the expression of protective proteins (such as heat shock proteins and antioxidant enzymes), enhancing cellular resilience to subsequent similar stimuli. Søberg views cold-heat exposure as a form of hormesis: the stimulus must reach a certain intensity to trigger adaptation, but excess may deplete cellular stress reserves, weakening benefits or even increasing risk. This framework also explains why there is an inverted U-shaped optimal range for cold-heat exposure dosing.

Cold Exposure During Sleep: A Controlled Experiment in a 19°C Room

The 2017 study by Hansen et al. cited in the material provides an easier-to-implement protocol. Subjects first slept in a 24°C room and underwent PET-CT to measure baseline brown fat, then slept in a 19°C room for one month (with blankets and clothing), during which brown fat activity and insulin sensitivity both increased; returning to 24°C for one month, the metrics fell back; in the fourth month, raised to 27°C, brown fat activation and insulin sensitivity declined further.

This result shows that even if only parts such as the face are exposed to cooler air, it is enough to produce systemic metabolic effects. Søberg thus points out multiple low-threshold pathways: walking in cold wind in short sleeves, using a cooling vest, or even just immersing your hands in cold water can all trigger brown fat activation.

From Metabolism to Inflammation: The Long-Term Significance of Cold-Heat Exposure

Søberg positions cold-heat exposure as "hormesis"—a healthy form of cellular stress. The induction of heat shock proteins and cold shock proteins makes cells more robust the next time they face similar stimuli, which is structurally similar to the effect of strength training on muscle cells. She believes that modern people, who remain in temperature-neutral environments for long periods and sit indoors, have precisely lost this type of natural stressor, and the rise in obesity rates highly coincides in time with this lifestyle shift.

On a longer timescale, the reduction in inflammation is seen as a key mediator: improvements in blood pressure, insulin sensitivity, and glucose balance may collectively point to a reduced risk of metabolic diseases such as type 2 diabetes; Søberg also mentions the associations in the literature between inflammation and depression, anxiety, and Alzheimer's disease, but makes clear that this is inferential extrapolation rather than a conclusion directly confirmed by her research.

She also candidly points out research gaps: there is currently insufficient evidence to quantify the specific magnitude of brown fat activation across different cold exposure protocols, and the optimal combination of water temperature, duration, and frequency still awaits more controlled studies. For ordinary people, the actionable conclusion is simple and clear—no need to pursue extremes; maintain repeated temperature changes and end with cold.

One-line Summary

Cold-heat alternation activates brown fat; a small amount of cold water plus sauna each week can improve metabolism.

Paragraph Summary

Dr. Susanna Søberg of the University of Copenhagen points out that the key to the metabolic benefits of cold exposure is not absolute low temperature but repeated temperature changes. Cold stimuli rapidly activate brown fat above the collarbone through the hypothalamus and direct pathways, and shivering as a second heat-generating mechanism is also beneficial. She proposes the "end with cold" principle, keeping the body generating heat for hours afterward. Her research shows that winter swimmers can improve insulin sensitivity, blood pressure, and inflammatory markers with about 11 minutes of cold water plus 57 minutes of sauna per week. A study of sleeping in a 19°C room further proves that low-threshold cold exposure is effective. However, excessive exposure depletes cellular stress reserves, so there is an upper dose limit.