Affiliation: Founder, The Soeberg Institute. Researcher in metabolic health, brown adipose tissue, and thermal exposure. Ph.D., University of Copenhagen.
Susanna Søberg’s research is the reason Sisu’s contrast therapy protocol works the way it does. Her doctoral and postdoctoral work at the University of Copenhagen and Rigshospitalet established the foundational dose-response data for cold and heat exposure as metabolic interventions. The protocol most Sisu members follow at the end of a cardiovascular training session, sauna first, cold plunge after, ending on cold and allowing the body to rewarm without external heat, traces directly to her published findings.
Key contribution
Søberg’s 2021 study in Cell Reports Medicine measured altered brown adipose tissue thermoregulation and enhanced cold-induced thermogenesis in winter swimmers. Her cohort demonstrated that repeated cold exposure recruits and activates brown fat, the metabolically active fat tissue that burns glucose and fatty acids to generate heat. The activation correlates with improved insulin sensitivity, lower fasting glucose, and improved metabolic health markers across the full population.
The dose-response work that followed established the practical numbers Sisu uses to educate members. Approximately 11 minutes of cold exposure per week, distributed across 2 to 3 short sessions of 1 to 2 minutes each, paired with approximately 57 minutes of sauna per week across 2 to 3 sessions. Repeated short exposures preserve the hormetic stress response. Long single sessions adapt the body to the discomfort and lose the signal.
How Sisu applies this work
Three elements of Sisu’s contrast therapy protocol are drawn directly from Søberg’s research.
- End on cold. The body’s effort to rewarm is the metabolic benefit. Toweling off and dressing is fine. A hot shower or returning to the sauna defeats the purpose.
- Short, repeated exposures. Sisu coaches the 1 to 2 minute cold dose as the standard, not the 5-plus-minute endurance test. The hormetic stimulus comes from the first minutes.
- Weekly accumulation targets. Members are educated toward the 11 and 57 minute weekly targets, not pressured into single-session heroics.
The full protocol breakdown is in our Contrast Therapy guide, with the deeper science in our Contrast Therapy white paper.
Where to learn more
- Primary site: The Soeberg Institute
- Book: Winter Swimming: The Nordic Way Toward a Healthier and Happier Life (2022)
- Key paper: Søberg, S., et al. (2021). Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men. Cell Reports Medicine, 2(10), 100408. DOI: 10.1016/j.xcrm.2021.100408
Podcast appearance: The Huberman Lab Podcast with Dr. Susanna Søberg on the science of cold and heat exposure
