Dr. Susanna Søberg is making a precise argument here, and I want to make sure it lands correctly. This isn't just "cold is good for you." Her claim is more specific: cold exposure activates brown adipose tissue — brown fat — through a norepinephrine surge, and that activation has cascading metabolic consequences that extend well beyond the cold itself. The cold is the trigger. Brown fat is the lever. And the lever moves more than most people realize.
The 2.5-fold increase in norepinephrine Søberg references is consistent with what I see across the knowledge base. A 2013 paper on cryostimulation found a 76.2% increase in plasma norepinephrine following whole-body cryotherapy — and that's a different modality entirely, colder and shorter than a typical cold plunge. The sympathetic nervous system response is remarkably consistent across cold exposure types. What varies is the magnitude, the duration of effect, and critically, what you do with it.
The 2019 research on brown adipose tissue alterations goes deeper than Søberg has space to cover here. Cold doesn't just activate BAT — it induces dynamic, heterogeneous changes in its composition. BAT ramps up oxidative metabolism, increases glucose uptake, pulls in free fatty acids. This is your body burning stored energy to generate heat, and it does so in a way that's metabolically distinct from skeletal muscle thermogenesis. Brown fat is efficient in a way muscle isn't. It's a dedicated furnace.
There's broad consensus on the sympathetic activation piece. Cold hits skin receptors, norepinephrine surges, brown fat activates, heat is generated. That chain is well-established. The disagreement lives in the dose and the long-term effects. Søberg's own admission that "we cannot really take everyone under one man" — meaning one minute isn't a universal threshold — reflects something I see repeatedly in this knowledge base: individual variability is enormous, and we're still building the map. Uldis Udris, a practitioner with seven years of immersion experience, makes the same point from a practitioner's angle. The protocol is a starting point, not a prescription.
The gender differences in brown fat distribution are genuinely underresearched. Women appear to carry more brown fat than men — possibly related to muscle mass differences, possibly hormonal, possibly neither. Søberg is honest that there's no clean answer yet. That kind of intellectual humility from a researcher is worth noting.
Start with one to two minutes of cold exposure — cold shower, plunge, open water, whatever is accessible to you. Do it consistently, not heroically. The adaptation curve matters more than the single session. Your vascular response improves, your brown fat becomes more responsive, the initial shock diminishes. This is the point where people often stop, thinking they've "gotten used to it." That's actually when the metabolic benefits are beginning to compound.
Here's what strikes me most about Søberg's work in context: brown fat responds to cold with heterogeneous changes — meaning different regions of BAT activate differently, at different intensities. This mirrors something we see in sauna research with heat shock proteins. Thermal stress, whether hot or cold, doesn't produce uniform biological responses. It produces regional, cell-specific adaptations. Your body is not a single system reacting to temperature. It's thousands of systems, each calibrating independently. Cold therapy isn't a switch you flip. It's a conversation you start — and the body responds with extraordinary specificity if you give it the right conditions to do so.