Essentials: Cold & Heat Exposure for Metabolic Health | Dr. Susanna Søberg
Dr. Susanna Søberg, a University of Copenhagen-trained metabolic scientist, explores how cold and heat exposure affect the body. They discuss cold shock, shivering, brown fat, thermogenesis, insulin sensitivity, winter swimming, sauna routines, inflammation, and practical ways to combine temperature stressors for metabolic health and resilience.
Top snips
Cold Shock Is the Point
0:47 – 2:10
Deliberate cold should feel uncomfortably cold — not comfortable, and not necessarily extreme.
Your first response is the cold shock:
- Faster breathing and possible hyperventilation
- Sympathetic nervous system activation
- A surge of catecholamines, including norepinephrine
With repeated exposure, the gasping response can subside. Adaptation turns an overwhelming shock into a trainable stress.
How Cold Activates Brown Fat
2:35 – 6:06
Cold water covers more of the body than cold air, so it activates skin cold receptors more rapidly and intensely.
Those receptors signal the hypothalamus, increasing norepinephrine and activating brown fat — a heat-producing tissue that raises metabolism.
Muscles join in later through shivering. Together, brown fat and muscle thermogenesis help defend your core temperature.
Don’t Fear the Shiver
7:53 – 10:51
After cold-water immersion, blood vessels reopen as you warm up. Cold blood from the skin can then return to the core, creating an “after-drop” in core temperature.
That may trigger shivering — and shivering is not automatically a problem. It activates muscles and brown fat, creating a form of healthy cellular stress, or hormesis.
Cold Training Changes Metabolism
11:02 – 13:13
Repeated exposure gradually builds cold adaptation.
Brown fat develops more mitochondria and becomes more efficient at producing heat. Skin capillaries also improve their ability to constrict, while the stress response becomes less intense.
The metabolic payoff may include higher insulin sensitivity, better glucose clearance, lower blood pressure, and a lower heart rate over time.
A Practical Cold + Sauna Protocol
20:49 – 24:37
Susanna Søberg studied experienced winter swimmers using short, repeated exposures:
- Cold water: 1–2 minutes per dip
- Sauna: 10–15 minutes at about 80°C
- Frequency: 2–3 times per week
The sessions produced improved insulin sensitivity and faster glucose clearance. The goal is not endurance or extremes — it is repeated temperature change.
The Minimum Effective Dose
26:39 – 28:30
You do not need to spend half an hour in icy water.
The study’s approximate weekly targets were:
- 11 total minutes in cold water
- 57 total minutes in the sauna
Short sessions can be enough to create hormetic stress. Overdoing it may exhaust the cells and increase cardiovascular risk, so more exposure is not automatically better.
The Søberg Principle: End Cold
33:02 – 34:17
Ending a hot-and-cold session with cold forces your body to reheat itself afterward.
Brown fat and muscles remain active, producing heat for hours and continuing the metabolic challenge. Repeatedly moving between warm and cold may matter more than chasing one perfect temperature.
You can even split exposure into short rounds — cold, recover, then cold again.