You have fat tissue whose only job is to burn other fat, and cold activates it

Brown adipose tissue (BAT) is a thermogenic fat depot named for its colour, which comes from its exceptional mitochondrial density. While white fat stores energy, brown fat burns it using UCP1 to release energy as heat. Until 2009, adults were assumed to have negligible brown fat. PET-CT imaging then revealed that virtually all adults retain active brown fat deposits — and that cold exposure dramatically increases their metabolic activity.

How to Activate Brown Fat

• Cold water immersion (11–15°C) for 2–11 minutes, 3–4 times per week
• Cold showers — the final 1–2 minutes cold provides a meaningful stimulus
• Sleeping in cooler rooms (17–19°C) chronically increases BAT activity
• Fasted cold exposure produces larger thermogenic responses than fed state