Metabolism Review

Thermogenesis

Brown fat and cold exposure

Brown fat is genuinely fascinating physiology and a genuinely small contributor to how much energy you burn.

Updated 2 min read 9 citations Evidence strength 3/5

What brown fat does

Brown adipose tissue is packed with mitochondria expressing UCP1, a protein that uncouples the respiratory chain from ATP production. The energy that would have been captured chemically is released as heat instead. It is a deliberate inefficiency, and its evolutionary purpose is keeping small mammals and human infants warm.

Adults were long thought to lose it. Imaging showed otherwise, and a meta-analysis has since quantified prevalence and the factors associated with having more of it [3]. A translational review covers the broader picture [7].

What activates it

Cold, principally. A systematic review of habitual cold exposure found it increases brown adipose tissue activity in humans [1]. Exercise effects on UCP1 expression have been meta-analysed [5], with less impressive results than the "browning" enthusiasm suggests. Dietary interventions have been systematically reviewed [4] and the effects are small.

One finding deserves emphasis because it undermines a lot of protocol advice: cold-induced thermogenesis varies across the day [6]. An ice bath at 6am and one at 6pm are not the same stimulus, which means most published protocols are not controlling a variable that matters.

Cold exposure activates brown fat

The arithmetic problem

Here is why this does not become a weight-loss intervention. Even generous estimates of maximally activated adult brown fat put its contribution in the low tens of kilocalories per day — on the order of a biscuit. Achieving that requires sustained cold discomfort, and shivering thermogenesis in ordinary muscle contributes more than brown fat does at the temperatures involved.

Measurement caveats

Most brown fat quantification uses PET-CT with a radioactive glucose tracer, which measures glucose uptake rather than heat production and involves radiation exposure that limits repeat measurement. Infrared thermography has been proposed as an alternative and systematically reviewed [2], with mixed conclusions about validity. This methodological difficulty is a large part of why the field moves slowly.

Common questions

Will cold showers help me lose weight?
The measured energetic contribution is far too small to matter for body weight.
Can I turn white fat into brown fat?
"Beiging" is real in rodents. Human evidence is much weaker, and the exercise meta-analysis on UCP1 is unimpressive [5].
Do supplements activate brown fat?
Dietary influences have been systematically reviewed [4] and effects are small and inconsistent.
Does it matter what time I do cold exposure?
Apparently yes — cold-induced thermogenesis has a diurnal rhythm [6].

References

Every citation below links to the original peer-reviewed record on PubMed or via DOI. Nothing here is a substitute for medical advice.

  1. Effect of habitual cold exposure on brown adipose tissue activity in Arctic adults: a systematic review Motzfeldt Jensen M, Jørgensen MG, Elberling Almasi C, et al. · International journal of circumpolar health · 2025 · Systematic review DOIPubMed 40804739Full text
  2. A systematic review on the role of infrared thermography in the Brown adipose tissue assessment Brasil S, Renck AC, de Meneck F, et al. · Reviews in endocrine & metabolic disorders · 2020 · Systematic review DOIPubMed 31965434
  3. Prevalence and Associated Factor of Brown Adipose Tissue: Systematic Review and Meta-Analysis Worku MG, Seretew WS, Angaw DA, et al. · BioMed research international · 2020 · Meta-analysis DOIPubMed 32685543Full text
  4. Effects of Nutrition/Diet on Brown Adipose Tissue in Humans: A Systematic Review and Meta-Analysis Heenan KA, Carrillo AE, Fulton JL, et al. · Nutrients · 2020 · Meta-analysis DOIPubMed 32927664Full text
  5. Exercise-induced effects on UCP1 expression in classical brown adipose tissue: a systematic review Flouris AD, Dinas PC, Valente A, et al. · Hormone molecular biology and clinical investigation · 2017 · Meta-analysis DOIPubMed 28085671
  6. Cold-Induced Thermogenesis Shows a Diurnal Variation That Unfolds Differently in Males and Females Straat ME, Martinez-Tellez B, Sardjoe Mishre A, et al. · The Journal of clinical endocrinology and metabolism · 2022 · Randomised controlled trial DOIPubMed 35176767Full text
  7. Brown Adipose Tissue-A Translational Perspective Carpentier AC, Blondin DP, Haman F, et al. · Endocrine reviews · 2023 · Review DOIPubMed 35640259Full text
  8. Brown adipose tissue: function and physiological significance Cannon B, Nedergaard J · Physiological reviews · 2004 · Review DOIPubMed 14715917
  9. Brown-fat-mediated tumour suppression by cold-altered global metabolism Seki T, Yang Y, Sun X, et al. · Nature · 2022 · Journal article DOIPubMed 35922508Full text