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 19 citations Evidence strength 3/5

Side view of a mechanical personal weighing scale
PantheraLeo1359531 · CC BY 4.0 · Wikimedia Commons

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.

Bare feet standing on a bathroom weighing scale
Photograph: Frank C. Müller, Baden-Baden · CC BY-SA 4.0 · Wikimedia Commons

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].
The evidence behind this page A stacked bar showing the composition of the 19 publications cited on this page by study type. 7921meta-analysis (7)randomised trial (9)review (2)other (1)
19 publications, 2004–2026. That is a mix with both trials and syntheses in it, which is the position from which a claim about cause is reasonable. Source: this page’s own citation list, below.

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. The association of the adipokine zinc-alpha2-glycoprotein with non-alcoholic fatty liver disease and related risk factors: A comprehensive systematic review Namkhah Z, Naeini F, Ostadrahimi A, et al. · International journal of clinical practice · 2021 · Systematic review DOIPubMed 33404166
  3. 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
  4. 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
  5. 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
  6. Brown Adipose Tissue (BAT) detection by (18)F-FDG PET and thyroid hormone level(s)-a systematic review Santhanam P, Ahima RS, Mammen JS, et al. · Endocrine · 2018 · Systematic review DOIPubMed 30066287
  7. 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
  8. Changes in circulating bile acid levels during cold exposure are associated with brown adipose tissue in humans: a secondary analysis from the ACTIBATE study Jurado-Fasoli L, Martinez-Tellez B, Kohler I, et al. · Journal of physiology and biochemistry · 2026 · Randomised controlled trial DOIPubMed 41762355Full text
  9. Acute Cold Exposure Improves Glucose Tolerance and Induces Beta-Cell Secretion Response Linked to Lipid Utilization in Young Male with Obesity Chamorro R, Bertozzi B, Backhaus J, et al. · Experimental and clinical endocrinology & diabetes : official journal, German Society of Endocrinology [and] German Diabetes Association · 2025 · Randomised controlled trial DOIPubMed 41125105
  10. Effects of boysenberry on postprandial energy metabolism in healthy adults: A randomized controlled crossover trial Furuuchi R, Kato S, Maejima D, et al. · PloS one · 2025 · Randomised controlled trial DOIPubMed 40844985Full text
  11. Effect of high-dose glucocorticoid treatment on human brown adipose tissue activity: a randomised, double-blinded, placebo-controlled cross-over trial in healthy men Maushart CI, Sun W, Othman A, et al. · EBioMedicine · 2023 · Randomised controlled trial DOIPubMed 37659283Full text
  12. 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
  13. Diurnal variations of cold-induced thermogenesis in young, healthy adults: A randomized crossover trial Acosta FM, Sanchez-Delgado G, Martinez-Tellez B, et al. · Clinical nutrition (Edinburgh, Scotland) · 2021 · Randomised controlled trial DOIPubMed 34536639
  14. No evidence for brown adipose tissue activation after creatine supplementation in adult vegetarians Connell NJ, Doligkeit D, Andriessen C, et al. · Nature metabolism · 2021 · Randomised controlled trial DOIPubMed 33462512
  15. Glucocorticoids suppress brown adipose tissue function in humans: A double-blind placebo-controlled study Thuzar M, Law WP, Ratnasingam J, et al. · Diabetes, obesity & metabolism · 2018 · Randomised controlled trial DOIPubMed 29119718
  16. Glucocorticoids modulate human brown adipose tissue thermogenesis in vivo Scotney H, Symonds ME, Law J, et al. · Metabolism: clinical and experimental · 2017 · Randomised controlled trial DOIPubMed 28403937Full text
  17. Brown Adipose Tissue-A Translational Perspective Carpentier AC, Blondin DP, Haman F, et al. · Endocrine reviews · 2023 · Review DOIPubMed 35640259Full text
  18. Brown adipose tissue: function and physiological significance Cannon B, Nedergaard J · Physiological reviews · 2004 · Review DOIPubMed 14715917
  19. 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