The Science of Brown Fat: What more is Yet to be Known?

Brown fat is said to be “good”. It is known that it plays an important role in thermogenesis and maintains body temperature when exposed to cold conditions. An increase in amount of BAT and/or its activation has been shown to be positively correlated with improvement of cardiometabolic health. Animal studies have shown that the brown fat can be increased/activated by exposure to cold conditions, reduced exposure to light and/or upregulation of specific genes. Further research and extensive human trials are needed to establish the importance of increased activation of BAT in improving cardiometabolic health. 

Brown fat is also called brown adipose tissue or BAT in short. It is a special type of body fat that is turned on (activated) when we experience cold. The heat produced by brown fat helps maintain our body temperature in cold conditions. The function of BAT is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency is of great significance for the body. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., among new-borns soon after birth and during fever when body temperature increases. Brown fat cells possess multilocular lipid droplets and large numbers of mitochondria that contain a unique protein called uncoupling protein 1 (UCP1) ⁽¹⁾. The development of brown adipose tissue along with its uncoupling protein-1 (UCP1), is probably responsible for the evolutionary success of mammals as homeothermic creatures, as its thermogenesis enhances neonatal survival and allows for active life under cold conditions ⁽²⁾. 

The presence of BAT has been positively associated with cardiometabolic health. Individuals with BAT have reduced obesity and have less prevalence of type 2 diabetes (increased insulin sensitivity), dyslipidaemia, coronary artery disease, cerebrovascular disease, congestive heart failure and hypertension. These findings were supported by improved blood glucose (low values), and increased high-density lipoprotein values. Moreover, the beneficial effects of BAT were more pronounced in obese individuals, indicating that BAT might also play a role in mitigating the harmful effects of obesity ⁽³⁾. The presence and function of BAT may have implications for the recent pandemic caused by COVID-19. It is becoming increasingly clear that obese individuals with more white adipose tissue (WAT) may be more prone to having and contracting severe COVID-19 ⁽⁴⁾ and it may be postulated that presence of BAT may have a beneficial effect with regards to contracting COVID-19 disease. 

Recent research evidence suggests that using therapeutic interventions such as use of mirabegron, a beta 3 adrenergic receptor agonist, can improve obesity-related metabolic disease by increasing brown adipose tissue (BAT) thermogenesis. In fact, results of chronic mirabegron therapy showed increased BAT metabolic activity, without significant changes in body weight or composition. In addition, plasma levels of beneficial lipoprotein biomarkers HDL and ApoA1 (apolipoprotein A1) were found to be higher. Adiponectin, a WAT-derived hormone that has antidiabetic and anti-inflammatory capabilities, also showed a 35% increase upon completion of the study. These were coupled with higher insulin sensitivity and insulin secretion⁽⁵⁾. 

What are the implications of the presence or beneficial effects of BAT for the common man? Can we activate BAT by reduced exposure to light or by upregulating genes expressed in BAT or by exposure to cold conditions? At least, the research on mice shed some light on these ^(6,7) and may pave a way for further initiation of studies on humans.

Does that mean that exposure to colder temperatures activates BAT and/or increases BAT volume? A randomised trial of cold exposure in humans for 1hr per day for 6 weeks resulted in increased volume of BAT ⁽⁸⁾. 

Further research and extensive human trials are required to bring out the beneficial effects of BAT on humans.  

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References:  

1. Liangyou R. 2017. Brown and Beige adipose tissues in health and disease. Compr Physiol. 2017 Sep 12; 7(4): 1281–1306. DOI: https://doi.org/10.1002/cphy.c17001 

2. Cannon B., and Jan Nedergaard J., 2004. Brown adipose tissue: function and physiological significance. Physiological Review. 2004 Jan;84(1):277-359. DOI: https://doi.org/10.1152/physrev.00015.2003  

3. Becher, T., Palanisamy, S., Kramer, D.J. et al. 2021 Brown adipose tissue is associated with cardiometabolic health. Published: 04 January 2021. Nature Medicine (2021). DOI: https://doi.org/10.1038/s41591-020-1126-7 

4. Dugail I, Amri E-Z and Vitale N. High prevalence for obesity in severe COVID-19: Possible links and perspectives towards patient stratification, Biochimie, Volume 179, 2020, Pages 257-265, ISSN 0300-9084. DOI: https://doi.org/10.1016/j.biochi.2020.07.001. 

5. O’Mara A., Johnson J., Linderman J., 2020. Chronic mirabegron treatment increases human brown fat, HDL cholesterol, and insulin sensitivity. Published January 21, 2020. Journal of Clinical Investigation Volume 130, Issue 5 on May 1, 2020, 2209–2219. DOI: https://doi.org/10.1172/JCI131126  

6. Shultz D. Could turning out the lights help you burn fat? Biology. 2015, DOI: https://doi.org/10.1126/science.aac4580 

7. Houtkooper R., 2018. Fat up to BAT. Science Translational Medicine 04 Jul 2018: Vol. 10, Issue 448, eaau1972. DOI: https://doi.org/10.1126/scitranslmed.aau1972  

8. A randomized trial of cold-exposure on energy expenditure and supraclavicular brown adipose tissue volume in humans. DOI: https://doi.org/10.1016/j.metabol.2016.03.012 

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