Abstract
Only with the development of the uncoupling protein 1 (UCP1)-ablated mouse has it become possible to strictly delineate the physiological significance of the thermogenic capacity of brown adipose tissue. Considering the presence of active brown adipose tissue in adult humans, these insights may have direct human implications. In addition to classical nonshivering thermogenesis, all adaptive adrenergic thermogeneses, including diet-induced thermogenesis, is fully dependent on brown adipocyte activity. Any weight-reducing effect of β(3)-adrenergic agonists is fully dependent on UCP1 activity, as is any weight-reducing effect of leptin (in excess of its effect on reduction of food intake). Consequently, in the absence of the thermogenic activity of brown adipose tissue, obesity develops spontaneously. The ability of brown adipose tissue to contribute to glucose disposal is also mainly related to thermogenic activity. However, basal metabolic rate, cold-induced thermogenesis, acute cold tolerance, fevers, nonadaptive adrenergic thermogenesis and processes such as angiogenesis in brown adipose tissue itself are not dependent on UCP1 activity. Whereas it is likely that these conclusions are also qualitatively valid for adult humans, the quantitative significance of brown adipose tissue for human metabolism--and the metabolic consequences for a single individual possessing more or less brown adipose tissue--awaits clarification.
Highlights
The present realization that brown adipose tissue is found in a significant fraction of the adult human population[1,2,3,4,5,6] and that the tissue is metabolically active in these adults[1,3,4,6] brings to the fore the issue of the functional importance of the tissue for our understanding of human metabolism
E-mail: barbara.cannon@wgi.su.se knownFthat is, since the 1960sFit was really only after the introduction of the uncoupling protein 1 (UCP1)-ablated mouse[7] that conclusive experiments could be designed to establish which physiological phenomena were dependent on UCP1
We define all UCP1-containing cells as being brown adipocytes, without distinguishing whether they are found in classical brown adipose tissue depots or elsewhere in the body
Summary
The present realization that brown adipose tissue is found in a significant fraction of the adult human population[1,2,3,4,5,6] and that the tissue is metabolically active in these adults (and, correspondingly, that it is not present and is not active in an significant number of adults)[1,3,4,6] brings to the fore the issue of the functional importance of the tissue for our understanding of human metabolism. The recruitment of brown adipose tissue that occurs successively in chronic cold is the basis for the increasing ability of the animal to produce heat by adaptive nonshivering thermogenesis.
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