Abstract
Brown adipose tissue undergoes a dynamic, heterogeneous response to cold exposure that can include the simultaneous synthesis, uptake, and oxidation of fatty acids. The purpose of this work was to quantify these changes in brown adipose tissue lipid content (fat-signal fraction (FSF)) using fat-water magnetic resonance imaging during individualized cooling to 3 °C above a participant’s shiver threshold. Eight healthy men completed familiarization, perception-based cooling, and MRI-cooling visits. FSF maps of the supraclavicular region were acquired in thermoneutrality and during cooling (59.5 ± 6.5 min). Brown adipose tissue regions of interest were defined, and voxels were grouped into FSF decades (0–10%, 10–20%…90–100%) according to their initial value. Brown adipose tissue contained a heterogeneous morphology of lipid content. Voxels with initial FSF values of 60–100% (P < 0.05) exhibited a significant decrease in FSF while a simultaneous increase in FSF occurred in voxels with initial FSF values of 0–30% (P < 0.05). These data suggest that in healthy young men, cold exposure elicits a dynamic and heterogeneous response in brown adipose tissue, with areas initially rich with lipid undergoing net lipid loss and areas of low initial lipid undergoing a net lipid accumulation.
Highlights
Human brown adipose tissue exhibits a variety of neural, vascular, and metabolic responses to cold exposure
X-ray computed tomography can distinguish between brown and white adipose tissues based on their Hounsfield units18,19. 1H-magnetic resonance (MR) spectroscopy has been used to characterize the content and degree of unsaturation of the lipids within brown vs. white adipocytes, revealing reduced levels of unsaturation and polyunsaturation in the lipids stored in brown adipose tissue[20]
There is a lack of consensus concerning the range of fat signal fraction (FSF) values that define human brown adipose tissue[13] with the ideal range apparently varying in a subject-specific manner[26]
Summary
Human brown adipose tissue exhibits a variety of neural, vascular, and metabolic responses to cold exposure. Biomedical imaging and spectroscopy are preeminent methods for studying the spatial distribution of many physiological and biochemical processes in vivo, and they provide complementary information about the structure and function of brown adipose tissue[13,14] Of these methods, PET, X-ray computed tomography, and magnetic resonance (MR) imaging and spectroscopy allow investigators to study brown adipose tissue lipid content. FSF can be used to detect brown adipose tissue independent of its activation status meaning that cold exposure is not required to estimate brown adipose tissue’s distribution in the body[19] Another important advantage of MRI is that it is non-invasive, non-destructive, and absent of ionizing radiation. There is a lack of consensus concerning the range of FSF values that define human brown adipose tissue[13] with the ideal range apparently varying in a subject-specific manner[26]
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