Summary: Introduction: New strategies for weight loss and weight maintenance in humans are needed. Human brown adipose tissue (BAT) can stimulate energy expenditure and may be a potential therapeutic target for obesity and type 2 diabetes. However, whether exercise training is an efficient stimulus to activate and recruit BAT remains to be explored. This study aimed to evaluate whether regular exercise training affects cold‐stimulated BAT metabolism and, if so, whether this was associated with changes in plasma metabolites.Methods: Healthy sedentary men (n = 11; aged 31 [SD 7] years; body mass index 23 [0.9] kg m⁻²; VO_{2 max} 39 [7.6] mL min⁻¹ kg⁻¹) participated in a 6‐week exercise training intervention. Fasting BAT and neck muscle glucose uptake (GU) were measured using quantitative [¹⁸F]fluorodeoxyglucose positron emission tomography–magnetic resonance imaging three times: (1) before training at room temperature and (2) before and (3) after the training period during cold stimulation. Cervico‐thoracic BAT mass was measured using MRI signal fat fraction maps. Plasma metabolites were analysed using nuclear magnetic resonance spectroscopy.Results: Cold exposure increased supraclavicular BAT GU by threefold (p < 0.001), energy expenditure by 59% (p < 0.001) and plasma fatty acids (p < 0.01). Exercise training had no effect on cold‐induced GU in BAT or neck muscles. Training increased aerobic capacity (p = 0.01) and decreased visceral fat (p = 0.02) and cervico‐thoracic BAT mass (p = 0.003). Additionally, training decreased very low‐density lipoprotein particle size (p = 0.04), triglycerides within chylomicrons (p = 0.04) and small high‐density lipoprotein (p = 0.04).Conclusions: Although exercise training plays an important role for metabolic health, its beneficial effects on whole body metabolism through physiological adaptations seem to be independent of BAT activation in young, sedentary men.

Clustering time activity curves of PET images have been used to separate clinically relevant areas of the brain or tumours. However, PET image segmentation in multiorgan level is much less studied due to the available total-body data being limited to animal studies. Now, the new PET scanners providing the opportunity to acquire total-body PET scans also from humans are becoming more common, which opens plenty of new clinically interesting opportunities. Therefore, organ-level segmentation of PET images has important applications, yet it lacks sufficient research. In this proof of concept study, we evaluate if the previously used segmentation approaches are suitable for segmenting dynamic human total-body PET images in organ level. Our focus is on general-purpose unsupervised methods that are independent of external data and can be used for all tracers, organisms, and health conditions. Additional anatomical image modalities, such as CT or MRI, are not used, but the segmentation is done purely based on the dynamic PET images. The tested methods are commonly used building blocks of the more sophisticated methods rather than final methods as such, and our goal is to evaluate if these basic tools are suited for the arising human total-body PET image segmentation. First, we excluded methods that were computationally too demanding for the large datasets from human total-body PET scanners. These criteria filtered out most of the commonly used approaches, leaving only two clustering methods, -means and Gaussian mixture model (GMM), for further analyses. We combined -means with two different preprocessing approaches, namely, principal component analysis (PCA) and independent component analysis (ICA). Then, we selected a suitable number of clusters using 10 images. Finally, we tested how well the usable approaches segment the remaining PET images in organ level, highlight the best approaches together with their limitations, and discuss how further research could tackle the observed shortcomings. In this study, we utilised 40 total-body [¹⁸F] fluorodeoxyglucose PET images of rats to mimic the coming large human PET images and a few actual human total-body images to ensure that our conclusions from the rat data generalise to the human data. Our results show that ICA combined with -means has weaker performance than the other two computationally usable approaches and that certain organs are easier to segment than others. While GMM performed sufficiently, it was by far the slowest one among the tested approaches, making -means combined with PCA the most promising candidate for further development. However, even with the best methods, the mean Jaccard index was slightly below 0.5 for the easiest tested organ and below 0.2 for the most challenging organ. Thus, we conclude that there is a lack of accurate and computationally light general-purpose segmentation method that can analyse dynamic total-body PET images.

Context Glucose-dependent insulinotropic peptide (GIP) and meal ingestion increase subcutaneous adipose tissue (SAT) perfusion in healthy individuals. The effects of GIP and a meal on visceral adipose tissue (VAT) perfusion are unclear. Objective Our aim was to investigate the effects of meal and GIP on VAT and SAT perfusion in obese individuals with type 2 diabetes mellitus (T2DM) before and after bariatric surgery. Methods We recruited 10 obese individuals with T2DM scheduled for bariatric surgery and 10 control individuals. Participants were studied under 2 stimulations: meal ingestion and GIP infusion. SAT and VAT perfusion was measured using ¹⁵O-H₂O positron emission tomography–magnetic resonance imaging at 3 time points: baseline, 20 minutes, and 50 minutes after the start of stimulation. Obese individuals were studied before and after bariatric surgery. Results Before bariatric surgery the responses of SAT perfusion to meal (P = .04) and GIP-infusion (P = .002) were blunted in the obese participants compared to controls. VAT perfusion response did not differ between obese and control individuals after a meal or GIP infusion. After bariatric surgery SAT perfusion response to a meal was similar to that of controls. SAT perfusion response to GIP administration remained lower in the operated-on than control participants. There was no change in VAT perfusion response after bariatric surgery. Conclusion The vasodilating effects of GIP and meal are blunted in SAT but not in VAT in obese individuals with T2DM. Bariatric surgery improves the effects of a meal on SAT perfusion, but not the effects of GIP. Postprandial increase in SAT perfusion after bariatric surgery seems to be regulated in a GIP-independent manner.

Context: Short-chain fatty acids (SCFAs) produced from dietary fiber fermentation can regulate adipose tissue metabolism through signaling pathways involving G protein–coupled receptors and histone deacetylase inhibition. While preclinical studies suggest they enhance thermogenesis, their role in human brown adipose tissue (BAT) under different thermal conditions remains unclear. Objective: This study explores the associations between circulating SCFAs and human BAT metabolism at room temperature and after cold exposure. Methods: This cross-sectional study included data from 71 adults (aged 20-55 years, body mass index 19-44 kg/m2). Dynamic [15O]O2, [15O]H2O, [¹⁸F]FDG, and [¹⁸F]FTHA positron emission tomography/computed tomography scans were used to assess BAT metabolism. Serum SCFAs were quantified using liquid chromatography–mass spectrometry, and gene expression in biopsy-excised BAT samples (n = 14) was analyzed. Participants were stratified into low- and high-BAT groups based on [¹⁸F]FDG or [¹⁸F]FTHA uptakes. Results: Cold-induced acetate and propionate were positively associated with key in vivo BAT metabolism indicators, namely nonesterified fatty acid (NEFA) uptake and oxygen consumption. Only in the high-BAT group were circulating SCFAs maintained after cold exposure. BAT transcriptome revealed that genes involved in SCFA metabolism (such as conversion to acetyl-CoA) correlated with thermogenic and lipid metabolism genes exclusively in the high-BAT group, suggesting a distinct molecular link between SCFA pathways and BAT function. Conclusion: Circulating SCFAs are linked with BAT oxidative metabolism and NEFA uptake during cold exposure. The observed correlations between SCFA catabolic genes and thermogenic markers suggest that metabolically active BAT may selectively engage SCFA-related pathways, pointing to a potential mechanistic role of SCFAs in supporting BAT function in humans.

Human brain metabolism is susceptible to temperature changes. It has been suggested that the supraclavicular brown adipose tissue (BAT) protects the brain from these fluctuations by regulating heat production through the presence of uncoupling protein 1 (UCP-1). It remains unsolved whether inter-individual variation in the expression of UCP-1, which represents the thermogenic capacity of the supraclavicular BAT, is linked with brain metabolism during cold stress. Ten healthy human participants underwent ¹⁸F-FDG PET scanning of the brain under cold stimulus to determine brain glucose uptake (BGU). On a separate day, an excision biopsy of the supraclavicular fat—the fat proximal to the carotid arteries supplying the brain with warm blood—was performed to determine the mRNA expression of the thermogenic protein UCP-1. Expression of UCP-1 in supraclavicular BAT was directly related to the whole brain glucose uptake rate determined under cold stimulation (rho = 0.71, p = 0.03). In sub-compartmental brain analysis, UCP-1 expression in supraclavicular BAT was directly related to cold-stimulated glucose uptake rates in the hypothalamus, medulla, midbrain, limbic system, frontal lobe, occipital lobe, and parietal lobe (all rho ≥ 0.67, p < 0.05). These relationships were independent of body mass index and age. When analysing gene expressions of BAT secretome, we found a positive correlation between cold-stimulated BGU and DIO2. These findings provide evidence of functional links between brain metabolism under cold stimulation and UCP-1 and DIO2 expressions in BAT in humans. More research is needed to evaluate the importance of these findings in clinical outcomes, for instance, in examining the supporting role of BAT in cognitive functions under cold stress.