Cold-induced activity of brown adipose tissue in young lean men of South-Asian and European origin

Abstract

Aims/hypothesis South Asians have a disproportionately high risk of developing abdominal obesity, insulin resistance and type 2 diabetes. Brown adipose tissue (BAT) has been identified as a possible target to fight obesity and protect against metabolic disturbance. We explored whether lower BAT activity in South Asians compared with Europids may contribute to the high risk of metabolic disturbance. Methods We studied 20 healthy men (ten Europids/ten South Asians, BMI 19-25 kg/m 2 , age 18-32 years). Following 2 h of cold exposure (16-18°C) after an overnight fast, 18 F-fluorodeoxyglucose ( 18 F-FDG) positronemission tomography-computed tomography (CT) and 123 I-metaiodobenzylguanidine ( 123 I-MIBG) single-photon emission computed tomography-CT were performed to visualise metabolic BAT activity and sympathetic stimulation of BAT. Metabolic BAT activity was defined as maximal standardised uptake value (SUV max ) of 18 F-FDG, and sympathetic stimulation of BAT as semiquantitative uptake value (SQUV) of 123 I-MIBG. We performed hyperinsulinaemic-euglycaemic clamps to assess insulin sensitivity. Spearman's correlations for SUV max of 18 F-FDG and both SQUV of 123 I-MIBG and insulin sensitivity were determined. Results The median (interquartile range) SUV max of 18 F-FDG in South Asians (7.5 [2.2-10.6] g/ml) was not different from the median SUV max obtained in Europids (4.5 [2.2-8.4] g/ml; p=0.59). There was no correlation between BAT activity and insulin sensitivity. Correlations between SQUV of 123 I-MIBG and SUV max of 18 F-FDG were positive, both in the total population (ρ=0.80, p<0.001) and after stratification by ethnicity (Europids, ρ =0.65, p = 0.04; South Asians, ρ=0.83, p=0.01). Conclusions/interpretation This is the first study to prospectively investigate ethnic differences in metabolic BAT activity during cold exposure. We did not find differences in BAT activity between South Asians and Europids. Therefore, it seems unlikely that BAT plays an important role in the development of unfavourable metabolic profiles in South Asians.

Brown adipose tissue thermogenesis in humans

“…In line with previous reports [10,13], Admiraal et al [11] did not find any correlation between whole body insulin sensitivity measured using euglycaemic clamp and BAT activity. The mechanisms by which BAT influences insulin sensitivity are indirect and complex.…”

“…A wide variation in values during cold exposure has also been found when FDG data have been modelled [4,10]. The main result of Admiraal et al was that the expected difference in BAT activity between ethnic groups was not found [11]. The low number of participants included in the study may be responsible for the lack of evidence supporting the hypothesis that BAT specifically contributes to weight gain in the South-Asian population.…”

“…In this issue of Diabetologia, Admiraal et al report the results of a study in which they compared BAT activation during cold-exposure between two different ethnic groups: South Asians and Europids [11]. In the UK, the risk of type 2 diabetes is four-to sixfold higher in the South-Asian population than in non-South-Asian populations [12].…”

“…Admiraal et al recruited healthy young men with normal BMIs; ten Europid and ten South Asian [11]. BAT activity was measured after cold exposure using static FDG PETcomputed tomography scanning.…”

Insights into Brown Adipose Tissue Physiology as Revealed by Imaging Studies

“…13 Cold-stimulated BAT activity was found to be similar in Caucasians (n D 10) and South Asians (n D 10). 16 In another study, although cold-induced BAT activity was similar between age-and BMI-matched male Caucasians (n D 11) and South Asians (n D 12), the volume of activated BAT was significantly lower in South Asians. 17 Additionally, cold-induced/non-shivering thermogenesis increased in Caucasians by 20% but did not increase in South Asians exposed to the same conditions.…”