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

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) positron-emission 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.